Topic 4: Exchange and Transport — Edexcel A-Level Biology B

Topic 4 establishes why size and metabolic activity necessitate specialised exchange surfaces and mass transport systems in multicellular organisms. As organisms increase in size, their surface area to volume ratio decreases, rendering simple diffusion across body surfaces insufficient to sustain metabolic demands.

The topic analyses the physiological adaptations of gas exchange systems across mammals, bony fish, and insects, followed by an in-depth study of mammalian circulation and plant vascular transport. Students master the mechanics of the cardiac cycle, haemoglobin dissociation, cohesion-tension theory in xylem, and the mass flow hypothesis in phloem.

• →Fick's Law of diffusion dictates that rate of exchange is proportional to (surface area * concentration difference) / thickness of exchange surface.
• →Countercurrent exchange mechanisms in teleost fish gills maximize oxygen diffusion by preventing dynamic equilibrium between blood and water.
• →The cardiac cycle is coordinated by intrinsic myogenic stimulation initiating at the sinoatrial node (SAN) and propagating via the atrioventricular node (AVN) and Purkyne tissue.
• →The Bohr effect causes a rightward shift in the haemoglobin dissociation curve at high partial pressures of CO2, facilitating oxygen unloading in respiring tissues.
• →Transpiration relies on cohesion between polar water molecules and adhesion to xylem walls, generating tension under negative hydrostatic pressure.

• Surface area to volume ratio is the surface area of an organism divided by its volume, and it decreases as body size increases.
• A large surface area to volume ratio allows efficient diffusion of molecules across the body surface, which is adequate for small organisms.
• A small surface area to volume ratio means diffusion across the outer surface is too slow to meet the metabolic needs of the whole organism.
• Larger organisms need a mass transport system to move substances to and from cells throughout the body.
• Larger organisms need specialised gas exchange surfaces, such as lungs or gills, to increase the area available for gas exchange.
• Specialised exchange surfaces and transport systems help maintain diffusion gradients and reduce the diffusion distance for molecules.
• The fluid mosaic model describes a phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward, with proteins, cholesterol and glycolipids or glycoproteins arranged within it.
• Diffusion is the passive net movement of particles down a concentration gradient; small non-polar molecules such as oxygen and carbon dioxide cross the bilayer directly.
• Facilitated diffusion is passive movement down a concentration gradient through carrier proteins or protein channels, used by polar or charged substances.
• Osmosis is the passive movement of water from higher to lower water potential across a partially permeable membrane.
• Solubility, size and charge determine the transport route: non-polar small molecules diffuse directly, while large or charged molecules need proteins.
• Endocytosis forms a vesicle from the cell surface membrane to take large molecules in; exocytosis fuses a vesicle with the membrane to release large molecules out.
• Active transport moves substances against a concentration gradient, from lower to higher concentration.
• Active transport requires energy supplied by ATP hydrolysis.
• Carrier proteins in the membrane bind the substance and change shape using the energy released.
• ATP is hydrolysed to ADP and inorganic phosphate, releasing energy.
• Hydrolysis of ATP provides an accessible supply of energy for biological processes.
• Phosphorylation of ADP requires energy and regenerates ATP.
• The sodium-potassium pump is an example of active transport using ATP.
• Active transport is selective because specific carrier proteins transport specific substances.
• Insects: air enters through spiracles, travels along tracheae and tracheoles, and diffuses directly to respiring tissues without a blood carrier.
• Fish: gills have filaments bearing lamellae, giving a large surface area; counter-current flow between water and blood maintains a diffusion gradient along the gill.
• Mammals: alveoli provide a large, thin, moist surface with a dense capillary network; ventilation and blood flow maintain concentration gradients.
• All three groups share thin, moist, large-surface-area exchange surfaces and a maintained concentration gradient, but differ in how the gradient is sustained.
• Double circulation separates oxygenated and deoxygenated blood, allowing higher pressure to the body compared to a fish's single system.
• The heart has atria, ventricles and valves; arteries have thick elastic walls, veins have valves, and capillaries have a single-cell endothelium.
• The cardiac cycle includes diastole, atrial systole and ventricular systole, driven by pressure changes and myogenic stimulation (SAN, AVN, bundle of His).
• Erythrocytes are biconcave and lack a nucleus for transport; leucocytes include phagocytic neutrophils and monocytes, eosinophils for parasites, and lymphocytes for specific immune defence.
• High hydrostatic pressure at the arteriole end of capillaries forces plasma out to form tissue fluid; excess fluid drains into lymphatic vessels to form lymph.
• Platelets release thromboplastin, which catalyses prothrombin to thrombin; thrombin converts soluble fibrinogen to an insoluble fibrin mesh.
• Atherosclerosis begins with endothelial damage and atheroma formation, leading to health effects like angina, myocardial infarction or stroke.
• Factors increasing atherosclerosis risk include smoking, high blood pressure, high dietary cholesterol, and genetics.
• Haemoglobin exhibits cooperative binding, meaning the binding of one oxygen molecule changes the tertiary structure, making it easier for subsequent molecules to bind.
• A high partial pressure of carbon dioxide causes the oxygen dissociation curve to shift to the right, decreasing haemoglobin's affinity for oxygen.
• Fetal haemoglobin's dissociation curve is shifted to the left compared to adult haemoglobin, enabling oxygen transfer from mother to fetus at the placenta.
• Myoglobin consists of a single polypeptide chain and one haem group, whereas haemoglobin has four polypeptide chains and four haem groups.
• Myoglobin does not exhibit a sigmoidal dissociation curve because it cannot undergo cooperative binding.
• Tissue fluid forms when hydrostatic pressure in the capillary exceeds oncotic pressure, forcing water and small solutes out through capillary wall gaps.
• Oncotic pressure is the osmotic pressure exerted by plasma proteins, which are too large to leave the capillary.
• At the venous end, oncotic pressure exceeds hydrostatic pressure, so water is reabsorbed into the capillary by osmosis.
• Tissue fluid not reabsorbed is collected by lymph capillaries and returned to the blood via the lymph system.
• Beetroot cells contain a red pigment in vacuoles; intact membranes retain it, so pigment in the surrounding water indicates membrane damage.
• Rinsing cut beetroot removes pigment released by cells damaged during cutting, so only leakage from intact tissue is measured.
• Temperature is the independent variable; pigment concentration or absorbance is the dependent variable.
• Controlled variables include volume of water, mass or length of beetroot, time in the water and pH.
• Higher temperatures increase membrane permeability because the phospholipid bilayer becomes more fluid and membrane proteins denature.
• A colorimeter measures absorbance, or a colour standard enables visual comparison, to quantify pigment release and plot a graph against temperature.
• Water potential is the tendency of water to move by osmosis, measured in kPa.
• ψ = P + π, where P is turgor pressure (positive, outward pressure against the wall) and π is osmotic potential (negative).
• In a turgid plant cell, P is large and positive, π is negative, and ψ is close to zero.
• In a flaccid or plasmolysed cell, P is zero, so ψ equals π.
• Core Practical 6 uses a series of sucrose solutions of known concentration to bathe plant tissue.
• The tissue is blotted, weighed or measured, and the change in mass or length is recorded.
• The point of no change in mass or length identifies the sucrose concentration with the same water potential as the cell contents.
• A calibration curve of sucrose concentration against known water potential values allows ψ to be read from the graph.
• Core Practical 7: dissect an insect to show the gas exchange system, following safe and ethical use of organisms, including careful handling and disposal.
• Stomata are pores, mainly on leaves, that allow carbon dioxide to diffuse in and oxygen to diffuse out, and can open and close to control water loss.
• The leaf has a large internal surface area, especially in the spongy mesophyll, for efficient gas exchange.
• Lenticels are pores in woody stems that allow gas exchange between the atmosphere and internal tissues.
• Xylem tissue is adapted for transport with dead, hollow cells and lignified walls; phloem tissue contains sieve tube elements and companion cells for sugar transport.
• Water moves through plant cells by the apoplastic pathway along cell walls and the symplastic pathway through the cytoplasm via plasmodesmata.
• The cohesion-tension model explains water transport from roots to shoots: transpiration pull creates tension, and cohesion between water molecules draws water up the xylem.
• Temperature, light, humidity and movement of air affect the rate of transpiration by changing the water potential gradient and evaporation rate.
• The mass-flow hypothesis explains sugar movement through phloem by pressure gradients; a strength is sap exuding from cut aphid stylets, but a weakness is bidirectional flow in the same sieve tube.
• Core Practical 8 uses a potometer, assembled underwater to prevent air bubbles, to measure the distance moved by an air bubble over time to investigate factors affecting water uptake.

• 💡Use the words surface area, volume and ratio explicitly when explaining why diffusion becomes insufficient in larger organisms.
• 💡Give a named example, such as a mammal using lungs and a circulatory system, to show how structure relates to size.
• 💡When comparing two organisms, state which has the larger surface area to volume ratio and link this to the rate of diffusion.
• 💡Link each named molecule to its route and justify the choice using solubility, size and charge.
• 💡State the direction of movement as down a concentration or water potential gradient for every passive process.
• 💡Use the phrase partially permeable membrane when describing osmosis, and name the vesicle in endocytosis and exocytosis answers.
• 💡Use the terms 'against the concentration gradient' and 'ATP hydrolysis' when describing active transport.
• 💡Link the role of ATP to specific processes such as the sodium-potassium pump, muscle contraction or protein synthesis.
• 💡Remember that ATP is hydrolysed to ADP and Pi, and that ADP can be phosphorylated back to ATP using energy from respiration.
• 💡In multiple-choice questions, eliminate options that describe passive processes when the question asks about active transport.
• 💡Read the stem carefully to identify which organism is being described before choosing an option.
• 💡Eliminate options that confuse structures between groups, such as spiracles in fish or alveoli in insects.
• 💡Check whether an option describes a feature that maintains a concentration gradient, as this is central to efficient gas exchange.
• 💡When interpreting ECG and pressure traces, align P, QRS and T waves with atrial and ventricular pressure changes and the opening or closing of each valve.
• 💡Use the terms 'soluble' and 'insoluble' explicitly when describing the conversion of fibrinogen to fibrin in blood clotting.
• 💡When comparing single and double circulatory systems, explicitly state that double systems maintain a higher blood pressure to the systemic circulation.
• 💡Link the stages of atherosclerosis to specific health effects, such as how an atheroma narrowing a coronary artery can lead to myocardial infarction.
• 💡When drawing or interpreting oxygen dissociation curves, always check the axes; the x-axis is usually the partial pressure of oxygen and the y-axis is percentage saturation.
• 💡Clearly distinguish between 'affinity' and 'saturation' when explaining how haemoglobin unloads oxygen in respiring tissues.
• 💡Remember to mention the structural difference of one polypeptide versus four when comparing myoglobin and haemoglobin.
• 💡When explaining tissue fluid formation, state the direction of movement and name the pressure that is greater at that point.
• 💡Use the terms hydrostatic pressure and oncotic pressure precisely, and link oncotic pressure to plasma proteins.
• 💡For multiple-choice questions, check whether the option describes the arterial end, venous end or lymph system before selecting it.
• 💡State clearly how each controlled variable is kept constant and why it matters.
• 💡Describe the trend as temperature rises and link it to increased phospholipid fluidity and protein denaturation.
• 💡Explain that a colorimeter reading measures absorbance of the pigment, not the pigment mass directly.
• 💡Learn the equation ψ = P + π and be ready to use it to explain why a turgid cell has ψ close to zero.
• 💡In practical questions, describe how you would find the point of no change in mass or length and how you would use a calibration curve to read water potential.
• 💡Use the correct units: kPa for water potential and mol dm⁻³ for sucrose concentration.
• 💡When explaining results, link changes in mass or length to water movement by osmosis and to the water potential gradient.
• 💡For practical questions, link each step to safety or ethical considerations, such as using a sharp instrument away from the body or disposing of biological material correctly.
• 💡When describing plant gas exchange, name the specific structure (stoma, spongy mesophyll, lenticel) rather than saying 'the leaf' generally.
• 💡Check whether the question asks about gas exchange for photosynthesis, respiration or both, as this affects the direction of gas movement.
• 💡When describing the cohesion-tension model, link transpiration pull, tension in xylem and cohesion between water molecules in a logical sequence.
• 💡For questions on the mass-flow hypothesis, be prepared to evaluate it by citing specific evidence, such as aphid stylet experiments (strength) or bidirectional flow in the same sieve tube (weakness).
• 💡For Core Practical 8, describe how a potometer measures water uptake (distance moved by meniscus/bubble per unit time) and identify the independent and dependent variables.
• 💡When explaining tissue fluid formation, always distinguish between hydrostatic pressure pushing fluid out at the arterial end and oncotic pressure pulling fluid back in at the venous end.
• 💡Use accurate anatomical terminology when describing the heart: refer to atrioventricular (bicuspid/tricuspid) and semilunar valves rather than vague terms like 'flaps' or 'doors'.

• Thinking that surface area to volume ratio increases with body size; it decreases because volume grows faster than surface area.
• Assuming large organisms can rely on diffusion across the body surface alone; their small surface area to volume ratio makes this too slow.
• Confusing a mass transport system with a gas exchange surface; the transport system moves substances around the body, while the exchange surface provides a large area for diffusion.
• Believing that a specialised gas exchange surface removes the need for a transport system; both are required in large organisms.
• Thinking facilitated diffusion needs ATP: it is passive and driven by the concentration gradient, unlike active transport.
• Believing all small molecules cross the bilayer freely: small polar or charged particles such as ions need channels or carriers.
• Confusing osmosis with diffusion of solute: osmosis is the movement of water down a water potential gradient.
• Assuming endocytosis and exocytosis are passive: both require energy and involve vesicle formation or fusion.
• Thinking active transport does not require energy. Correction: active transport moves substances against their concentration gradient and therefore requires ATP hydrolysis.
• Confusing active transport with facilitated diffusion. Correction: facilitated diffusion is passive and moves substances down a concentration gradient through channel or carrier proteins, whereas active transport uses ATP and moves substances against the gradient.
• Believing ATP hydrolysis creates energy. Correction: ATP hydrolysis releases energy that was stored in the phosphate bond; it does not create new energy.
• Thinking phosphorylation of ADP releases energy. Correction: phosphorylation of ADP requires energy and stores it in ATP, while hydrolysis of ATP releases energy.
• Thinking insects use blood to carry oxygen to tissues: in insects, tracheoles deliver oxygen directly by diffusion, and blood does not transport oxygen in the same way as in mammals.
• Believing fish gills work by parallel flow: the correct arrangement is counter-current flow, where water and blood move in opposite directions to maintain a gradient.
• Assuming larger animals simply have larger exchange surfaces: the key is surface area to volume ratio, which decreases as body size increases, so specialised surfaces and ventilation are needed.
• Stating that the AVN initiates the heartbeat; correction: the SAN acts as the pacemaker and initiates the heartbeat, while the AVN delays the impulse.
• Confusing fibrin and fibrinogen; correction: fibrinogen is the soluble plasma protein, whereas fibrin is the insoluble protein that forms the clot mesh.
• Believing tissue fluid contains erythrocytes; correction: erythrocytes are too large to pass through capillary walls, so tissue fluid contains plasma and some leucocytes.
• Failing to distinguish between leucocyte types; correction: neutrophils and monocytes are phagocytes, whereas lymphocytes are involved in specific immunity (antibodies).
• Mistake: Stating that the Bohr effect shifts the dissociation curve to the left. Correction: The Bohr effect shifts the curve to the right, indicating a lower affinity for oxygen.
• Mistake: Claiming that myoglobin transports oxygen around the body. Correction: Myoglobin acts as an oxygen store in muscle tissue, not as a transport protein in the blood.
• Mistake: Believing fetal haemoglobin has a lower affinity for oxygen than adult haemoglobin. Correction: Fetal haemoglobin has a higher affinity for oxygen, allowing it to load oxygen at lower partial pressures.
• Thinking tissue fluid is reabsorbed at the arterial end; correction: reabsorption occurs mainly at the venous end where oncotic pressure exceeds hydrostatic pressure.
• Confusing oncotic pressure with hydrostatic pressure; correction: oncotic pressure is osmotic pressure due to plasma proteins, while hydrostatic pressure is the fluid pressure exerted on the capillary wall.
• Believing all tissue fluid is reabsorbed into capillaries; correction: some tissue fluid enters lymph capillaries and is returned to the blood via the lymph system.
• Stating that phospholipids denature: only proteins denature, whereas phospholipids gain kinetic energy and become more fluid.
• Forgetting to rinse the cut beetroot: surface pigment from cutting would falsely raise readings.
• Using different masses, volumes or times for each temperature: these must be controlled to make the comparison valid.
• Assuming pigment leaks because cells actively pump it out: leakage reflects loss of membrane integrity, not active transport.
• Thinking that turgor pressure is inward. Correction: P is the outward pressure of the cell contents against the cell wall.
• Believing that water potential is always negative. Correction: pure water at standard pressure has ψ = 0 kPa, and plant cells usually have negative ψ.
• Confusing water potential with solute concentration. Correction: water potential is measured in kPa, while concentration is measured in mol dm⁻³; the two are related but not identical.
• Assuming the point of no change means the cell is dead. Correction: it means the external solution and cell contents have the same water potential, so there is no net water movement.
• Confusing stomata with lenticels: stomata are mainly on leaves and some stems, while lenticels are found on woody stems and allow gas exchange through bark.
• Thinking plants only exchange gases during photosynthesis: they also respire continuously, so gas exchange occurs day and night, though the net direction of oxygen and carbon dioxide movement varies.
• Believing the insect dissection requires no ethical consideration: the practical must use organisms safely and ethically, including appropriate selection, handling and disposal.
• Thinking xylem transports sugars; correction: xylem transports water and mineral ions, while phloem transports sugars.
• Confusing apoplastic and symplastic pathways; correction: apoplastic pathway is along cell walls, symplastic pathway is through the cytoplasm via plasmodesmata.
• Believing transpiration rate increases with high humidity; correction: high humidity reduces the water potential gradient and decreases transpiration rate.
• Failing to explain how to set up a potometer; correction: always state that the shoot must be cut underwater and the apparatus assembled underwater to maintain the continuous water column.
• Believing all arteries carry oxygenated blood and all veins carry deoxygenated blood; the pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood to the heart.
• Assuming transpiration pulls water upward via a positive pumping pressure or vacuum suction, rather than cohesive tension created by water evaporation from mesophyll cells.
• Thinking that oxygen dissociation curves shift to the left during exercise; high carbon dioxide and lower pH cause a shift to the *right* (Bohr effect), lowering haemoglobin affinity for oxygen.

1. 1Week 1 (Days 1-3): Review surface area to volume ratio calculations and comparative gas exchange systems (mammalian alveoli, fish gills, insect tracheae).
2. 2Week 1 (Days 4-7): Master the circulatory system, including blood vessel histology, tissue fluid dynamics, and pressure-volume changes during the cardiac cycle.
3. 3Week 2 (Days 8-10): Focus on oxygen transport, plotting and interpreting oxygen haemoglobin dissociation curves, fetal haemoglobin, and the Bohr effect.
4. 4Week 2 (Days 11-14): Consolidate plant transport mechanisms, contrasting cohesion-tension in xylem with mass flow and active loading in phloem, followed by past paper questions.

• 📋Data analysis and graphical interpretation: Reading Wiggers diagrams (cardiac pressure/volume traces) to identify valve opening/closing points.
• 📋Haemoglobin dissociation curve questions: Explaining curve shifts (Bohr effect, altitude adaptation, fetal haemoglobin) in terms of affinity.
• 📋Experimental method evaluation: Analysing potometer investigations measuring transpiration rates, addressing control variables and calculation of water uptake.
• 📋Extended response (6-mark) comparative questions: Comparing structural adaptations between mammalian alveoli and insect tracheal systems.