Exchange and Transport

Exchange and Transport

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1. Surface Area to Volume Ratio

1.1 The Fundamental Constraint

As an organism increases in size, its volume grows faster than its surface area. For a cube of side length $l$:

$\mathrm{Surface\ area} = 6l^2, \quad \mathrm{Volume} = l^3, \quad \frac◆LB◆\mathrm{SA}◆RB◆◆LB◆\mathrm{V}◆RB◆ = \frac{6}{l}$

The SA:V ratio therefore decreases as size increases. This has critical implications: the surface area available for exchange of gases, nutrients, and heat becomes proportionally smaller relative to the metabolic demand (proportional to volume).

For single-celled organisms, diffusion alone is sufficient because the SA:V ratio is large and diffusion distances are short. Larger, multicellular organisms require specialised exchange surfaces and mass transport systems.

1.2 Adaptations for Efficient Exchange

All gas exchange surfaces share common features:

1. Large surface area (folding, branching, alveoli) to maximise the rate of diffusion.
2. Thin barrier (short diffusion pathway) to minimise diffusion distance.
3. Steep concentration gradient maintained by ventilation (animals) or air flow (plants).
4. Moist surface to dissolve gases for diffusion through the membrane.
5. Dense blood supply (in animals) to carry away exchanged gases and maintain the gradient.

These features all derive from Fick's law of diffusion:

$J = -D \frac◆LB◆\Delta C◆RB◆◆LB◆\Delta x◆RB◆$

where $J$ is the flux (rate of diffusion per unit area), $D$ is the diffusion coefficient, $\Delta C$ is the concentration difference, and $\Delta x$ is the diffusion distance.

2. Gas Exchange in Humans

2.1 The Mammalian Ventilatory System

Air enters through the nasal cavity (warmed, moistened, filtered by ciliated epithelium and mucus) $\to$ trachea $\to$ bronchi $\to$ bronchioles $\to$ alveoli.

The trachea and bronchi are supported by C-shaped rings of cartilage that keep the airway open while allowing flexibility. The walls contain ciliated epithelium and goblet cells that produce mucus to trap dust and pathogens. The cilia beat upwards, moving mucus towards the pharynx (the mucus escalator).

Bronchioles lack cartilage but have smooth muscle in their walls that can constrict (reducing airflow) or dilate (increasing airflow), regulated by the autonomic nervous system and local chemical signals.

2.2 Alveoli

Alveoli are the sites of gas exchange. Each lung contains approximately $350$ million alveoli, giving a total surface area of approximately $70\ \mathrm{m^2}$.

Alveolar epithelium: a single layer of thin, flattened squamous epithelial cells (thickness $\approx 1\ \mu\mathrm{m}$).

Capillary endothelium: a single layer of endothelial cells, closely pressed against the alveolar epithelium (the combined barrier is $\approx 0.5$--$1.0\ \mu\mathrm{m}$).

Ventilation maintains a steep concentration gradient: incoming air has a higher $p\mathrm{O_2}$ ($\approx 13.3\ \mathrm{kPa}$) than deoxygenated blood ($\approx 5.3\ \mathrm{kPa}$), and a lower $p\mathrm{CO_2}$ ($\approx 0.04\ \mathrm{kPa}$) than blood ($\approx 6.0\ \mathrm{kPa}$).

2.3 Ventilation Mechanism

Breathing is driven by pressure changes in the thorax:

Inspiration (active): the external intercostal muscles contract, moving the ribs upwards and outwards. The diaphragm contracts and flattens. Thoracic volume increases, so intrapulmonary pressure decreases below atmospheric pressure. Air flows in.

Expiration (passive at rest): the external intercostal muscles and diaphragm relax. Elastic recoil of the lungs and thoracic wall decreases thoracic volume, increasing intrapulmonary pressure above atmospheric pressure. Air flows out.

Forced expiration: internal intercostal muscles contract (ribs move down and in), and abdominal muscles contract (pushing the diaphragm up).

2.4 Pulmonary Ventilation Rate

$\mathrm{Pulmonary\ ventilation\ rate} = \mathrm{Tidal\ volume} \times \mathrm{breathing\ rate}$

Tidal volume is the volume of air inhaled or exhaled in one normal breath (approximately $500\ \mathrm{cm^3}$ at rest). Breathing rate is the number of breaths per minute (approximately 15 at rest). Resting ventilation rate is therefore approximately $500 \times 15 = 7500\ \mathrm{cm^3\ min^{-1}}$.

warning

warning the maximum volume of air that can be exhaled after a maximum inhalation ($\approx 4500\ \mathrm{cm^3}$). Residual volume is the air remaining in the lungs after maximum exhalation ($\approx 1500\ \mathrm{cm^3}$).

3. Gas Exchange in Other Organisms

3.1 Insects: The Tracheal System

Insects do not use blood for gas transport. Instead, they have a system of branching tracheae that deliver air directly to tissues.

Air enters through spiracles (valved openings on the thorax and abdomen) $\to$ tracheae $\to$ tracheoles (finer branches, diameter $\approx 1\ \mu\mathrm{m}$) that penetrate individual cells.

Gas exchange occurs at the tips of tracheoles, where they are filled with fluid. During activity, lactic acid builds up in tissues, lowering the water potential; water is drawn out of tracheoles by osmosis, exposing more tracheole surface to air and increasing gas exchange.

Ventilation: larger insects use rhythmic abdominal movements to pump air through the tracheal system. The limits of diffusion in tracheae constrain insect body size.

3.2 Fish: Gills

Fish use gills for gas exchange. Water enters through the mouth, passes over the gills, and exits through opercula (bony flaps on each side).

Each gill consists of many gill filaments (primary lamellae), covered in gill plates (secondary lamellae) that provide a large surface area. Blood flows through the gill plates in the opposite direction to the flow of water.

Countercurrent flow: blood and water flow in opposite directions. This maintains a concentration gradient along the entire length of the gill plate, maximising oxygen uptake. Approximately 80% of dissolved oxygen is extracted from water (compared to approximately 25% in mammalian lungs).

Feature	Countercurrent	Concurrent (parallel)
Gradient maintained?	Yes, along entire length	No; equilibrium reached quickly
$\mathrm{O_2}$ extraction	$\approx 80\%$	$\approx 50\%$
Mechanism	Blood flows opposite to water	Blood flows same direction as water

3.3 Plants: Stomata and Leaf Structure

Gas exchange in plants occurs primarily through stomata (pores, typically on the underside of leaves), each surrounded by a pair of guard cells.

Guard cells control stomatal aperture: when guard cells take up water (by osmosis), they swell and buckle, opening the stomata. When they lose water, they become flaccid and the stomata close. This is regulated by:

• Light: stimulates $\mathrm{K^+}$ uptake by guard cells, lowering their water potential.
• $\mathrm{CO_2}$ concentration: high internal $\mathrm{CO_2}$ causes closure.
• Abscisic acid (ABA): produced during water stress, triggers closure.

Leaf adaptations for gas exchange:

• Large surface area (broad, flat lamina).
• Thin leaves (short diffusion distance).
• Air spaces in the spongy mesophyll (create a large internal surface area in contact with cells).
• Stomata density is higher on the lower surface (reduces water loss).

The conflict between gas exchange and water loss is the central constraint on plant gas exchange. Plants must balance $\mathrm{CO_2}$ uptake for photosynthesis against transpirational water loss.

4. Transport in Plants

4.1 Xylem

Xylem transports water and mineral ions from roots to leaves. Xylem vessels are dead, hollow tubes formed from columns of cells whose end walls and cytoplasm have broken down. Their walls are reinforced with lignin, which:

• Provides mechanical strength to resist the negative pressure (tension) of transpiration.
• Prevents the walls from collapsing inward.
• Is deposited in spiral, annular, or reticulate patterns, allowing some flexibility.

Cohesion-tension theory: water is pulled up the xylem by transpiration pull. Water molecules form hydrogen bonds with each other (cohesion) and with the walls of the xylem (adhesion). As water evaporates from mesophyll cell walls in the leaves, the continuous water column is placed under tension, pulling water upward from the roots. This is a passive process driven by the energy of evaporation.

Root pressure: active transport of mineral ions into the xylem lowers its water potential, causing water to enter by osmosis. This creates a positive pressure that can push water upward (root pressure). This is a minor contribution compared to transpiration pull.

4.2 Phloem

Phloem transports organic solutes (primarily sucrose, but also amino acids) from source to sink. Phloem consists of:

• Sieve tube elements: living cells with no nucleus, few organelles, and perforated end plates (sieve plates) that allow sap to flow between cells. Cytoplasm is continuous through the sieve plates.
• Companion cells: adjacent to sieve tube elements; provide metabolic support (ATP, proteins) through plasmodesmata.

Mass flow hypothesis (pressure flow hypothesis, Munch 1930):

1. Sucrose is actively loaded into sieve tube elements at the source (e.g., photosynthesising leaves) by companion cells. This lowers the water potential inside the sieve tube.
2. Water enters the sieve tube by osmosis from the xylem and surrounding cells, creating high hydrostatic pressure at the source.
3. At the sink (e.g., growing root tips, meristems), sucrose is unloaded (actively or passively), raising the water potential. Water leaves the sieve tube by osmosis.
4. The pressure gradient drives bulk flow (mass flow) of sap from source to sink.

warning

warning a model with limitations. Translocation is faster than predicted by simple diffusion, and some solutes appear to move bidirectionally, which the model does not easily explain. The current consensus is that mass flow is the primary mechanism but is supplemented by cytoplasmic streaming and active transport.

4.3 Transpiration

Transpiration is the evaporation and loss of water vapour from the aerial parts of a plant, primarily through stomata.

Factors affecting transpiration rate:

Factor	Effect	Mechanism
Temperature	Increased rate	Higher kinetic energy of water molecules; faster diffusion
Humidity	Decreased rate at high humidity	Smaller concentration gradient (water potential) between leaf and air
Wind speed	Increased rate	Removes saturated air from leaf surface; maintains steep gradient
Light intensity	Increased rate (indirect)	Opens stomata for photosynthesis

Transpiration is not a useful process for the plant -- it is an inevitable consequence of opening stomata for gas exchange. However, the evaporative cooling it provides can prevent overheating, and the transpiration stream carries mineral ions from roots to shoots.

5. Mammalian Circulatory System

5.1 Double Circulation

Mammals have a closed, double circulatory system:

1. Pulmonary circulation: right ventricle $\to$ pulmonary artery $\to$ lungs $\to$ pulmonary vein $\to$ left atrium. Deoxygenated blood is oxygenated.
2. Systemic circulation: left ventricle $\to$ aorta $\to$ body tissues $\to$ vena cava $\to$ right atrium. Oxygenated blood delivers $\mathrm{O_2}$ and nutrients to tissues.

The blood passes through the heart twice per complete circuit, which is efficient because the heart can maintain different pressures for the two circuits. The systemic circulation requires higher pressure (left ventricle has thicker walls) than the pulmonary circulation.

5.2 Blood Vessels

Feature	Artery	Vein	Capillary
Function	Carry blood away from heart at high pressure	Return blood to heart at low pressure	Exchange of materials with tissues
Wall thickness	Thick (muscle and elastic tissue)	Thin	One cell thick (endothelium only)
Lumen diameter	Narrow (relative to wall)	Wide (relative to wall)	Very narrow ($\approx 8\ \mu\mathrm{m}$)
Valves	Absent (except semilunar in heart)	Present (prevent backflow)	Absent
Blood flow	Pulsatile, high pressure	Steady, low pressure	Slow (facilitates exchange)
Blood type	Oxygenated (except pulmonary)	Deoxygenated (except pulmonary)	Both

Arterioles have smooth muscle in their walls that can constrict (vasoconstriction) or dilate (vasodilation) to redistribute blood flow to tissues with the greatest demand. This is controlled by the sympathetic nervous system and local chemical signals (e.g., $\mathrm{CO_2}$, low pH, low $\mathrm{O_2}$).

5.3 Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat:

1. Ventricular diastole (relaxation): the heart muscle relaxes. Blood flows passively from the atria into the ventricles through the open atrioventricular (AV) valves. The semilunar (SL) valves in the aorta and pulmonary artery are closed.
2. Atrial systole (contraction): the atria contract, pushing the remaining blood into the ventricles.
3. Ventricular systole: the ventricles contract from the apex (base) upwards. The increase in pressure closes the AV valves (producing the first heart sound, "lub") and opens the SL valves. Blood is ejected into the aorta and pulmonary artery. When ventricular pressure falls below arterial pressure, the SL valves close (second heart sound, "dub").

Pressure and volume changes can be represented on a pressure curve: atrial pressure peaks during atrial systole; ventricular pressure rises sharply during ventricular systole (exceeding aortic pressure to open the SL valves); aortic pressure peaks during ejection and falls during diastole.

warning

warning They contract from the base (apex) upwards, which efficiently pushes blood towards the arteries at the top of the heart.

5.4 Cardiac Output

$\mathrm{Cardiac\ output} = \mathrm{stroke\ volume} \times \mathrm{heart\ rate}$

Stroke volume is the volume of blood ejected by one ventricle per beat (approximately $70\ \mathrm{cm^3}$ at rest). Heart rate is approximately $72\ \mathrm{beats\ min^{-1}}$ at rest. Resting cardiac output is therefore approximately $70 \times 72 = 5040\ \mathrm{cm^3\ min^{-1}}$.

During exercise, cardiac output can increase to approximately $25\ \mathrm{dm^3\ min^{-1}}$ through increases in both stroke volume and heart rate.

5.5 Tissue Fluid and Lymph

Tissue fluid is the fluid that surrounds cells in tissues. It is formed by ultrafiltration at the arterial end of capillaries:

1. Hydrostatic pressure (generated by the heart) forces fluid out of capillaries.
2. At the venous end, hydrostatic pressure is lower and the oncotic pressure (due to plasma proteins that cannot leave the capillary) draws fluid back in.
3. The net result is that approximately 90% of fluid leaving at the arterial end returns at the venous end. The remaining 10% enters the lymphatic system and is returned to the circulation via the subclavian veins.

Tissue fluid is similar to blood plasma but has no plasma proteins and fewer blood cells. It exchanges substances with cells by diffusion.

6. Blood

6.1 Components of Blood

Component	Function	Key Features
Red blood cells (erythrocytes)	Transport $\mathrm{O_2}$ and $\mathrm{CO_2}$	Biconcave disc (SA:V ratio); no nucleus; contain haemoglobin
White blood cells (leucocytes)	Defence against pathogens	Phagocytes (engulf pathogens); lymphocytes (produce antibodies)
Platelets (thrombocytes)	Blood clotting	Cell fragments; no nucleus; release clotting factors
Plasma	Transport medium	Water, dissolved substances (glucose, amino acids, urea, hormones, antibodies)

6.2 Haemoglobin and Oxygen Transport

Haemoglobin (Hb) is a quaternary protein with four polypeptide chains (two $\alpha$, two $\beta$), each associated with a haem group containing an iron(II) ion ($\mathrm{Fe^{2+}}$) that binds one $\mathrm{O_2}$ molecule. Each Hb can carry up to four $\mathrm{O_2}$ molecules.

$\mathrm{Hb} + 4\mathrm{O_2} \rightleftharpoons \mathrm{HbO_8}$

The oxygen dissociation curve is sigmoidal (S-shaped) because haemoglobin exhibits cooperative binding: the binding of the first $\mathrm{O_2}$ molecule changes the conformation of Hb, increasing the affinity of the remaining haem groups for $\mathrm{O_2}$.

Factors shifting the curve right (decreasing affinity):

• Higher $p\mathrm{CO_2}$ (Bohr effect)
• Lower pH (higher $\mathrm{H^+}$ concentration)
• Higher temperature
• Higher concentration of 2,3-BPG (2,3-bisphosphoglycerate)

These conditions occur in actively respiring tissues, facilitating $\mathrm{O_2}$ unloading. In the lungs, the reverse conditions (high $p\mathrm{O_2}$, low $p\mathrm{CO_2}$, lower temperature) shift the curve left, facilitating $\mathrm{O_2}$ loading.

Fetal haemoglobin (HbF) has a higher affinity for $\mathrm{O_2}$ than adult haemoglobin (the curve is shifted left). This allows fetal blood to extract $\mathrm{O_2}$ from maternal blood across the placenta, where $p\mathrm{O_2}$ is lower than in the maternal lungs.

6.3 Carbon Dioxide Transport

$\mathrm{CO_2}$ is transported in three ways:

1. Dissolved in plasma (approximately 5%): $\mathrm{CO_2}$ is slightly soluble.
2. Bound to haemoglobin as carbaminohaemoglobin (approximately 10%): $\mathrm{CO_2}$ binds to amino groups on the globin chains (not the haem group).
3. As hydrogencarbonate ions ($\mathrm{HCO_3^-}$) (approximately 85%): $\mathrm{CO_2}$ enters red blood cells and is catalysed by carbonic anhydrase:

$$\mathrm{CO_2} + \mathrm{H_2O} \rightleftharpoons \mathrm{H_2CO_3} \rightleftharpoons \mathrm{H^+} + \mathrm{HCO_3^-}$$

The $\mathrm{H^+}$ ions are taken up by haemoglobin (acting as a buffer), causing $\mathrm{O_2}$ to be released (the Bohr effect). \mathrm{HCO_3^- ions are exchanged for $\mathrm{Cl^-}$ ions (chloride shift) to maintain electrochemical neutrality.

7. Quantitative Gas Exchange: Fick's Law Applications

7.1 Applying Fick's Law

Fick's first law of diffusion states:

$J = -D \frac◆LB◆\Delta C◆RB◆◆LB◆\Delta x◆RB◆$

where:

• $J$ is the flux (rate of diffusion per unit area, in $\mathrm{mol\ m^{-2}\ s^{-1}}$)
• $D$ is the diffusion coefficient (in $\mathrm{m^2\ s^{-1}}$), which depends on the molecule and the medium
• $\Delta C$ is the concentration difference across the barrier (in $\mathrm{mol\ m^{-3}}$)
• $\Delta x$ is the diffusion distance (in $\mathrm{m}$)

The total rate of diffusion across a surface of area $A$ is:

$\text{Rate} = J \times A = D \cdot A \cdot \frac◆LB◆\Delta C◆RB◆◆LB◆\Delta x◆RB◆$

This equation directly explains the adaptations of gas exchange surfaces: maximising $A$ (large surface area), minimising $\Delta x$ (thin barrier), and maximising $\Delta C$ (maintaining a steep concentration gradient via ventilation and blood flow) all increase the rate of diffusion.

7.2 Worked Examples

Worked Example 1. The diffusion coefficient of $\mathrm{O_2}$ in water is $D = 1.8 \times 10^{-9}\ \mathrm{m^2\ s^{-1}}$. An alveolus has a surface area of $A = 200\ \mu\mathrm{m^2}$ and a barrier thickness of $\Delta x = 0.5\ \mu\mathrm{m}$. The concentration of $\mathrm{O_2}$ in alveolar air corresponds to $p\mathrm{O_2} = 13.3\ \mathrm{kPa}$, and in deoxygenated blood $p\mathrm{O_2} = 5.3\ \mathrm{kPa}$. Estimate the rate of $\mathrm{O_2}$ diffusion across this alveolus.

Using Henry's law, $\mathrm{O_2}$ concentration in water is proportional to partial pressure. The solubility of $\mathrm{O_2}$ in water at $37\ ^\circ\mathrm{C}$ is approximately $\alpha = 1.3 \times 10^{-3}\ \mathrm{mol\ m^{-3}\ kPa^{-1}}$.

$\Delta C = \alpha \times \Delta p\mathrm{O_2} = 1.3 \times 10^{-3} \times (13.3 - 5.3) = 0.0104\ \mathrm{mol\ m^{-3}}$

$\text{Rate} = D \cdot A \cdot \frac◆LB◆\Delta C◆RB◆◆LB◆\Delta x◆RB◆ = 1.8 \times 10^{-9} \times 200 \times 10^{-12} \times \frac◆LB◆0.0104◆RB◆◆LB◆0.5 \times 10^{-6}◆RB◆$

$= 1.8 \times 10^{-9} \times 2 \times 10^{-10} \times 20800 = 7.5 \times 10^{-15}\ \mathrm{mol\ s^{-1}}$

Per single alveolus. With $\approx 350$ million alveoli per lung, the total rate is enormous, which is why the human respiratory system can meet the body's $\mathrm{O_2}$ demand.

Worked Example 2. Compare the rate of $\mathrm{O_2}$ diffusion through the alveolar barrier ($\Delta x = 0.5\ \mu\mathrm{m}$) with diffusion through a layer of connective tissue ($\Delta x = 5.0\ \mu\mathrm{m}$), all else being equal.

$\frac◆LB◆J_{\mathrm{alveolus}}◆RB◆◆LB◆J_{\mathrm{tissue}}◆RB◆ = \frac◆LB◆D \cdot \Delta C / 0.5◆RB◆◆LB◆D \cdot \Delta C / 5.0◆RB◆ = \frac{5.0}{0.5} = 10$

The alveolar barrier is 10 times more efficient at gas exchange because it is 10 times thinner. This quantifies the critical importance of a thin diffusion barrier.

7.3 Diffusion Coefficients: A Note on Medium

The diffusion coefficient of $\mathrm{O_2}$ is much larger in air ($D \approx 2.1 \times 10^{-5}\ \mathrm{m^2\ s^{-1}}$) than in water ($D \approx 1.8 \times 10^{-9}\ \mathrm{m^2\ s^{-1}}$). The ratio is approximately $12000:1$. This means that organisms living in water face a fundamentally more challenging gas exchange problem than air-breathing organisms, which is why fish gills must be extremely efficient (countercurrent flow, large surface area) to extract sufficient $\mathrm{O_2}$ from water.

warning

Common Pitfall Students often state that gas exchange is faster in air "because air is less dense." While this is directionally correct, the precise reason is that the diffusion coefficient in air is approximately four orders of magnitude larger than in water. This is a consequence of the kinetic theory of gases: gas molecules travel further between collisions in a gas than in a liquid.

8. Water Potential Calculations

8.1 Osmosis and Water Potential

Water potential ($\Psi$) determines the direction of water movement. Water always moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential.

For plant cells:

$\Psi_{\mathrm{cell}} = \Psi_s + \Psi_p$

8.2 Worked Examples

Worked Example 1. A plant cell with $\Psi_s = -1200\ \mathrm{kPa}$ and $\Psi_p = +400\ \mathrm{kPa}$ is placed in a solution with $\Psi_{\mathrm{solution}} = -500\ \mathrm{kPa}$. Determine the direction of net water movement and the cell's equilibrium state.

$\Psi_{\mathrm{cell}} = -1200 + 400 = -800\ \mathrm{kPa}$

Since $\Psi_{\mathrm{solution}} = -500\ \mathrm{kPa} > \Psi_{\mathrm{cell}} = -800\ \mathrm{kPa}$, water moves from the solution into the cell. As water enters, $\Psi_p$ increases (the cell becomes more turgid). Equilibrium when $\Psi_{\mathrm{cell}} = -500\ \mathrm{kPa}$:

$\Psi_s + \Psi_p = -500$

$-1200 + \Psi_p = -500$

$\Psi_p = +700\ \mathrm{kPa}$

The cell reaches equilibrium at a pressure potential of $+700\ \mathrm{kPa}$.

Worked Example 2. A plant cell is placed in a solution of sucrose with concentration $0.3\ \mathrm{mol\ dm^{-3}}$ at $20\ ^\circ\mathrm{C}$. The cell has $\Psi_s = -1000\ \mathrm{kPa}$ and $\Psi_p = +200\ \mathrm{kPa}$. Describe what happens.

The solute potential of the external solution is approximately:

$\Psi_s = -iCRT$

where $i = 1$ (sucrose does not ionise), $C = 0.3\ \mathrm{mol\ dm^{-3}} = 300\ \mathrm{mol\ m^{-3}}$, $R = 8.314\ \mathrm{J\ mol^{-1}\ K^{-1}}$, $T = 293\ \mathrm{K}$.

$\Psi_{\mathrm{solution}} = -1 \times 300 \times 8.314 \times 293 = -730800\ \mathrm{Pa} = -731\ \mathrm{kPa}$

$\Psi_{\mathrm{cell}} = -1000 + 200 = -800\ \mathrm{kPa}$

Since $\Psi_{\mathrm{solution}} = -731\ \mathrm{kPa} > \Psi_{\mathrm{cell}} = -800\ \mathrm{kPa}$, water moves into the cell. The cell gains water, $\Psi_p$ increases until equilibrium is reached at $\Psi_p = +269\ \mathrm{kPa}$.

Worked Example 3. A student carries out an experiment to determine the solute potential of potato cells. Potato cylinders are placed in a range of sucrose solutions. The results are:

Sucrose concentration ($\mathrm{mol\ dm^{-3}}$)	0.0	0.1	0.2	0.3	0.4	0.5
Percentage change in mass (%)	+12	+6	-1	-8	-15	-22

The concentration at which there is no change in mass lies between $0.1$ and $0.2\ \mathrm{mol\ dm^{-3}}$. By interpolation: the equilibrium concentration is approximately $0.17\ \mathrm{mol\ dm^{-3}}$.

The solute potential of the potato cells at this concentration:

$\Psi_s = -iCRT = -1 \times 170 \times 8.314 \times 293 = -414000\ \mathrm{Pa} \approx -414\ \mathrm{kPa}$

At equilibrium (no net water movement), $\Psi_{\mathrm{cell}} = \Psi_{\mathrm{solution}}$ and $\Psi_p = 0$ (the cell is at the point of incipient plasmolysis). Therefore, $\Psi_s \approx -414\ \mathrm{kPa}$.

warning

Common Pitfall Students sometimes forget that the formula $\Psi_s = -iCRT$ gives the solute potential of the solution, not the cell. The cell's solute potential is only equal to this value at equilibrium when $\Psi_p = 0$. In a turgid cell, $\Psi_p > 0$ and $\Psi_{\mathrm{cell}}$ is less negative than $\Psi_s$.

9. Detailed Cardiac Cycle and Pressure Curves

9.1 Pressure Relationships

Understanding the pressure changes during the cardiac cycle requires tracking three pressures simultaneously: atrial pressure ($P_{\mathrm{atrium}}$), ventricular pressure ($P_{\mathrm{ventricle}}$), and aortic pressure ($P_{\mathrm{aorta}}$).

The fundamental rule governing valve behaviour:

• AV valves open when $P_{\mathrm{atrium}} > P_{\mathrm{ventricle}}$; close when $P_{\mathrm{ventricle}} > P_{\mathrm{atrium}}$.
• Semilunar valves open when $P_{\mathrm{ventricle}} > P_{\mathrm{aorta}}$; close when $P_{\mathrm{aorta}} > P_{\mathrm{ventricle}}$.

9.2 Pressure Curve Analysis

The pressure curve is divided into phases:

1. Ventricular filling (mid-to-late diastole): $P_{\mathrm{ventricle}}$ is low and slowly rising as blood flows passively from atria. AV valves open; SL valves closed.

2. Atrial systole: $P_{\mathrm{atrium}}$ rises sharply (the "a wave"), pushing the last 20--30% of blood into the ventricles ("atrial kick").

3. Isovolumetric contraction: $P_{\mathrm{ventricle}}$ rises rapidly but both sets of valves are closed (AV valves just closed due to $P_{\mathrm{ventricle}} > P_{\mathrm{atrium}}$; SL valves not yet open). Ventricular volume is constant (iso-volumetric). This is the brief period where $P_{\mathrm{ventricle}}$ is between $P_{\mathrm{atrium}}$ and $P_{\mathrm{aorta}}$.

4. Ventricular ejection: $P_{\mathrm{ventricle}}$ exceeds $P_{\mathrm{aorta}}$, SL valves open, blood is ejected. $P_{\mathrm{ventricle}}$ peaks and then begins to fall as ejection proceeds. Aortic pressure rises to a peak (systolic pressure).

5. Isovolumetric relaxation: $P_{\mathrm{ventricle}}$ falls below $P_{\mathrm{aorta}}$, SL valves close (the "dicrotic notch" on the aortic curve, caused by backflow against closed aortic valve). $P_{\mathrm{ventricle}}$ continues to fall but is still above $P_{\mathrm{atrium}}$, so AV valves remain closed. Volume is again constant.

6. Return to filling: $P_{\mathrm{ventricle}}$ falls below $P_{\mathrm{atrium}}$, AV valves open, and passive filling resumes.

9.3 Worked Example: Cardiac Output During Exercise

A 20-year-old student has the following measurements at rest:

• Heart rate $= 68\ \mathrm{beats\ min^{-1}}$
• Stroke volume $= 72\ \mathrm{cm^3}$
• End-diastolic volume $= 120\ \mathrm{cm^3}$

During maximal exercise:

• Heart rate $= 195\ \mathrm{beats\ min^{-1}}$
• End-diastolic volume $= 140\ \mathrm{cm^3}$
• End-systolic volume $= 40\ \mathrm{cm^3}$

(a) Calculate cardiac output at rest and during exercise.

At rest: $\mathrm{CO} = 72 \times 68 = 4896\ \mathrm{cm^3\ min^{-1}} = 4.9\ \mathrm{dm^3\ min^{-1}}$.

During exercise: Stroke volume $= 140 - 40 = 100\ \mathrm{cm^3}$. $\mathrm{CO} = 100 \times 195 = 19500\ \mathrm{cm^3\ min^{-1}} = 19.5\ \mathrm{dm^3\ min^{-1}}$.

(b) Calculate the ejection fraction at rest.

Ejection fraction $= \frac◆LB◆\mathrm{Stroke\ volume}◆RB◆◆LB◆\mathrm{End-diastolic\ volume}◆RB◆ \times 100\% = \frac{72}{120} \times 100\% = 60\%$.

(c) The cardiac output increased by a factor of $\frac{19.5}{4.9} \approx 4.0\times$. This is achieved primarily through increased heart rate (from 68 to $195\ \mathrm{beats\ min^{-1}}$, a factor of $2.9\times$) and increased stroke volume (from 72 to $100\ \mathrm{cm^3}$, a factor of $1.4\times$).

warning

Common Pitfall Students sometimes add the increases in heart rate and stroke volume multiplicatively and state the increase as $2.9 \times 1.4 = 4.1\times$. While this gives approximately the right answer, the correct approach is to calculate the cardiac output at each state separately and then compare, as shown above. This avoids rounding errors and is methodologically correct.

10. The Chloride Shift and Bicarbonate Buffer System

10.1 Mechanism of $\mathrm{CO_2}$ Transport in Detail

When $\mathrm{CO_2}$ enters a red blood cell, carbonic anhydrase catalyses its hydration:

$\mathrm{CO_2} + \mathrm{H_2O} \rightleftharpoons \mathrm{H_2CO_3} \rightleftharpoons \mathrm{H^+} + \mathrm{HCO_3^-}$

The $\mathrm{HCO_3^-}$ ions are transported out of the red blood cell in exchange for $\mathrm{Cl^-}$ ions entering from the plasma. This is the chloride shift (Hamburger shift), mediated by the anion exchanger protein (Band 3) in the red blood cell membrane.

The exchange is 1:1: for every $\mathrm{HCO_3^-}$ that leaves, one $\mathrm{Cl^-}$ enters. This maintains electrochemical neutrality inside the cell.

The $\mathrm{H^+}$ ions are buffered by haemoglobin:

$\mathrm{HbO_8} + \mathrm{H^+} \rightleftharpoons \mathrm{HHb} + 4\mathrm{O_2}$

This is the molecular basis of the Bohr effect: the binding of $\mathrm{H^+}$ to haemoglobin reduces its affinity for $\mathrm{O_2}$, promoting $\mathrm{O_2}$ unloading in respiring tissues.

10.2 Reversal in the Lungs

In the pulmonary capillaries, the process reverses. The high $p\mathrm{O_2}$ promotes $\mathrm{O_2}$ binding to haemoglobin, which releases $\mathrm{H^+}$. The $\mathrm{H^+}$ combines with $\mathrm{HCO_3^-}$ to form $\mathrm{H_2CO_3}$, which is broken down by carbonic anhydrase to $\mathrm{CO_2}$ and $\mathrm{H_2O}$. The $\mathrm{CO_2}$ diffuses out into the alveolar air. $\mathrm{Cl^-}$ exits the red blood cell (reverse chloride shift).

warning

Common Pitfall Students sometimes write that $\mathrm{CO_2}$ "binds to haemoglobin" in the same way that $\mathrm{O_2}$ does. $\mathrm{CO_2}$ binds to the amino groups of the globin chains (forming carbaminohaemoglobin), not to the haem groups. $\mathrm{O_2}$ binds to the iron in the haem groups. These are distinct binding sites and mechanisms.

Practice Problems

Details

Problem 1

Explain how the countercurrent flow mechanism in fish gills is more efficient for gas exchange than parallel flow. Use a numerical example.

Answer. In countercurrent flow, water and blood flow in opposite directions. At every point along the gill plate, the $p\mathrm{O_2}$ in water exceeds that in blood, so there is always a diffusion gradient driving $\mathrm{O_2}$ from water into blood. If water enters with $p\mathrm{O_2} = 15\ \mathrm{kPa}$ and blood enters with $p\mathrm{O_2} = 5\ \mathrm{kPa}$, by the time water exits its $p\mathrm{O_2}$ may be $7\ \mathrm{kPa}$, but blood at that point has risen to nearly $15\ \mathrm{kPa}$. In parallel flow, water and blood flow in the same direction; equilibrium is reached partway along the gill, and no further $\mathrm{O_2}$ transfer occurs. The countercurrent system achieves approximately 80% oxygen extraction versus approximately 50% for parallel flow.

If you get this wrong, revise: Fish: Gills

Details

Problem 2

Describe the mechanism by which water is transported through the xylem from roots to leaves. In your answer, refer to the cohesion-tension theory and the role of transpiration.

Answer. The cohesion-tension theory explains water movement in xylem. Water evaporates from the cell walls of mesophyll cells in the leaf into the sub-stomatal air space and exits through stomata (transpiration). As water molecules leave, the remaining water forms a concave meniscus in the cell walls, generating negative pressure (tension). This tension is transmitted through the continuous column of water in the cell walls, through the cytoplasm, and into the xylem vessels. Water molecules are held together by hydrogen bonds (cohesion) and to the hydrophilic xylem walls (adhesion). The tension pulls the entire water column upward from the roots. At the roots, water enters by osmosis following the water potential gradient created by the tension in the xylem. Root pressure (from active transport of ions into the xylem) provides a minor additional push.

If you get this wrong, revise: Xylem

Details

Problem 3

Explain the Bohr effect and its significance in ensuring efficient oxygen delivery to respiring tissues.

Answer. The Bohr effect describes the decrease in haemoglobin's affinity for oxygen in the presence of increased $p\mathrm{CO_2}$ and decreased pH. In actively respiring tissues, cells produce $\mathrm{CO_2}$, which is converted to $\mathrm{H^+}$ and $\mathrm{HCO_3^-}$ by carbonic anhydrase in red blood cells. The increased $\mathrm{H^+}$ concentration lowers pH, which causes a conformational change in haemoglobin that reduces its affinity for $\mathrm{O_2}$. This shifts the oxygen dissociation curve to the right, meaning that at any given $p\mathrm{O_2}$, more $\mathrm{O_2}$ is unloaded from haemoglobin. The significance is that tissues with the highest metabolic rate (and therefore highest $\mathrm{CO_2}$ production) receive the most $\mathrm{O_2}$ delivery. In the lungs, the reverse conditions (low $p\mathrm{CO_2}$, higher pH) shift the curve left, increasing affinity and facilitating $\mathrm{O_2}$ loading.

If you get this wrong, revise: Haemoglobin and Oxygen Transport

Details

Problem 4

Describe how tissue fluid is formed and returned to the circulatory system. Explain why tissue fluid has a different composition from blood plasma.

Answer. Tissue fluid is formed by ultrafiltration at the arterial end of capillaries. The hydrostatic pressure of the blood (generated by cardiac contraction) forces fluid out through the capillary walls, which are permeable to water and small solutes but not to large plasma proteins. This filtrate (tissue fluid) bathes the cells. At the venous end, hydrostatic pressure has fallen but oncotic pressure (due to plasma proteins remaining in the capillary) is unchanged. The oncotic pressure draws fluid back in. Approximately 90% returns this way; the remaining 10% enters the lymphatic system and drains back into the subclavian veins. Tissue fluid differs from blood plasma in that it contains no plasma proteins (too large to leave capillaries) and very few blood cells (too large to leave). It contains water, glucose, amino acids, ions, and other small dissolved molecules at similar concentrations to plasma.

If you get this wrong, revise: Tissue Fluid and Lymph

Details

Problem 5

Explain how the structure of an insect's tracheal system is adapted for efficient gas exchange. Why does this system limit the maximum body size of insects?

Answer. The insect tracheal system consists of air-filled tubes (tracheae) that branch into smaller tracheoles, delivering air directly to cells without requiring a circulatory transport system. Adaptations include: spiracles with valves to control air entry and reduce water loss; tracheoles with very small diameter ($\approx 1\ \mu\mathrm{m}$) that penetrate individual cells, providing a very short diffusion pathway; and in larger insects, rhythmic abdominal movements that ventilate the system (mass flow rather than relying on diffusion alone). The system limits body size because gas transport within tracheae relies on diffusion for the final stages, and diffusion is only effective over very short distances (a few hundred micrometres). As body size increases, the distance from spiracles to the deepest tissues increases, and diffusion becomes insufficient to meet metabolic demands. This is why giant insects existed in the Carboniferous period when atmospheric $\mathrm{O_2}$ was higher (approximately 35% vs. 21% today), increasing the diffusion gradient.

If you get this wrong, revise: Insects: The Tracheal System

Details

Problem 6

A person has a resting stroke volume of $65\ \mathrm{cm^3}$ and a heart rate of $70\ \mathrm{beats\ min^{-1}}$. During exercise, their stroke volume increases to $110\ \mathrm{cm^3}$ and heart rate to $160\ \mathrm{beats\ min^{-1}}$. Calculate the cardiac output at rest and during exercise. By what factor does cardiac output increase?

Answer. At rest: cardiac output $= 65 \times 70 = 4550\ \mathrm{cm^3\ min^{-1}} = 4.55\ \mathrm{dm^3\ min^{-1}}$. During exercise: cardiac output $= 110 \times 160 = 17600\ \mathrm{cm^3\ min^{-1}} = 17.6\ \mathrm{dm^3\ min^{-1}}$. Factor of increase $= 17.6 / 4.55 \approx 3.9\times$.

If you get this wrong, revise: Cardiac Output

Details

Problem 7

A student investigates the effect of temperature on the rate of diffusion of a dye through agar gel. The dye diffuses $3.2\ \mathrm{mm}$ in 20 minutes at $20\ ^\circ\mathrm{C}$ and $5.1\ \mathrm{mm}$ in 20 minutes at $40\ ^\circ\mathrm{C}$. (a) Calculate the rate of diffusion at each temperature. (b) The $Q_{10}$ coefficient is defined as the ratio of rates at temperatures differing by $10\ ^\circ\mathrm{C}$. Estimate $Q_{10}$ for this process. (c) Explain the effect of temperature on the rate of diffusion.

Answer. (a) Rate at $20\ ^\circ\mathrm{C}$: $\frac{3.2}{20} = 0.16\ \mathrm{mm\ min^{-1}}$.

Rate at $40\ ^\circ\mathrm{C}$: $\frac{5.1}{20} = 0.255\ \mathrm{mm\ min^{-1}}$.

(b) $Q_{10} = \left(\frac{v_{40}}{v_{20}}\right)^{\frac{10}{40-20}} = \left(\frac{0.255}{0.16}\right)^{0.5} = (1.594)^{0.5} = 1.26$.

The rate of diffusion increases by approximately 26% for each $10\ ^\circ\mathrm{C}$ increase in temperature.

(c) Increasing temperature increases the kinetic energy of dye molecules, causing them to move faster. According to the kinetic theory, the average kinetic energy of molecules is proportional to absolute temperature ($\mathrm{KE} = \frac{3}{2}k_BT$). Higher kinetic energy means more frequent and more energetic collisions, increasing the rate of diffusion. Note that this effect is relatively modest for simple diffusion ($Q_{10} \approx 1.2$--$1.4$) compared with enzyme-catalysed reactions ($Q_{10} \approx 2$--$3$), because diffusion does not involve the conformational changes that make enzyme rates so temperature-sensitive.

If you get this wrong, revise: Quantitative Gas Exchange: Fick's Law Applications

Details

Problem 8

Describe and explain the mechanism of mass flow in the phloem. Include reference to the role of active transport, water potential, and hydrostatic pressure. Why is the mass flow hypothesis considered incomplete?

Answer. At the source (photosynthesising leaves), sucrose is actively loaded into sieve tube elements by companion cells using ATP. This lowers the water potential inside the sieve tube (sucrose is a solute, so $\Psi_s$ becomes more negative). Water enters the sieve tube by osmosis from the xylem (which has a higher water potential), creating high hydrostatic pressure. At the sink (e.g., growing root tips), sucrose is unloaded from sieve tubes (actively or passively). This raises the water potential inside the sieve tube. Water leaves by osmosis into surrounding cells. The resulting hydrostatic pressure gradient drives bulk flow (mass flow) of sap from source to sink through the phloem.

The mass flow hypothesis is considered incomplete because: (1) it does not easily explain bidirectional transport (sucrose moving in opposite directions in different sieve tubes simultaneously); (2) the calculated flow rates are sometimes lower than observed translocation rates; (3) sieve plates would be expected to impede flow, yet translocation is rapid. Current models supplement mass flow with cytoplasmic streaming and active transport along sieve tubes.

If you get this wrong, revise: Phloem

Details

Problem 9

A spirometer trace from a student shows the following measurements: tidal volume $= 450\ \mathrm{cm^3}$, vital capacity $= 4200\ \mathrm{cm^3}$, breathing rate $= 16\ \mathrm{breaths\ min^{-1}}$, and respiratory minute ventilation $= 7.2\ \mathrm{dm^3\ min^{-1}}$. After 5 minutes of exercise, the breathing rate increases to $28\ \mathrm{breaths\ min^{-1}}$ and tidal volume to $750\ \mathrm{cm^3}$. (a) Verify the resting respiratory minute ventilation. (b) Calculate the respiratory minute ventilation during exercise. (c) Calculate the volume of air that ventilates the anatomical dead space per minute at rest and during exercise, given a dead space volume of $150\ \mathrm{cm^3}$.

Answer. (a) Respiratory minute ventilation $= \text{tidal volume} \times \text{breathing rate} = 450 \times 16 = 7200\ \mathrm{cm^3\ min^{-1}} = 7.2\ \mathrm{dm^3\ min^{-1}}$. This matches the stated value.

(b) During exercise: $750 \times 28 = 21000\ \mathrm{cm^3\ min^{-1}} = 21.0\ \mathrm{dm^3\ min^{-1}}$.

(c) Dead space ventilation (volume of air that does not reach the alveoli) $= \text{dead space volume} \times \text{breathing rate}$.

At rest: $150 \times 16 = 2400\ \mathrm{cm^3\ min^{-1}} = 2.4\ \mathrm{dm^3\ min^{-1}}$.

During exercise: $150 \times 28 = 4200\ \mathrm{cm^3\ min^{-1}} = 4.2\ \mathrm{dm^3\ min^{-1}}$.

The alveolar ventilation (air actually reaching the gas exchange surface) is:

At rest: $7.2 - 2.4 = 4.8\ \mathrm{dm^3\ min^{-1}}$.

During exercise: $21.0 - 4.2 = 16.8\ \mathrm{dm^3\ min^{-1}}$.

The alveolar ventilation increased by a factor of $\frac{16.8}{4.8} = 3.5\times$, ensuring sufficient $\mathrm{O_2}$ uptake and $\mathrm{CO_2}$ removal during exercise.

If you get this wrong, revise: Pulmonary Ventilation Rate

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11. The Mammalian Heart in Detail

11.1 Cardiac Muscle Structure

Cardiac muscle cells (cardiomyocytes) are branched, striated (contain actin and myosin arranged in sarcomeres), and connected by intercalated discs containing:

• Desmosomes: mechanical connections that prevent cells from separating during contraction.
• Gap junctions: channels that allow ions to pass freely between adjacent cells, enabling rapid spread of electrical impulses. This means cardiac muscle functions as a syncytium -- a single functional unit where all cells contract together.

Cardiac muscle is myogenic: it can initiate its own contractions without nervous stimulation (unlike skeletal muscle, which requires motor neuron input).

11.2 The Cardiac Conduction System

1. Sinoatrial node (SAN): the pacemaker, located in the wall of the right atrium near the opening of the superior vena cava. The SAN generates waves of electrical depolarisation at approximately 70 impulses per minute (resting heart rate). It sets the rhythm because its intrinsic rate is faster than that of other pacemaker tissue.
2. The wave of depolarisation spreads across both atria, causing atrial systole (contraction).
3. Non-conducting tissue (fibrous skeleton) prevents the wave from passing directly to the ventricles.
4. The wave reaches the atrioventricular node (AVN), located in the septum between the atria. The AVN delays the impulse for approximately 0.1 seconds, ensuring the atria have fully emptied before the ventricles contract.
5. The impulse travels down the bundle of His (in the interventricular septum).
6. The bundle branches into the Purkinje fibres in the ventricular walls.
7. The ventricles contract from the apex (bottom) upwards, squeezing blood efficiently towards the arteries.

11.3 The Electrocardiogram (ECG)

The ECG records the electrical activity of the heart from the body surface:

Wave	Event	Description
P wave	Atrial depolarisation	Atrial systole follows shortly after
QRS complex	Ventricular depolarisation (and atrial repolarisation, masked)	Ventricular systole; QRS is large because the ventricles have more muscle mass
T wave	Ventricular repolarisation	Ventricular relaxation (diastole)

Key calculations from ECG:

• Heart rate (from ECG): count the number of QRS complexes in a known time period. If the ECG speed is $25\ \mathrm{mm\ s^{-1}}$ and the distance between two QRS complexes is $20\ \mathrm{mm}$:

$\text{Time per beat} = \frac{20}{25} = 0.8\ \mathrm{s}$

$\text{Heart rate} = \frac{60}{0.8} = 75\ \mathrm{bpm}$

11.4 Pressure and Volume Changes in the Cardiac Cycle

Phase	Atrial Pressure	Ventricular Pressure	Aortic Pressure	AV Valves	Semilunar Valves
Late diastole (ventricular filling)	Rising (atrial systole pushes blood in)	Rising slowly	Falling slowly	Open	Closed
Isovolumetric contraction	Slight rise	Rising rapidly (but still below aortic)	No change	Closed (bulge into atria = "c wave")	Closed
Ventricular ejection	Falling	Peaks above aortic pressure	Rising	Closed	Open
Isovolumetric relaxation	Rising	Falling rapidly (below aortic)	Falling (dicrotic notch)	Closed	Closed
Early diastole	Rising	Falling to very low	Falling slowly	Open (rapid filling)	Closed

12. Transport in Plants: Advanced Topics

12.1 The Cohesion-Tension Theory

The cohesion-tension theory explains how water is pulled up through the xylem from roots to leaves:

1. Transpiration (evaporation of water from the mesophyll cell walls inside the leaf) creates a negative pressure (tension) in the xylem.
2. Water molecules are cohesive (hydrogen bonds between them) and adhesive (hydrogen bonds with the xylem walls). The cohesion means that when one water molecule is pulled into the leaf by transpiration, it pulls the next molecule behind it, transmitting the tension all the way down the xylem to the roots.
3. The tension pulls water up the xylem against gravity. This is a passive process -- no energy input is required from the plant (the energy comes from the Sun, which drives evaporation).

Evidence for the cohesion-tension theory:

• Cutting a stem causes the xylem sap to retract (due to tension being released).
• Measuring xylem pressure with a pressure probe shows negative pressure (tension).
• The rate of transpiration correlates with the rate of water uptake.

12.2 Translocation in the Phloem: The Mass Flow Hypothesis

The mass flow (pressure flow) hypothesis explains how organic solutes (mainly sucrose) are transported in the phloem:

1. Loading: sucrose is actively transported (via companion cells) from photosynthetic cells into the phloem sieve tubes at the source (e.g., leaves). This lowers the water potential inside the sieve tube.
2. Water entry: water enters the sieve tube from the xylem by osmosis (the xylem has a higher water potential).
3. High pressure: the influx of water creates high hydrostatic pressure at the source.
4. Mass flow: the pressure gradient drives the bulk flow of sap (sucrose solution) from the source to the sink (e.g., roots, growing tips, storage organs).
5. Unloading: sucrose is removed from the sieve tube at the sink (by active transport or diffusion), increasing the water potential. Water leaves the sieve tube by osmosis, reducing the hydrostatic pressure at the sink.

Evidence for the mass flow hypothesis:

• Aphids feeding on phloem secrete honeydew (sucrose-rich), confirming phloem sap contains high sugar concentrations.
• Ringing a tree (removing a strip of bark including phloem) causes swelling above the ring (sucrose accumulates) and the tissue below dies (no sucrose supply).
• Radioactively labelled $\mathrm{^{14}CO_2}$ fed to a leaf appears in the phloem and can be traced to sinks.

Problems with the mass flow hypothesis:

• Sieve plates (perforated end walls between sieve tube elements) would create significant resistance to flow.
• Sucrose is transported to all sinks, not just those closest to the source (which would be expected if flow were purely pressure-driven).
• Bidirectional transport can occur in the same sieve tube (though this is debated).

13. Haemoglobin and Oxygen Dissociation Curves: Advanced Analysis

13.1 The Bohr Effect in Detail

The Bohr effect describes the rightward shift of the oxygen dissociation curve at lower pH (higher $\mathrm{CO_2}$ concentration):

• In actively respiring tissues, $\mathrm{CO_2}$ is produced, which diffuses into red blood cells and is converted to $\mathrm{H^+}$ and $\mathrm{HCO_3^-}$ by carbonic anhydrase: $\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}$
• The increase in $\mathrm{H^+}$ (lower pH) reduces haemoglobin's affinity for $\mathrm{O_2}$, causing it to release $\mathrm{O_2}$ more readily.
• $\mathrm{H^+}$ binds to amino acid residues on haemoglobin, stabilising the T-state (tense, deoxygenated) conformation.
• In the lungs, $\mathrm{CO_2}$ diffuses out, the pH rises, and haemoglobin's affinity for $\mathrm{O_2}$ increases, facilitating $\mathrm{O_2}$ loading.

13.2 Foetal Haemoglobin

Foetal haemoglobin (HbF) has a higher affinity for $\mathrm{O_2}$ than adult haemoglobin (HbA). This is because HbF has two $\gamma$ chains instead of two $\beta$ chains, and the $\gamma$ chain binds 2,3-BPG less effectively. Since 2,3-BPG stabilises the T-state (reducing $\mathrm{O_2}$ affinity), reduced binding of 2,3-BPG means HbF stays in the R-state (high-affinity state) more easily.

This ensures that foetal haemoglobin can "steal" $\mathrm{O_2}$ from maternal haemoglobin in the placenta, where maternal and foetal blood run in close proximity (but do not mix) in the chorionic villi.

13.3 Comparing Haemoglobin Types

Haemoglobin Type	$P_{50}$ ($\mathrm{kPa}$)	Relative Affinity	Adaptation
Human HbA (adult)	$\approx 3.5$	Medium	General purpose
Human HbF (foetal)	$\approx 2.5$	High	Extracts $\mathrm{O_2}$ from maternal blood
Llama Hb	$\approx 2.0$	Very high	High altitude (low $p\mathrm{O_2}$)
Bar-headed goose Hb	Very low	Very high	Migratory flight over Himalayas
Antarctic fish Hb (icefish)	N/A	N/A	No haemoglobin; uses dissolved $\mathrm{O_2}$ directly

The $P_{50}$ value is the partial pressure of $\mathrm{O_2}$ at which haemoglobin is 50% saturated. A lower $P_{50}$ means higher affinity (the curve is shifted to the left).

warning

Common Pitfall Students often confuse "left shift" and "right shift" of the oxygen dissociation curve. Remember: Left = Loads $\mathrm{O_2}$ more readily (high affinity, e.g., foetal Hb, low temperature, low $\mathrm{CO_2}$). Right = Releases $\mathrm{O_2}$ more readily (low affinity, e.g., adult Hb in muscle, high temperature, high $\mathrm{CO_2}$, high 2,3-BPG).

15. Plant Transport: Advanced Topics

15.1 Xylem Structure and Adaptations

Xylem vessels are dead, hollow tubes formed from files of cells arranged end to end. The end walls (cross walls) break down completely, forming continuous columns.

Adaptations for water transport:

• Lignified walls: deposited in spirals, rings, or continuous layers, providing strength and preventing collapse under the tension (negative pressure) of transpiration.
• Wide lumen: reduces resistance to water flow (Poiseuille's law: flow rate is proportional to $r^4$, so doubling the radius increases flow rate 16-fold).
• No end walls: continuous tubes with no obstructions to flow.
• Pits: thin areas in the lateral walls where lignin is absent, allowing lateral water movement between vessels.

15.2 Transpiration Rate: Factors and Calculations

The rate of transpiration is affected by:

Factor	Effect on Transpiration Rate	Mechanism
Temperature	Increased	Higher temperature increases kinetic energy of water molecules (more evaporation); increases the water vapour concentration gradient
Humidity	Decreased	Higher humidity reduces the water vapour concentration gradient between the leaf and the air
Wind speed	Increased	Wind removes the boundary layer of humid air near the leaf surface, maintaining the concentration gradient
Light intensity	Increased	Light causes stomata to open (via photosynthesis producing ATP for guard cell ion pumps), increasing the pathway for water loss

Using a potometer: a potometer measures water uptake by a plant shoot (which approximates transpiration rate). The distance moved by an air bubble in the capillary tube is recorded over time:

$\text{Transpiration rate} = \frac◆LB◆\text{Volume of water taken up}◆RB◆◆LB◆\text{Time}◆RB◆ = \frac◆LB◆\pi r^2 d◆RB◆◆LB◆t◆RB◆$

Where $r$ = radius of capillary tube, $d$ = distance bubble moved, $t$ = time.

Worked Example. The bubble in a potometer with capillary tube radius $0.5\ \mathrm{mm}$ moves $12\ \mathrm{mm}$ in 5 minutes.

Volume $= \pi \times (0.5)^2 \times 12 = \pi \times 0.25 \times 12 = 9.42\ \mathrm{mm^3}$.

Rate $= \frac{9.42}{5} = 1.88\ \mathrm{mm^3\ min^{-1}}$.

15.3 Stomatal Mechanism

Guard cells control the opening and closing of stomata:

Opening stomata (in the light):

1. Light drives photosynthesis in guard cell chloroplasts, producing ATP.
2. ATP powers the active transport of $\mathrm{K^+}$ into guard cells (via $\mathrm{K^+}$ channels).
3. $\mathrm{Cl^-}$ ions enter through channels, maintaining electrical neutrality.
4. The accumulation of ions lowers the water potential inside guard cells.
5. Water enters guard cells by osmosis from neighbouring epidermal cells.
6. Guard cells swell and become turgid. Because the inner cell wall is thicker and less elastic than the outer wall, the guard cells curve apart, opening the stomatal pore.

Closing stomata (in the dark or during water stress):

1. $\mathrm{K^+}$ ions leave guard cells (passive efflux through channels).
2. Water potential inside guard cells increases.
3. Water leaves guard cells by osmosis.
4. Guard cells become flaccid, and the stomatal pore closes.

Abscisic acid (ABA) is produced by roots in response to water stress and transported to leaves. ABA triggers stomatal closure by:

• Increasing $\mathrm{Ca^{2+}}$ concentration in guard cell cytoplasm.
• $\mathrm{Ca^{2+}}$ activates anion channels, allowing $\mathrm{Cl^-}$ and malate to leave.
• This depolarises the membrane, opening $\mathrm{K^+}$ efflux channels.
• $\mathrm{K^+}$ leaves, water follows, guard cells become flaccid.

16. Human Circulatory System: Advanced Topics

16.1 Blood Composition in Detail

Plasma (approximately 55% of blood volume):

Component	Concentration	Function
Water	90--92%	Solvent for transport
Proteins (albumin, globulins, fibrinogen)	$60$--$80\ \mathrm{g\ L^{-1}}$	Osmotic pressure (albumin), immunity (globulins), clotting (fibrinogen)
Ions ($\mathrm{Na^+}$, $\mathrm{K^+}$, $\mathrm{Ca^{2+}}$, $\mathrm{Cl^-}$, $\mathrm{HCO_3^-}$)	Variable	Osmoregulation, pH buffering, nerve impulse transmission

• Nutrients (glucose, amino acids, lipids) | Variable | Transport from digestive system to cells |
• Urea, uric acid | Variable | Waste products transported to kidneys |
• Hormones | Trace | Chemical signalling |
• Dissolved gases ($\mathrm{O_2}$, $\mathrm{CO_2}$, $\mathrm{N_2}$) | Variable | Gas transport |

16.2 Tissue Fluid and Lymph

Tissue fluid is formed by filtration from blood capillaries:

At the arterial end of a capillary:

• Hydrostatic pressure (blood pressure) $\approx 4.3\ \mathrm{kPa}$ (pushes fluid out).
• Oncotic pressure (due to plasma proteins, mainly albumin) $\approx 3.3\ \mathrm{kPa}$ (pulls fluid in).
• Net outward pressure $= 4.3 - 3.3 = 1.0\ \mathrm{kPa}$.
• Fluid is forced out of the capillary into the tissue spaces, forming tissue fluid.

At the venous end of a capillary:

• Hydrostatic pressure has fallen to approximately $1.6\ \mathrm{kPa}$ (resistance of the capillary reduces pressure).
• Oncotic pressure remains approximately $3.3\ \mathrm{kPa}$ (proteins cannot leave the capillary).
• Net inward pressure $= 3.3 - 1.6 = 1.7\ \mathrm{kPa}$.
• Approximately 90% of the tissue fluid is reabsorbed into the capillary.

The remaining 10% of tissue fluid is returned to the circulation via the lymphatic system:

1. Tissue fluid enters lymphatic capillaries (blind-ended vessels with overlapping endothelial cells that act as one-way valves).
2. The fluid (now called lymph) is transported through larger lymphatic vessels by:
   • Contraction of surrounding skeletal muscles (squeezing the vessels).
   • Pressure changes during breathing.
   • Valves in the lymphatic vessels preventing backflow.
3. Lymph passes through lymph nodes, where lymphocytes and macrophages filter out pathogens and debris.
4. Lymph is returned to the blood via the thoracic duct (left) and right lymphatic duct, which empty into the subclavian veins near the heart.

16.3 Oedema

Oedema (swelling) occurs when excess tissue fluid accumulates in the tissues. Causes:

• Increased capillary hydrostatic pressure (e.g., venous thrombosis, heart failure).
• Decreased plasma oncotic pressure (e.g., liver disease -- reduced albumin synthesis; nephrotic syndrome -- albumin lost in urine).
• Increased capillary permeability (e.g., inflammation, allergic reactions -- histamine causes capillaries to become more permeable).
• Lymphatic obstruction (e.g., filariasis -- parasitic worms block lymphatic vessels, causing elephantiasis).

16.4 Atherosclerosis and Coronary Heart Disease

Risk factors for CHD:

Modifiable Risk Factors	Non-Modifiable Risk Factors
Smoking (damages endothelium, reduces HDL)	Age (risk increases with age)
High blood pressure (damages endothelium)	Sex (males have higher risk pre-menopause)
High LDL cholesterol	Family history (genetic predisposition)
Obesity, diabetes (insulin resistance)
Physical inactivity
High saturated fat diet

Prevention and treatment:

• Statins: inhibit HMG-CoA reductase (rate-limiting enzyme in cholesterol synthesis in the liver), reducing LDL cholesterol.
• Aspirin: inhibits cyclooxygenase (COX), reducing thromboxane A2 production (a potent platelet aggregator), reducing clot formation.
• Blood pressure control: ACE inhibitors, beta-blockers, diuretics.
• Lifestyle changes: smoking cessation, exercise, Mediterranean diet (high in monounsaturated fats, omega-3 fatty acids, fibre).

17. Gas Exchange: Comparative Physiology

17.1 Insect Tracheal System

Insects do not use blood for gas transport. Instead, they have a tracheal system -- a network of air-filled tubes that deliver oxygen directly to the tissues.

Structure	Description	Function
Spiracles	Paired openings on the thorax and abdomen	Entry and exit points for air; can be opened and closed by valves to reduce water loss
Tracheae	Main tubes branching from spiracles	Air conduits
Tracheoles	Fine, terminal branches ($< 1\ \mu\mathrm{m}$ diameter) that penetrate tissues and cells	Deliver oxygen directly to cells; extend to within 1 cell diameter of every cell
Air sacs	Thin-walled, expandable chambers	Act as air reservoirs; improve ventilation (especially during flight)

Ventilation in insects: larger insects actively ventilate their tracheal system by rhythmic abdominal movements (alternating compression and expansion of the tracheae and air sacs, similar to a bellows).

Limitations: the reliance on diffusion limits the maximum body size of insects. This is why insects are generally small (though some extinct insects were much larger, possibly because atmospheric $\mathrm{O_2}$ levels were higher in the Carboniferous).

17.2 Fish vs Mammalian Gas Exchange

Feature	Fish Gills	Mammalian Lungs
Medium	Water (3000 times more viscous; 30 times less $\mathrm{O_2}$ per litre)	Air
Surface area	Large (many secondary lamellae)	Very large (alveoli; $\approx 70\ \mathrm{m^2}$)
Flow mechanism	Countercurrent	Tidal
$\mathrm{O_2}$ extraction	$\approx 80\%$	$\approx 25\%$
Circulation	Single circuit	Double circuit

17.3 Single vs Double Circulation

Fish (single circulation): heart $\to$ gills $\to$ body $\to$ heart. Blood passes through the heart once per circuit. Blood pressure drops after gills, limiting metabolic rate.

Mammals (double circulation): pulmonary circuit (right ventricle $\to$ lungs $\to$ left atrium) and systemic circuit (left ventricle $\to$ body $\to$ right atrium). Blood is pumped at high pressure twice, supporting high metabolic rates.

18. The Mammalian Heart: Detailed Structure and Function

18.1 Cardiac Muscle and the Cardiac Cycle

Cardiac muscle (myocardium) has unique properties:

• Autorhythmicity: cardiac muscle can generate its own action potentials without nervous stimulation (myogenic). The sinoatrial node (SAN) is the pacemaker.
• Intercalated discs: gap junctions allow electrical coupling between cardiac muscle cells, so action potentials spread rapidly through the myocardium.
• Long refractory period: prevents tetanus (sustained contraction), which would stop blood flow.

The cardiac cycle (for a heart rate of 75 bpm, one cycle $= 0.8\ \mathrm{s}$):

Phase	Duration	Atria	Ventricles	AV Valves	Semilunar Valves
Atrial systole	0.1 s	Contract	Relax (filling)	Open	Closed
Ventricular systole	0.3 s	Relax	Contract	Closed	Open
Diastole	0.4 s	Relax (filling)	Relax (filling)	Open	Closed

18.2 Electrical Activity of the Heart

Structure	Role
Sinoatrial node (SAN)	Pacemaker; generates waves of depolarisation that spread across both atria, causing atrial systole
Atrioventricular node (AVN)	Delays the electrical impulse for approximately 0.1 s (allowing ventricles to fill completely before contracting)
Bundle of His	Conducts the impulse from the AVN down the septum to the apex of the heart
Purkyne fibres	Spread the impulse up the ventricular walls from the apex, causing ventricular contraction from the apex upwards (efficient emptying)

18.3 ECG (Electrocardiogram)

Feature	Description	Clinical Significance
P wave	Atrial depolarisation	Enlarged P wave may indicate atrial enlargement
QRS complex	Ventricular depolarisation (and atrial repolarisation, masked)	Widened QRS may indicate bundle branch block; elevated ST segment suggests myocardial infarction
T wave	Ventricular repolarisation	Inverted T wave may indicate ischaemia
PR interval	Time from atrial depolarisation to ventricular depolarisation (0.12--0.20 s)	Prolonged PR interval indicates heart block (delay at AVN)

18.4 Pressure and Volume Changes During the Cardiac Cycle

Stage	Atrial Pressure	Ventricular Pressure	Aortic Pressure	AV Valves	Semilunar Valves
Mid-diastole (filling)	Rising (venous return)	Rising slowly (passive filling)	Falling (run-off into arteries)	Open	Closed
Atrial systole	Peak	Rising (last 25% of filling)	Falling	Open	Closed
Isovolumetric contraction	Falling	Rising rapidly (above atrial, below aortic)	Minimal change	Closed (bulge into atria)	Closed
Ventricular ejection	Low	Peak ($\approx 120\ \mathrm{mmHg}$)	Rising ($\approx 120\ \mathrm{mmHg}$)	Closed	Open
Isovolumetric relaxation	Rising	Falling rapidly	Minimal change	Closed	Closed (brief backflow causes dicrotic notch)

18.5 Coronary Heart Disease (CHD)

Atherosclerosis: the build-up of fatty plaques (atheromas) in the walls of the coronary arteries:

1. Endothelial damage: caused by high blood pressure, smoking, or high LDL cholesterol. The inner lining (endothelium) of the artery is damaged.
2. Inflammatory response: macrophages accumulate at the site of damage and accumulate LDL cholesterol, becoming foam cells.
3. Plaque formation: a fibrous cap forms over the fatty deposit (atheroma). The plaque narrows the lumen of the artery, restricting blood flow.
4. Ischaemia: reduced blood flow to the heart muscle (myocardium) causes angina (chest pain), especially during exercise.
5. Thrombosis: if the fibrous cap ruptures, a blood clot (thrombus) forms, which can completely block the coronary artery, causing a myocardial infarction (heart attack).

Risk factors for CHD:

Non-modifiable	Modifiable
Age (risk increases with age)	High blood pressure (hypertension)
Sex (males at higher risk pre-menopause)	High LDL cholesterol; low HDL cholesterol
Family history	Smoking (damages endothelium, increases clotting)
Genetics (FH, APOE4)	Obesity, physical inactivity, high saturated fat diet
	Diabetes (high blood glucose damages endothelium)

19. Plant Transport: Translocation and Transpiration

19.1 Transpiration: The Cohesion-Tension Theory

Water moves up the xylem from roots to leaves by the cohesion-tension mechanism:

1. Evaporation: water evaporates from the mesophyll cell walls into the air spaces of the leaf, and exits through the stomata (transpiration).
2. Tension: evaporation creates a negative pressure (tension) in the xylem, pulling water upwards.
3. Cohesion: water molecules are attracted to each other by hydrogen bonds (cohesion), so the column of water in the xylem is pulled up as a continuous column.
4. Adhesion: water molecules are attracted to the hydrophilic walls of the xylem vessels (adhesion), helping to resist the downward pull of gravity.
5. Root pressure: in some plants (especially at night when transpiration is low), active transport of ions into the root xylem lowers the water potential, drawing water in by osmosis and creating a positive root pressure that pushes water upwards.

19.2 Factors Affecting Transpiration Rate

Factor	Effect	Explanation
Light intensity	Increases rate	Stomata open in light (for $\mathrm{CO_2}$ uptake for photosynthesis), increasing the diffusion pathway for water vapour
Temperature	Increases rate	Higher temperature increases kinetic energy of water molecules (faster evaporation); increases the water vapour potential gradient between leaf and air
Humidity	Decreases rate	Higher humidity reduces the water vapour potential gradient between leaf (saturated) and air
Wind speed	Increases rate	Wind removes the boundary layer of moist air around the leaf, maintaining a steep water vapour gradient
$\mathrm{CO_2}$ concentration	Decreases rate	High $\mathrm{CO_2}$ causes stomata to close (a guard cell response to prevent excessive water loss)

19.3 Xerophyte Adaptations

Xerophytes are plants adapted to dry conditions:

Adaptation	Function	Example
Thick waxy cuticle	Reduces cuticular transpiration	Holly leaves
Sunken stomata	Stomata in pits, reducing air flow and creating a humid microenvironment	Pine needles
Hairs on leaf surface	Trap moist air, reducing water vapour gradient	Lavandula (lavender)
Reduced leaf surface area	Fewer stomata; less surface for transpiration	Cactus spines (modified leaves)
Rolling of leaves	Exposes waterproof lower epidermis; traps moist air	Marram grass (Ammophila)
Crassulacean acid metabolism (CAM)	Stomata open at night (when transpiration is low) to take in $\mathrm{CO_2}$, which is stored as malic acid and used in photosynthesis during the day	Cacti, succulents
Succulence	Stores water in fleshy leaves or stems	Aloe vera
Deep or extensive root system	Accesses water deep underground or over a wide area	Mesquite tree

19.4 Mass Flow Hypothesis (Phloem Transport)

Sucrose is transported in the phloem from source (e.g., photosynthetic leaves) to sink (e.g., roots, growing tips, storage organs):

1. Loading: sucrose is actively transported (via companion cells) into the phloem sieve tube elements at the source. This lowers the water potential inside the sieve tube.
2. Water entry: water enters the sieve tube from the xylem by osmosis, creating high hydrostatic pressure at the source.
3. Mass flow: the pressure gradient drives bulk flow of sucrose solution (sap) from source to sink through the sieve tubes.
4. Unloading: sucrose is removed from the sieve tube at the sink (by active transport or diffusion), raising the water potential. Water exits the sieve tube by osmosis, reducing the pressure at the sink.

Evidence for mass flow:

• Aphid stylet experiments: aphids pierce sieve tubes and exude sap under pressure, confirming a pressure gradient.
• Ringing experiments (Malpighi, 1670s): removing a ring of bark (phloem) causes swelling above the ring (sucrose accumulates) and the plant dies below the ring (sucrose cannot reach roots).
• Radioactive tracer experiments: $^{14}$C-labelled $\mathrm{CO_2}$ fed to a source leaf appears in sink tissues.

20. Gas Exchange in Different Organisms

20.1 Adaptations for Gas Exchange

Organism	Gas Exchange Surface	Adaptations to Increase Rate of Exchange
Single-celled organism (e.g., Amoeba)	Cell surface membrane (whole cell)	Large surface area:volume ratio; short diffusion distance; direct contact with environment
Flatworm (e.g., Planaria)	Body surface (no specialised system)	Very flat body (short diffusion distance); large surface area:volume ratio
Earthworm	Skin (moist, vascularised)	Thin, moist skin (gases dissolve in water before diffusing); subcutaneous blood capillaries; blood contains haemoglobin
Insect	Tracheal system	Branched network of tracheae and tracheoles (air-filled tubes delivering $\mathrm{O_2}$ directly to cells); spiracles (openings) on body surface; can open/close spiracles to reduce water loss; thin walls of tracheoles (short diffusion distance); tracheal fluid at ends of tracheoles (recedes during exercise, exposing more cells to air)
Fish	Gills	Large surface area (many primary and secondary lamellae); short diffusion distance (lamellae are thin); countercurrent flow (maintains concentration gradient along entire length of gill); good blood supply; water flows over gills in one direction (ventilation by opercular pumping)
Human	Alveoli in lungs	Large surface area (approximately $70\ \mathrm{m^2}$); very thin walls (one cell thick; alveolar epithelium and capillary endothelium); dense capillary network; ventilation (tidal breathing) maintains concentration gradient; surfactant (reduces surface tension, prevents alveolar collapse)

20.2 The Insect Tracheal System

The insect tracheal system is an efficient gas exchange system that delivers air directly to cells, bypassing the circulatory system:

1. Spiracles: paired openings on the thorax and abdomen. Each spiracle has a valve that can open and close (regulated by muscles) to control gas exchange and water loss.
2. Tracheae: thick-walled tubes lined with chitin (provides structural support). Tracheae branch into smaller tubes.
3. Tracheoles: thin-walled (no chitin), blind-ending tubes that penetrate tissues and deliver $\mathrm{O_2}$ directly to cells. Tracheoles are filled with tracheal fluid at their tips.
4. Gas exchange: $\mathrm{O_2}$ diffuses along the tracheae and tracheoles to cells; $\mathrm{CO_2}$ diffuses in the opposite direction. During exercise, lactic acid produced by muscle cells lowers the water potential, causing water to be reabsorbed from the tracheoles by osmosis. This exposes more of the tracheole surface to air, increasing $\mathrm{O_2}$ delivery.

Limitations of the tracheal system:

• Diffusion is only effective over short distances (limits insect body size).
• Active ventilation is required for larger or more active insects (abdominal pumping, thoracic pumping).
• Tracheal system is vulnerable to desiccation (water loss through spiracles).

20.3 The Mammalian Alveolus

Adaptations for efficient gas exchange (features related to Fick's Law):

$\text{Rate of diffusion} \propto \frac◆LB◆\text{surface area} \times \text{concentration difference}◆RB◆◆LB◆\text{diffusion distance}◆RB◆$

Feature	How It Maximises Diffusion Rate
Large number of alveoli (approximately 300 million)	Massive total surface area ($\approx 70\ \mathrm{m^2}$)
Alveolar epithelium is one cell thick (squamous epithelium)	Very short diffusion distance
Capillary endothelium is one cell thick	Very short diffusion distance
Dense capillary network around each alveolus	Maintains steep concentration gradient (blood is continuously flowing)
Ventilation (breathing)	Refreshes air in alveoli, maintaining steep concentration gradient
Pulmonary circulation is low-pressure	Slower blood flow allows more time for gas exchange
Surfactant (secreted by type II pneumocytes)	Reduces surface tension; prevents alveolar collapse; reduces work of breathing

20.4 $\mathrm{O_2}$ Dissociation Curves

The $\mathrm{O_2}$ dissociation curve shows the relationship between the partial pressure of $\mathrm{O_2}$ ($p\mathrm{O_2}$) and the percentage saturation of haemoglobin with $\mathrm{O_2}$.

The curve is sigmoidal (S-shaped) because of cooperative binding.

Comparing different haemoglobins:

Haemoglobin Type	Curve Position	Explanation
Adult (HbA)	Standard position	Normal $\mathrm{O_2}$ affinity
Foetal (HbF)	Left-shifted	Higher affinity for $\mathrm{O_2}$ than HbA; allows efficient transfer of $\mathrm{O_2}$ from mother to foetus across the placenta
Myoglobin	Further left	Much higher affinity for $\mathrm{O_2}$; acts as an $\mathrm{O_2}$ store in muscles; releases $\mathrm{O_2}$ only at very low $p\mathrm{O_2}$ (during intense exercise)

21. Mass Transport in Animals

21.1 The Need for a Mass Transport System

Large, multicellular animals require a mass transport (circulatory) system because:

1. Small surface area:volume ratio: diffusion alone is too slow to supply all cells with $\mathrm{O_2}$ and nutrients and remove $\mathrm{CO_2}$ and waste.
2. Activity level: active animals have high metabolic rates and $\mathrm{O_2}$ demand.
3. Specialised tissues: some tissues (brain, heart, kidneys) have very high metabolic demands and require a dedicated blood supply.

21.2 Features of an Efficient Transport System

Feature	Why It Is Important
A suitable transport medium (blood)	Must carry $\mathrm{O_2}$, nutrients, $\mathrm{CO_2}$, waste, hormones
A pump (heart)	Creates pressure to drive flow
Vessels (arteries, veins, capillaries)	Form a closed system to direct flow
Valves	Prevent backflow (in veins and the heart)
Breathing mechanism	Maintains concentration gradient for gas exchange at the lungs

21.3 Blood Vessels: Structure and Function

Feature	Artery	Arteriole	Capillary	Venule	Vein
Wall thickness	Thick (muscle and elastic tissue)	Thin muscle layer	One cell thick (endothelium only)	Thin	Thin (some muscle)
Lumen diameter	Relatively small	Smaller	Very small (8--10 $\mu\mathrm{m}$)	Small	Large
Valves	None	None	None	Occasionally	Present (prevent backflow)
Blood pressure	High (80--120 mmHg)	Lower	Low (15--35 mmHg at arterial end)	Low	Very low (5--15 mmHg)
Blood flow	Pulsatile	Decreasing	Slow	Slow	Steady (smooth)
Function	Carry blood away from heart at high pressure	Distribute blood to specific organs	Exchange of substances with tissues (diffusion)	Collect blood from capillaries	Return blood to heart

21.4 Tissue Fluid Formation: Quantitative Example

At the arterial end of a capillary:

• Blood hydrostatic pressure (BHP) $= 35\ \mathrm{mmHg}$
• Blood oncotic pressure (BOP) $= -25\ \mathrm{mmHg}$ (pulls fluid in)
• Net filtration pressure $= 35 - 25 = +10\ \mathrm{mmHg}$ (fluid leaves capillary)

At the venous end:

• BHP $= 15\ \mathrm{mmHg}$ (pressure has dropped due to resistance)
• BOP $= -25\ \mathrm{mmHg}$ (unchanged; plasma proteins are too large to leave the capillary)
• Net filtration pressure $= 15 - 25 = -10\ \mathrm{mmHg}$ (fluid returns to capillary)

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tip

Diagnostic Test Ready to test your understanding of Exchange and Transport? The diagnostic test contains the hardest questions within the A-Level specification for this topic, each with a full worked solution.

Unit tests probe edge cases and common misconceptions. Integration tests combine Exchange and Transport with other biology topics to test synthesis under exam conditions.

See Diagnostic Guide for instructions on self-marking and building a personal test matrix.

22. Adaptations to Extreme Environments

22.1 High Altitude Adaptations

At high altitude, atmospheric pressure is lower, so the partial pressure of $\mathrm{O_2}$ is lower, making gas exchange less efficient.

Adaptation	Mechanism	Example
Increased ventilation rate	Hyperventilation in response to low $p\mathrm{O_2}$ (detected by peripheral chemoreceptors)	Humans at high altitude breathe faster and deeper
Increased red blood cell production	Kidney detects low $\mathrm{O_2}$; releases EPO; stimulates erythropoiesis in bone marrow	Tibetan highlanders have higher haemoglobin concentration than lowlanders
Increased capillary density	More capillaries in tissues reduce diffusion distance for $\mathrm{O_2}$	Llama has dense capillary network in muscle tissue
Larger lungs	Increased surface area for gas exchange	Bar-headed goose (migrates over the Himalayas at > 9,000 m) has 30% larger lungs than related species
Higher haemoglobin oxygen affinity	Haemoglobin variant with lower P50 (binds $\mathrm{O_2}$ more tightly at low $p\mathrm{O_2}$)	Tibetan antelope has a haemoglobin variant with increased $\mathrm{O_2}$ affinity

22.2 Deep-Sea Adaptations

Adaptation	Mechanism	Example
Pressure tolerance	Proteins and membranes adapted to high hydrostatic pressure (up to 1000 atm)	Barophilic bacteria have flexible membrane lipids
Bioluminescence	Light produced by chemical reaction (luciferin + $\mathrm{O_2}$, catalysed by luciferase)	Anglerfish (lure prey); deep-sea squid (counter-illumination camouflage)
Reduced metabolic rate	Lower energy demands in food-scarce environment	Deep-sea fish have low metabolic rates and slow growth
Specialised sensory systems	Enhanced lateral line (detect water movements); electroreception	Gulper eel; deep-sea sharks

22.3 Desert Adaptations (Plants and Animals)

Adaptation	Plant Example	Animal Example
Water conservation	Xerophytic adaptations (see Section 19.3)	Kangaroo rat: produces very concentrated urine; does not need to drink water (gets water from metabolic water and food)
Heat tolerance	CAM photosynthesis (stomata open at night)	Fennec fox: large ears (radiate heat); light-coloured fur (reflects heat)
Nocturnal lifestyle	Flowers open at night (pollinated by moths)	Many desert animals are nocturnal to avoid daytime heat
Burrowing	Deep root systems (access groundwater)	Desert tortoise: burrows to escape extreme surface temperatures

23. The Human Circulatory System: Blood Composition

23.1 Blood Components

Component	Percentage	Function
Plasma	55%	Transport medium; carries dissolved substances (glucose, amino acids, urea, hormones, antibodies, ions); distributes heat; contains fibrinogen and other clotting factors
Red blood cells (erythrocytes)	45%	Transport $\mathrm{O_2}$ (bound to haemoglobin) and $\mathrm{CO_2}$ (bound to haemoglobin as carbaminohaemoglobin, and converted to $\mathrm{HCO_3^-}$); no nucleus; biconcave disc shape maximises surface area
White blood cells (leukocytes)	< 1%	Defence: phagocytosis (neutrophils, macrophages); antibody production (B lymphocytes); cell-mediated immunity (T lymphocytes)
Platelets (thrombocytes)	< 1%	Blood clotting (release clotting factors that initiate the cascade leading to fibrin formation)

23.2 Red Blood Cells: Adaptations for Gas Transport

1. Biconcave disc shape: maximises surface area:volume ratio for gas exchange.
2. No nucleus: more room for haemoglobin (approximately 270 million molecules per cell).
3. No mitochondria: no $\mathrm{O_2}$ consumed by the cell itself; all $\mathrm{O_2}$ carried is available for delivery to tissues.
4. Flexible membrane: can squeeze through narrow capillaries (diameter 7--8 $\mu\mathrm{m}$; capillaries as narrow as 5 $\mu\mathrm{m}$).
5. Lifespan: approximately 120 days; destroyed by macrophages in the spleen and liver.

23.3 Carbon Dioxide Transport

$\mathrm{CO_2}$ is transported in the blood in three forms:

Form	Percentage of $\mathrm{CO_2}$ Transport	Description
Hydrogencarbonate ions ($\mathrm{HCO_3^-}$)	$\approx 70\%$	$\mathrm{CO_2}$ diffuses into red blood cells; carbonic anhydrase catalyses: $\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}$. \mathrm{HCO_3^- moves out into the plasma (chloride shift: $\mathrm{Cl^-}$ moves in to maintain electrical neutrality).
Carbaminohaemoglobin	$\approx 23\%$	$\mathrm{CO_2}$ binds reversibly to the amino groups of haemoglobin (not the same site as $\mathrm{O_2}$).
Dissolved in plasma	$\approx 7\%$	$\mathrm{CO_2}$ is directly dissolved in blood plasma.

23.4 The Bohr Effect in Action

In actively respiring tissues:

• High $p\mathrm{CO_2}$ $\to$ lower pH $\to$ reduced haemoglobin affinity for $\mathrm{O_2}$ $\to$ $\mathrm{O_2}$ is unloaded where it is most needed.
• High temperature (from metabolic activity) further reduces haemoglobin affinity.
• High 2,3-BPG (produced by active muscles) also reduces affinity.

In the lungs (reverse Bohr effect):

• Low $p\mathrm{CO_2}$ (expired air) $\to$ higher pH $\to$ increased haemoglobin affinity for $\mathrm{O_2}$ $\to$ $\mathrm{O_2}$ is loaded.
• \mathrm{HCO_3^- diffuses back into red blood cells; carbonic anhydrase catalyses the reverse reaction; $\mathrm{CO_2}$ is exhaled.

24. Plant Transport: Detailed Vascular System

24.1 Xylem Structure

Feature	Description
Cell type	Dead, hollow cells (vessels) -- no cytoplasm, no end walls at maturity (except perforation plates at the ends)
Cell walls	Lignified (thick, strong, waterproof)
Transport mechanism	Transpiration pull (cohesion-tension); capillary action (minor contribution)
Direction of transport	Unidirectional: roots $\to$ stems $\to$ leaves
Contents	Water and mineral ions; no sugars (phloem handles sugars)

24.2 Phloem Structure

Feature	Description
Cell type	Living cells (sieve tube elements and companion cells)
Cell walls	Cellulose (not lignified); perforated sieve plates at ends
Companion cells	Adjacent to sieve tube elements; provide metabolic support; load sucrose into phloem
Transport mechanism	Mass flow (pressure flow hypothesis); bidirectional (source to sink)
Direction of transport	Source (e.g., leaves) $\to$ sink (e.g., roots, growing tips, storage organs)
Contents	Sucrose (main transport sugar), amino acids, hormones, plant defence compounds

24.3 Evidence for the Mass Flow Hypothesis

Evidence	What It Shows
Aphid stylet exudation	Aphid pierces sieve tube; exudes sap under pressure, confirming a pressure gradient
Ringing experiment	Removing phloem (bark) causes swelling above the ring (sucrose accumulates) and death below (no sucrose reaches roots)
Radioactive tracer ($^{14}\mathrm{C}$)	$^{14}\mathrm{CO_2}$ fed to source leaf appears in sink tissues, confirming translocation direction
Microinjection	Fluorescent dye injected into phloem moves at rates consistent with mass flow

25. The Heart: Electrical Activity and the ECG

25.1 The Cardiac Conduction System

Structure	Location	Role
SAN (sinoatrial node)	Wall of right atrium, near vena cava	The pacemaker; initiates each heartbeat; sets heart rate (~60--100 bpm at rest)
AVN (atrioventricular node)	Septum between atria, near coronary sinus	Receives impulse from SAN; delays impulse (0.1 s) allowing ventricles to fill before contracting
Bundle of His	Septum between ventricles	Conducts impulse from AVN down the septum
Bundle branches	Left and right branches in the interventricular septum	Carry impulse to the apex of the heart
Purkyne fibres	Spread up from apex through ventricular walls	Cause ventricles to contract from the apex upwards (efficient emptying)

25.2 Reading an ECG

ECG Feature	What It Represents	Normal Duration
P wave	Atrial depolarisation (atria contract)	0.08--0.12 s
PR interval	Time from start of P wave to start of QRS complex; includes AVN delay	0.12--0.20 s
QRS complex	Ventricular depolarisation (ventricles contract); atrial repolarisation hidden within	0.06--0.10 s
T wave	Ventricular repolarisation (ventricles relax)	0.16 s
ST segment	Ventricles fully depolarised (plateau phase of cardiac action potential)	Flat (isoelectric)

25.3 Common ECG Abnormalities

Condition	ECG Appearance	Cause
Tachycardia	Heart rate > 100 bpm	Exercise, stress, fever, arrhythmia
Bradycardia	Heart rate < 60 bpm	Fitness, heart block, hypothyroidism
Atrial fibrillation	Irregular R-R intervals; no distinct P waves	Disorganised electrical activity in atria
Ventricular fibrillation	Chaotic, irregular waveform; no distinct QRS complexes	Disorganised ventricular electrical activity; fatal without defibrillation
Myocardial infarction	Elevated ST segment (STEMI); inverted T waves; pathological Q waves	Coronary artery blockage; myocardial ischaemia/necrosis

26. Blood Composition and Blood Cells

26.1 Blood Components

Component	Approximate Percentage	Function
Plasma	55%	Transport medium; contains dissolved substances (glucose, amino acids, urea, hormones, antibodies, ions); fibrinogen and other clotting factors
Red blood cells (erythrocytes)	45% of blood volume	Transport $\mathrm{O_2}$ (bound to haemoglobin) and some $\mathrm{CO_2}$ (as carbaminohaemoglobin); biconcave disc shape maximises surface area:volume ratio; no nucleus, no mitochondria (more space for haemoglobin)
White blood cells (leukocytes)	<1%	Immune defence: phagocytosis (neutrophils, macrophages); antibody production (B lymphocytes/plasma cells); cell-mediated immunity (T lymphocytes)
Platelets (thrombocytes)	<1%	Blood clotting (release thromboplastin; activate the clotting cascade)

26.2 Red Blood Cell Adaptations

Adaptation	Benefit
Biconcave disc shape	Increases surface area:volume ratio for gas exchange; allows flexibility to pass through narrow capillaries
No nucleus	More space for haemoglobin (approximately 270 million haemoglobin molecules per RBC)
No mitochondria	No aerobic respiration; no $\mathrm{O_2}$ consumption; all ATP from anaerobic glycolysis (prevents RBCs from using the $\mathrm{O_2}$ they carry)
Haemoglobin	Quaternary protein ($\alpha_2\beta_2$) with 4 haem groups; each haem binds one $\mathrm{O_2}$; cooperative binding (sigmoidal dissociation curve)
Thin cell membrane	Short diffusion distance for $\mathrm{O_2}$ and $\mathrm{CO_2}$

27. Gas Exchange in Fish

27.1 Structure of a Fish Gill

Feature	Description
Gill filaments	Thin, flat plates that increase surface area for gas exchange
Gill lamellae (secondary lamellae)	Thin, plate-like projections from each gill filament; this is where gas exchange actually occurs
Water flow direction	Water enters through the mouth, passes over the gills, exits through the operculum
Blood flow direction	Blood flows through the gill lamellae in the opposite direction to water flow (countercurrent)
Ventilation mechanism	Buccal cavity acts as a pump: mouth opens $\to$ buccal cavity expands $\to$ water enters; mouth closes $\to$ operculum opens $\to$ water flows over gills and out

27.2 Countercurrent Exchange in Fish Gills

Feature	Description
Countercurrent principle	Blood flows through the lamellae in the opposite direction to the flow of water
Effect on $\mathrm{O_2}$ gradient	Maintains a diffusion gradient along the entire length of the lamella; blood always meets water with a higher $\mathrm{O_2}$ concentration
$\mathrm{O_2}$ extraction efficiency	Approximately 80--90% of dissolved $\mathrm{O_2}$ is removed from water (compared to only ~50% in a concurrent system)

warning

Common Pitfall In concurrent (parallel) exchange, blood and water flow in the same direction. Equilibrium is reached quickly and no further diffusion occurs. In countercurrent exchange, a gradient is maintained along the entire length, maximising diffusion. Always specify countercurrent in your answers about fish gills.

28. Gas Exchange in Insects

28.1 The Tracheal System

Component	Description	Function
Spiracles	Pores on the body surface (typically one pair per body segment); can open and close	Air enters and exits; opening controlled by valves to minimise water loss
Tracheae	Branching tubes from spiracles into the body	Air transport; reinforced with chitin rings to prevent collapse
Tracheoles	Finest branches; penetrate tissues and cells; end blindly	Site of gas exchange (direct diffusion between air and cells); no blood or circulatory system involved
Tracheal fluid	Fluid at the ends of tracheoles	Gas dissolves in the fluid and diffuses into cells; during exercise, fluid is withdrawn into the body (increasing the gas-filled surface area)

28.2 Ventilation in Insects

Type	Mechanism	When Used
Passive diffusion	Air moves in and out of tracheae by diffusion alone (sufficient for small, inactive insects)	Rest
Active ventilation (mechanical)	Rhythmic abdominal movements: expanding the abdomen decreases pressure, drawing air in; compressing the abdomen increases pressure, pushing air out	During activity (flight, running)

28.3 Adaptations of the Insect Tracheal System

Adaptation	Benefit
Direct delivery of $\mathrm{O_2}$ to cells	No circulatory system needed for gas transport; faster than in vertebrates
Branched network of tracheae and tracheoles	Large surface area for gas exchange
Tracheoles penetrate individual cells	Very short diffusion distance (air is delivered directly to the mitochondria)
Spiracle valves	Can close to reduce water loss
Chitin rings on tracheae	Prevent tubes from collapsing when body pressure changes

29. Plant Gas Exchange: Stomata

29.1 Stomatal Structure

Component	Description
Guard cells	Paired kidney-shaped cells surrounding each stoma; contain chloroplasts; have unevenly thickened cell walls (thicker on the inner side)
Stoma (pore)	Gap between the two guard cells; allows gas exchange ($\mathrm{CO_2}$ in, $\mathrm{O_2}$ out) and water vapour loss (transpiration)
Subsidiary cells	Surrounding cells that may assist in stomatal opening/closing

29.2 Mechanism of Stomatal Opening

Step	What Happens
1	Light activates a proton pump in the guard cell membrane (actively pumps $\mathrm{H^+}$ out)
2	Inside the guard cell becomes more negative (membrane potential becomes more negative)
3	$\mathrm{K^+}$ ions enter the guard cell through potassium channels (following the electrochemical gradient)
4	Chloride ions ($\mathrm{Cl^-}$) also enter (to maintain electrical neutrality)
5	The guard cell's solute concentration increases; water potential becomes more negative
6	Water enters the guard cells by osmosis (from neighbouring epidermal cells)
7	Guard cells swell; the thin outer walls stretch more than the thick inner walls; the stoma opens

Condition	Stomata	Guard Cells
Light (day)	Open	Turgid (full of water)
Dark (night)	Closed	Flaccid (water has left)
High $\mathrm{CO_2}$	Closed	Flaccid (conserves water when photosynthesis is slow)
Low water availability	Closed	Flaccid (abscisic acid triggers closure)

30. Transpiration

30.1 What Is Transpiration?

Transpiration is the loss of water vapour from the aerial parts of a plant (mainly through stomata on the leaves).

Factor	Effect on Transpiration Rate	Mechanism
Temperature (increase)	Increases	Higher temperature $\to$ more kinetic energy $\to$ more water molecules evaporate; steeper water vapour concentration gradient
Humidity (increase)	Decreases	Higher humidity $\to$ smaller water vapour concentration gradient between leaf interior and air
Wind speed (increase)	Increases	Wind carries away water vapour from the leaf surface; maintains a steep concentration gradient
Light intensity (increase)	Increases	Light causes stomata to open; wider stomata = more water vapour escapes
Water availability (decrease)	Decreases	Low water availability $\to$ guard cells become flaccid $\to$ stomata close

30.2 Measuring Transpiration

Method	Description
Potometer	Measures the rate of water uptake by a plant shoot (assumes water uptake $\approx$ transpiration rate); the plant shoot is connected to a capillary tube containing water; the movement of an air bubble is measured over time
Weighing	Measures mass loss of a potted plant over time (mass loss = water lost by transpiration)
Leaf area	Used to standardise transpiration rate: rate per unit leaf area (e.g., $\mathrm{g\ H_2O\ m^{-2}\ h^{-1}}$)

31. Adaptations for Gas Exchange

31.1 Features of Efficient Gas Exchange Surfaces

Feature	Why It Is Needed
Large surface area	More surface for diffusion; higher rate of gas exchange
Thin diffusion pathway	Shorter distance for gases to diffuse; faster exchange
Steep concentration gradient	Maintained by ventilation (air flow) and blood flow (perfusion)
Good blood supply	Transports $\mathrm{O_2}$ away from the exchange surface (maintains the gradient) and $\mathrm{CO_2}$ towards it
Moist surface	Gases must dissolve in water before they can diffuse across the membrane

31.2 Adaptations of the Human Lungs

Adaptation	Description
Alveoli	Millions of tiny air sacs; provide enormous surface area (~70 m2)
Alveolar epithelium	One cell thick (squamous epithelium); very short diffusion distance
Capillary network	Dense capillary network surrounding each alveolus; short diffusion distance between air in alveolus and blood
Ventilation	Breathing movements (diaphragm and intercostal muscles) constantly refresh the air in the alveoli; maintains concentration gradient
Pulmonary circulation	Blood is pumped from the right ventricle to the lungs; maintains the blood flow (perfusion)
Surfactant	Secreted by type II alveolar cells; reduces surface tension in the alveoli; prevents alveolar collapse (atelectasis)

32. The Mammalian Heart: Structure and Function

32.1 Heart Chambers and Valves

Structure	Description
Right atrium	Receives deoxygenated blood from the vena cava (superior and inferior); thin muscular wall
Right ventricle	Pumps deoxygenated blood to the lungs via the pulmonary artery; muscular wall thinner than left ventricle (shorter distance to pump)
Left atrium	Receives oxygenated blood from the pulmonary veins; thin muscular wall
Left ventricle	Pumps oxygenated blood to the body via the aorta; thick muscular wall (must generate high pressure to pump blood around the systemic circulation)
Tricuspid valve	Between right atrium and right ventricle; prevents backflow
Bicuspid (mitral) valve	Between left atrium and left ventricle; prevents backflow
Semilunar valves	In the pulmonary artery and aorta; prevent backflow into the ventricles when the ventricles relax
Tendinous cords (chordae tendineae)	Attach the atrioventricular valves to the papillary muscles in the ventricle walls; prevent the valves from inverting (turning inside out) under pressure

32.2 Cardiac Cycle

Phase	What Happens	Atria	Ventricles	AV Valves	Semilunar Valves
Atrial systole	Atria contract; remaining blood is pushed into the ventricles	Contracting (high pressure)	Relaxing (filling)	Open	Closed
Ventricular systole	Ventricles contract; pressure rises; AV valves close; semilunar valves open; blood is ejected	Relaxing (filling from veins)	Contracting (high pressure)	Closed (prevents backflow)	Open
Diastole	Heart relaxes; all chambers relax; blood flows from veins into atria and from atria into ventricles passively	Relaxing (filling)	Relaxing (filling)	Open (blood flows passively)	Closed

33. Ventilation in Humans

33.1 Mechanism of Breathing

Phase	Muscles	Volume of Thorax	Pressure in Lungs	Air Movement
Inspiration (inhalation)	External intercostal muscles contract (ribs move up and out); diaphragm contracts and flattens	Increases	Decreases below atmospheric pressure	Air flows in
Expiration (exhalation) at rest	External intercostal muscles relax (ribs move down and in); diaphragm relaxes and domes upward	Decreases	Increases above atmospheric pressure	Air flows out
Forced expiration	Internal intercostal muscles contract (ribs move down and in more forcefully); abdominal muscles contract (push diaphragm up)	Decreases more	Increases more	Air forced out rapidly

33.2 Lung Volumes

Volume	Description	Typical Value
Tidal volume	Volume of air breathed in and out in one normal breath	~500 mL
Vital capacity	Maximum volume of air that can be breathed out after a maximum breath in	~4--5 L
Residual volume	Volume of air remaining in the lungs after maximum exhalation (lungs never fully empty)	~1.5 L
Inspiratory reserve volume	Maximum volume that can be inhaled after a normal tidal inspiration	~3 L
Expiratory reserve volume	Maximum volume that can be exhaled after a normal tidal expiration	~1.5 L
Minute ventilation	Tidal volume $\times$ breathing rate	~500 mL $\times$ 15 = ~7.5 L/min at rest

34. Haemoglobin and Oxygen Transport

34.1 Haemoglobin Dissociation Curve

Feature	Description
Shape	Sigmoidal (S-shaped) curve
Low $p\mathrm{O_2}$ (e.g., in tissues)	Haemoglobin has lower affinity for $\mathrm{O_2}$; $\mathrm{O_2}$ is unloaded (dissociates)
High $p\mathrm{O_2}$ (e.g., in lungs)	Haemoglobin has higher affinity for $\mathrm{O_2}$; $\mathrm{O_2}$ is loaded (binds)
Steep part of curve	Small changes in $p\mathrm{O_2}$ cause large changes in $\mathrm{O_2}$ saturation; this is where most $\mathrm{O_2}$ loading/unloading occurs
Plateau (top)	Haemoglobin is nearly saturated; small changes in $p\mathrm{O_2}$ cause little change in saturation

34.2 Fetal vs Adult Haemoglobin

Feature	Adult Haemoglobin (HbA)	Fetal Haemoglobin (HbF)
Subunits	$\alpha_2\beta_2$	$\alpha_2\gamma_2$
$\mathrm{O_2}$ affinity	Lower	Higher (left-shifted dissociation curve)
Why	--	Fetal Hb must extract $\mathrm{O_2}$ from maternal blood in the placenta; higher affinity ensures transfer of $\mathrm{O_2}$ from mother to fetus
Effect of 2,3-BPG	Binds and reduces affinity	Reduced binding of 2,3-BPG; contributes to higher $\mathrm{O_2}$ affinity

35. Plant Transport: Transpiration Stream

35.1 The Cohesion-Tension Theory

The cohesion-tension theory explains how water moves up the xylem against gravity:

Component	Description
Transpiration pull	Water evaporates from the spongy mesophyll cells in the leaf; this creates a negative pressure (tension) in the xylem
Cohesion	Water molecules are attracted to each other by hydrogen bonds; this allows the water column to be pulled up as a continuous stream without breaking
Adhesion	Water molecules are attracted to the hydrophilic walls of the xylem vessels; this helps water rise by capillary action
Result	A continuous column of water is pulled from the roots to the leaves; the driving force is evaporation at the leaf surface

35.2 Evidence for the Cohesion-Tension Theory

Evidence	What It Shows
Cut stem experiment	When a stem is cut, water is pulled up from below and air is drawn in from above; confirms the existence of tension
Potometer readings	Transpiration rate increases with temperature, wind, and light; consistent with increased evaporation driving the pull
Pressure probe	Direct measurement shows negative pressure (tension) in the xylem of transpiring plants
Cavitation	When the water column breaks (usually under drought stress), a clicking sound can be heard (acoustic emission); water transport is interrupted

36. The Human Circulatory System: Blood Vessels

36.1 Types of Blood Vessels

Feature	Arteries	Veins	Capillaries
Function	Carry blood away from the heart (usually oxygenated)	Carry blood back to the heart (usually deoxygenated)	Exchange of materials between blood and tissues ($\mathrm{O_2}$, $\mathrm{CO_2}$, glucose, urea, etc.)
Wall structure	Thick, muscular wall (tunica media contains smooth muscle and elastic tissue)	Thinner wall; less muscle	Single layer of endothelial cells (squamous epithelium); very thin walls
Lumen	Narrow (relative to wall thickness)	Wide (relative to wall thickness)	Very narrow (8--10 $\mu$m diameter; red blood cells squeeze through in single file)
Valves	None (except semilunar valves at the base of the aorta and pulmonary artery)	Valves present (prevent backflow; ensure one-way flow)	None
Blood pressure	High (generated by the heart)	Low (pressure decreases as blood passes through the arterioles and capillaries)	Very low; slow flow (allows time for diffusion)
Blood flow	Pulsatile (surges with each heartbeat)	Smooth (non-pulsatile)	Steady; slow

36.2 Arterioles and Venules

Structure	Description	Function
Arterioles	Small branches of arteries; have thick walls with a ring of smooth muscle	Control blood flow to different tissues (vasodilation and vasoconstriction)
Venules	Small branches of veins; thin walls	Collect blood from capillaries and merge into larger veins

37. Fick's Law of Diffusion

37.1 The Equation

$\text{Rate of diffusion} = \frac◆LB◆D \times A \times \Delta C◆RB◆◆LB◆\Delta x◆RB◆$

Symbol	Meaning	Unit
Rate of diffusion	Volume per unit time	\mathrm{mol\ s^{-1} or \mathrm{cm^3\ s^{-1}
$D$	Diffusion coefficient (a constant for a given substance in a given medium at a given temperature)	\mathrm{m^2\ s^{-1}
$A$	Surface area across which diffusion occurs	$\mathrm{m^2}$
$\Delta C$	Concentration gradient	\mathrm{mol\ mm^{-1} or \mathrm{mol\ cm^{-3}
$\Delta x$	Diffusion distance (thickness of membrane or tissue)	$\mathrm{m}$ or $\mu$m

37.2 Fick's Law Applied to Gas Exchange

For oxygen diffusing across the alveolar membrane:

Feature	Typical Value	Effect on Diffusion
$D$ (diffusion coefficient for $\mathrm{O_2}$ in water)	~1.7 \times 10^{-9}\ \mathrm{m^2\ s^{-1}	Increases with temperature; larger for smaller molecules
$A$ (alveolar surface area)	~70 m2 (combined total)	Larger area = faster diffusion
$\Delta C$ ($\mathrm{O_2}$ gradient)	Alveolar $\mathrm{pO_2}$ (~13 kPa) vs venous blood $\mathrm{pO_2}$ (~5 kPa)	Larger gradient = faster diffusion
$\Delta x$ (alveolar membrane thickness)	~1--2 $\mu$m	Shorter distance = faster diffusion

38. The Lymphatic System

38.1 Structure and Function

Feature	Description
What it is	A network of vessels that returns excess tissue fluid (interstitial fluid) to the blood circulatory system
Why needed	Not all tissue fluid returns to blood capillaries by osmosis; the remainder (~3 litres per day) drains into lymphatic capillaries
Lymphatic capillaries	Blind-ended vessels in tissues; very permeable walls (allow large molecules and cells to enter); wider than blood capillaries
Lymph	The fluid inside lymphatic vessels; similar in composition to blood plasma but with less protein and more lymphocytes
Lymph nodes	Bean-shaped structures along lymphatic vessels; filter lymph; contain lymphocytes and macrophages; sites of immune response
Lymphatic ducts	Two large ducts (thoracic duct and right lymphatic duct) that empty lymph into the subclavian veins

38.2 Lymph Flow

Feature	Description
Pump mechanism	No central pump (the heart does not directly pump lymph)
Movement	Skeletal muscle contraction squeezes lymphatic vessels; one-way valves prevent backflow; breathing movements create pressure changes that assist flow
Flow rate	Slow (~125 mL per hour); much slower than blood flow

39. Adaptations of Red Blood Cells

39.1 Structural Adaptations

Adaptation	Effect on Function
Biconcave disc shape	Increases surface area to volume ratio; maximises $\mathrm{O_2}$ and $\mathrm{CO_2}$ diffusion
No nucleus	More space inside the cell for haemoglobin (more $\mathrm{O_2}$ can be carried)
No mitochondria	Prevents RBCs from using the $\mathrm{O_2}$ they carry for their own respiration; all $\mathrm{O_2}$ is available for transport to tissues
Flexible membrane	Allows RBCs to squeeze through narrow capillaries (diameter ~7 $\mu$m; capillaries ~5 $\mu$m)
Haemoglobin (inside)	Red pigment that binds $\mathrm{O_2}$; each RBC contains ~270 million haemoglobin molecules

39.2 Red Blood Cell Production

Feature	Description
Site	Red bone marrow (spongy bone: sternum, ribs, vertebrae, pelvis, ends of long bones)
Stem cell	Haemopoietic stem cell (multipotent)
Process	Erythropoiesis: stem cell $\to$ proerythroblast $\to$ erythroblast $\to$ reticulocyte (immature RBC, still contains some organelles) $\to$ mature RBC (enucleated)
Lifespan	~120 days
Destruction	Old/damaged RBCs are destroyed by macrophages in the spleen and liver; haem is broken down to biliverdin (green) $\to$ bilirubin (yellow); iron is recycled
Control	Erythropoietin (EPO) is secreted by the kidneys in response to low $\mathrm{O_2}$ levels; stimulates RBC production

40. Haemoglobin and Oxygen Transport

40.1 Structure of Haemoglobin

Feature	Description
Quaternary structure	Globular protein with four polypeptide chains (2 alpha, 2 beta in adult Hb)
Haem groups	Each chain contains one haem group (iron-containing prosthetic group); each haem can bind one $\mathrm{O_2}$ molecule
Total $\mathrm{O_2}$ capacity	One Hb molecule can carry 4 $\mathrm{O_2}$ molecules
Reversible binding	$\mathrm{Hb + 4O_2 \rightleftharpoons Hb(O_2)_4}$; binding is cooperative: each $\mathrm{O_2}$ molecule that binds increases the affinity of the remaining haem groups for $\mathrm{O_2}$

40.2 The Oxygen Dissociation Curve

Feature	Description
Shape	Sigmoidal (S-shaped) due to cooperative binding
Steep region (middle)	Small changes in $\mathrm{pO_2}$ cause large changes in Hb saturation; this is the region where tissues unload $\mathrm{O_2}$ most efficiently
Flat region (top/left)	Hb is nearly saturated even when $\mathrm{pO_2}$ is moderately low; this provides a safety margin in the lungs
Bohr effect	In respiring tissues: high $\mathrm{CO_2}$ $\to$ lower pH $\to$ curve shifts to the right $\to$ Hb has lower affinity for $\mathrm{O_2}$ $\to$ more $\mathrm{O_2}$ is unloaded

40.3 Fetal vs Adult Haemoglobin

Feature	Adult Hb	Fetal Hb
Affinity for $\mathrm{O_2}$	Lower	Higher
Oxygen dissociation curve	To the right of fetal Hb	To the left of adult Hb
Why important	Unloads $\mathrm{O_2}$ to fetal Hb at the placenta	Takes up $\mathrm{O_2}$ from maternal blood at the placenta even though maternal $\mathrm{pO_2}$ is relatively low
Structure	2 alpha + 2 beta chains	2 alpha + 2 gamma chains (the gamma chains alter the conformation of the haem group, increasing $\mathrm{O_2}$ affinity)

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