AQA A-Level Biology: Organisms Exchange Substances With Their Environment (7402) – Revision Notes

These notes condense section 3.3.1 to 3.3.4 (Organisms exchange substances with their environment) of the AQA AS and A-level Biology specification, version 1.5, for the A-level Biology (7402) course. The content is part of topics 1 to 4, which the specification lists as assessable on Paper 1 and Paper 3 at A-level. For the fuller explanation, use the study guide, then test yourself with the practice questions. Course hub: AQA A-level Biology. Printable list: checklist. Unsure where to start? Try the free diagnostics.

Background for this topic is in biological molecules and lipids and protein structure.

3.3.1 Surface area to volume ratio

• Surface area to volume ratio (SA:V) falls as an object gets bigger, because volume grows faster than surface area.
• Small organisms: large SA:V, so diffusion across the body surface can meet demand. Large organisms: small SA:V, so they need exchange surfaces plus a mass transport system.

Cube side	Surface area	Volume	SA:V
1 cm	6 cm²	1 cm³	6 : 1
2 cm	24 cm²	8 cm³	3 : 1
4 cm	96 cm²	64 cm³	1.5 : 1

Doubling the side halves the ratio. A sphere of radius r has SA:V = 3/r.

Adaptations as the ratio falls: flattened bodies (more surface per unit volume), folded or branched exchange surfaces (gills, alveoli), and transport systems such as a circulatory system.

Metabolic rate link: a small animal loses heat quickly through its large relative surface, so it needs a high metabolic rate per gram to stay warm. It therefore takes up oxygen and food faster per gram than a large animal. A large animal has a lower rate per gram.

3.3.2 Gas exchange

Four exchange surfaces

Organism	Route and key structures	Why it works
Single-celled	Diffusion straight across the cell surface	High SA:V, short distance
Insect	Spiracles, tracheae, tracheoles; tracheole ends hold fluid and reach the muscle cells	Air-filled tubes carry gases close to cells; diffusion in air is fast
Fish	Water over gill filaments; each has many lamellae	Thin, richly supplied lamellae; counter-current flow
Dicot leaf	Stomata, air spaces, mesophyll	Air reaches the wet surfaces of mesophyll cells; guard cells control the pore

Counter-current principle: blood in the lamellae runs opposite to the water. The water always meets blood with a lower oxygen content, so a diffusion gradient exists along the full length of the lamella and a large share of the oxygen is taken up. Parallel flow would let the two streams reach equilibrium part-way along.

Water loss against gas exchange

• Insects: spiracles can close, which limits water loss, and a waterproof cuticle covers the body. The cost is less gas entry while closed.
• Xerophytes: thick cuticle, sunken stomata, rolled leaves and hairs trap humid air next to the stomata and cut the gradient for water vapour. The cost is a longer, more restricted diffusion path for CO2 and O2.

Human gas exchange system

Order of air flow: trachea, bronchi, bronchioles, alveoli, all inside the lungs.

Alveolar epithelium: one cell thick and flat, which gives a short diffusion distance. It sits against a dense capillary network, which keeps blood flowing and removes oxygen to maintain the gradient. There are very many alveoli, giving a large surface. Moist lining lets gases dissolve.

Breathing

	Inspiration	Expiration (at rest)
Diaphragm	Contracts, flattens	Relaxes, domes upward
External intercostals	Contract, ribs up and out	Relax
Internal intercostals	Relax	Contract during forced expiration only
Thoracic volume / pressure	Volume up, pressure below atmospheric	Volume down, pressure above atmospheric

The two sets of intercostals are antagonistic: one raises the ribs, the other lowers them.

Pulmonary ventilation rate (PVR) = tidal volume × breathing rate. Rearrange for either factor.

Lung disease and risk factors: be ready to read tables and graphs on lung disease (for example reduced lung elasticity, thickened or scarred tissue, fewer alveoli) and its effect on gas exchange or ventilation. A correlation between smoking or pollution and disease does not alone prove cause. Stronger evidence combines a dose-response trend, controlled comparison groups and an explained mechanism. Be able to evaluate how such data led to laws restricting a risk factor.

3.3.3 Digestion and absorption

Digestion is hydrolysis: large molecules are split by water into units small enough to cross membranes.

Food group	Enzymes	Product
Carbohydrate	Amylase (salivary, pancreatic); membrane-bound disaccharidases (maltase, sucrase, lactase)	Starch to maltose; then monosaccharides
Lipid	Lipase; bile salts emulsify	Monoglycerides and fatty acids
Protein	Endopeptidases cut inside the chain; exopeptidases remove terminal amino acids; membrane-bound dipeptidases	Amino acids

Absorption in the ileum

• Monosaccharides and amino acids: sodium ions are pumped out of the epithelial cell by active transport, which lowers the internal sodium concentration. Sodium then enters down its gradient through a carrier protein that carries glucose or an amino acid with it (co-transport). The solute builds up in the cell, then leaves by facilitated diffusion into blood.
• Lipids: bile salts help form tiny micelles holding monoglycerides and fatty acids. Micelles bring these to the cell surface, so they keep a high local concentration there. The lipids are lipid-soluble and diffuse across the membrane.

3.3.4.1 Mass transport in animals

Haemoglobin and oxygen

• Red blood cells carry haemoglobin, a protein with quaternary structure: four polypeptide chains, each with a haem group. Different organisms carry chemically similar but not identical haemoglobins with different oxygen properties.
• Loading happens in the lungs (high partial pressure of oxygen); unloading in respiring tissue (low).
• The S-shaped dissociation curve shows cooperative binding: the first oxygen alters the shape and makes the next ones bind more easily.
• Bohr effect: more CO2 shifts the curve right, so haemoglobin releases oxygen at a given partial pressure more readily.
• A curve to the left means higher affinity: useful at low oxygen availability (for example a high-altitude animal or a fetus). A curve to the right suits an active animal.

Circulation, heart and vessels

• Mammalian circulation is double: heart to lungs and back, then heart to body and back. Know the names of the coronary arteries and of the vessels entering and leaving the heart, lungs and kidneys (aorta, vena cava, pulmonary artery and vein, renal artery and vein).
• Heart: left and right atria and ventricles, atrioventricular valves, semilunar valves. The left ventricle wall is thicker.
• Cardiac cycle: atrial systole pushes blood into ventricles; ventricular systole raises pressure, shutting the atrioventricular valves and opening semilunar valves; diastole lets pressure fall, semilunar valves close and the atria refill. Valves open only when pressure behind exceeds pressure in front, so flow is one way.
• Cardiac output = stroke volume × heart rate.

Vessel	Structure	Function link
Artery	Thick muscle and elastic wall	Withstands and smooths pulses
Arteriole	Muscle in wall	Constricts or dilates to adjust flow
Vein	Thin wall, wide lumen, valves	Low pressure return; valves stop backflow
Capillary	One cell thick	Short path; beds are exchange surfaces

Tissue fluid: at the arteriole end, hydrostatic pressure forces fluid out through the capillary wall. Plasma proteins stay behind, so blood water potential falls. Toward the venule end, hydrostatic pressure has dropped and water re-enters by osmosis. Some fluid drains into the lymph system and returns to the blood.

Data skills: interpret pressure and volume curves of the cardiac cycle. For cardiovascular disease risk factors, analyse the incidence data, then weigh conflicting evidence by comparing sample size, control of other variables and whether a mechanism is known; remember that a correlation does not prove cause.

Required practical 5: dissection of a gas exchange or mass transport system, or an organ within one.

3.3.4.2 Mass transport in plants

• Xylem carries water up stems and leaves. Cohesion-tension: transpiration lowers pressure at the leaf; hydrogen bonds make water molecules cohere, so the column is pulled up under tension and adheres to the walls.
• Phloem carries organic substances. Mass flow: sucrose is loaded into the phloem at the source, water follows by osmosis, hydrostatic pressure rises, and sap flows toward the sink where sucrose is removed or used.
• Tracers: radioactive carbon in supplied CO2 is later detected in phloem sugars, showing direction and speed.
• Ringing: removing a ring of bark and phloem causes sugars to collect above the cut, supporting phloem transport of sugars; the xylem stays intact. Evaluate for and against mass flow: sap does flow under pressure, but sieve plates would be expected to slow bulk flow, and translocation stops when respiration is inhibited, which suggests energy is needed beyond simple pressure flow.

Quick self-test

1. A cube has sides of 5 cm. State its SA:V.
2. Why is a large animal’s metabolic rate per gram lower than a mouse’s?
3. Name the structures that carry air into an insect’s tissues, in order from outside.
4. What does counter-current flow maintain?
5. Tidal volume is 0.35 dm³ and breathing rate 16 per minute. Find PVR.
6. Name the enzyme type that completes protein digestion at the ileum surface.
7. Which ion gradient drives glucose co-transport?
8. A rightward shift of the dissociation curve means what?
9. Cardiac output is 5.4 dm³ per minute at 72 beats per minute. Find stroke volume in cm³.
10. In a ringing experiment, where do sugars collect?

Answers

1. 150 cm² divided by 125 cm³ = 1.2 : 1.
2. Smaller SA:V means less heat lost per gram, so less metabolic activity is needed.
3. Spiracles, tracheae, tracheoles.
4. A diffusion gradient along the whole lamella.
5. 0.35 × 16 = 5.6 dm³ per minute.
6. Membrane-bound dipeptidases.
7. The sodium ion gradient into the cell.
8. Oxygen is released more readily at a given partial pressure.
9. 5400 / 72 = 75 cm³.
10. Just above the ring.

Where marks are usually lost

• Writing “bigger surface” without saying that SA:V falls with size and why that matters.
• Describing counter-current flow without saying the water and blood move in opposite directions.
• Saying the insect “breathes through its skin” rather than naming spiracles, tracheae and tracheoles.
• Mixing up which intercostals contract during inspiration, or forgetting that pressure change drives air movement.
• Calling co-transport “passive” or leaving out sodium ions.
• Saying haemoglobin “has a high affinity for oxygen” without linking it to partial pressure.
• Stating a correlation proves cause in a risk-factor question.
• Confusing xylem (water, tension, no living cells) with phloem (sugars, pressure, energy).

Frequently asked questions

Why does surface area to volume ratio fall as an organism gets bigger? Volume increases faster than surface area as size increases, so SA:V falls. Doubling the side length of a cube halves the ratio.

What is the counter-current principle in fish gills? Blood in the gill lamellae flows in the opposite direction to the water passing over them, so a diffusion gradient is maintained along the whole length of the lamellae.

How do you calculate pulmonary ventilation rate? Pulmonary ventilation rate = tidal volume × breathing rate.

What does the Bohr effect do? More carbon dioxide shifts the oxyhaemoglobin dissociation curve to the right, so haemoglobin releases oxygen more readily in respiring tissues.

How do you calculate cardiac output? Cardiac output = stroke volume × heart rate.

Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), version 1.5, published 26 November 2021 by AQA. This page covers section 3.3.