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

This guide covers 3.3 Organisms exchange substances with their environment, the third of eight subject-content sections in AQA A-level Biology (7402). Sections 1-4 are first-year content and are also the whole of the AS (7401) subject content, so this section is assessed on both AS papers and, at A-level, on Paper 1 (topics 1-4) and Paper 3 (topics 1-8). It builds directly on the cell content of Topic 3.2 and underpins later A-level-only topics on homeostasis and energy transfer. These notes complement the site’s guides to Biological Molecules and Eukaryotic and Prokaryotic Cell Structure.

Where this fits in 7402

The internal environment of a cell or organism differs from its external environment, and exchange between the two happens at exchange surfaces – most substances must cross cell plasma membranes to truly enter or leave an organism. In large multicellular organisms, most cells sit too far from exchange surfaces for diffusion alone to work, so exchange surfaces are paired with mass transport systems that move substances around the body.

Syllabus coverage

AQA A-LEVEL BIOLOGY (7402) – 3.3 ORGANISMS EXCHANGE SUBSTANCES WITH THEIR ENVIRONMENT

• 3.3.1 Surface area to volume ratio: the relationship between organism/structure size and its surface area to volume ratio; changes in body shape and system development in larger organisms as adaptations that facilitate exchange as this ratio reduces; the relationship between surface area to volume ratio and metabolic rate
• 3.3.2 Gas exchange: adaptations of gas exchange surfaces across a single-celled organism’s body surface, an insect’s tracheal system (tracheae, tracheoles, spiracles), fish gills (lamellae, filaments, counter-current principle), and dicotyledonous leaves (mesophyll, stomata); the trade-off between efficient gas exchange and limiting water loss in terrestrial insects and xerophytic plants; the human gas exchange system (alveoli, bronchioles, bronchi, trachea, lungs); the alveolar epithelium; the mechanism of breathing including the diaphragm and antagonistic intercostal muscles
• 3.3.3 Digestion and absorption: hydrolysis of large molecules during digestion; digestion of carbohydrates (amylases, membrane-bound disaccharidases), lipids (lipase, bile salts) and proteins (endopeptidases, exopeptidases, membrane-bound dipeptidases); co-transport mechanisms for absorbing amino acids and monosaccharides; the role of micelles in lipid absorption
• 3.3.4.1 Mass transport in animals: haemoglobin structure and function; loading, transport and unloading of oxygen and the oxyhaemoglobin dissociation curve; cooperative binding; the Bohr effect; different haemoglobins in different organisms; the general pattern of mammalian blood circulation; the gross structure of the human heart and the cardiac cycle; the structure of arteries, arterioles, veins and capillaries; the formation and return of tissue fluid
• 3.3.4.2 Mass transport in plants: xylem and the cohesion-tension theory of water transport; phloem and the mass flow hypothesis for translocation; the use of tracers and ringing experiments

How to approach it

Every section in this topic returns to the same underlying principle from 3.3.1: as organisms get larger, their surface area to volume ratio falls, so diffusion alone can no longer supply cells fast enough, which is why gas exchange surfaces, digestive absorption surfaces, and mass transport systems all exist. Frame your revision around that single idea rather than memorising each system in isolation – exam questions frequently ask you to explain why a named adaptation increases surface area, reduces diffusion distance, or maintains a diffusion gradient, and the answer draws on the same underlying logic every time.

Official syllabus

AQA A-level Biology (7402) specification, first teaching 2015 – aqa.org.uk.

The exchange-surface principle

Efficient exchange surfaces share four features: a large surface area, a short diffusion distance (thin walls), a steep concentration gradient maintained by a supply mechanism, and often a good blood or fluid supply. Every named exchange surface in this topic – alveoli, gill lamellae, the villi lining the ileum – can be explained by checking which of these four features it maximises.

Gas exchange: comparing four systems

Single-celled organisms exchange gases directly across their body surface, since their surface area to volume ratio is high enough that diffusion alone suffices. Insects use a tracheal system that delivers air directly to tissues, bypassing a circulatory gas-transport role entirely. Fish gills use a counter-current arrangement – blood and water flow in opposite directions – which maintains a diffusion gradient along the whole length of the gill lamellae, extracting more oxygen than a parallel-flow system could. Mammalian lungs rely on a huge alveolar surface area, a two-cell-thick exchange membrane, and continuous ventilation to maintain the gradient.

Digestion is hydrolysis followed by absorption: co-transport or diffusion

Carbohydrates, lipids and proteins are all broken down by hydrolysis into units small enough to cross cell membranes, then absorbed by mechanisms specific to each: monosaccharides and amino acids by co-transport with sodium ions (driven by the sodium ion gradient that active transport of sodium out of the epithelial cell maintains), lipids via micelles that carry monoglycerides and fatty acids to the epithelial cell membrane, across which these lipid-soluble molecules then move by simple diffusion.

Worked example: interpreting an oxyhaemoglobin dissociation curve

A dissociation curve shifted to the right (the Bohr effect) at a respiring tissue means:

Higher CO2 concentration at the tissue
-> lower pH (more carbonic acid formed)
-> haemoglobin's affinity for oxygen decreases
-> oxygen is unloaded MORE READILY at that tissue

This is adaptive: actively respiring tissue produces more CO2,
which is exactly where more oxygen unloading is needed.

A rightward shift always means “unloads oxygen more easily at a given partial pressure” – learn that direction cold, since curve-shift questions are a recurring exam feature.

Common mistakes

Explaining an adaptation only as “increases surface area” without linking it to the underlying surface-area-to-volume-ratio principle. Confusing the counter-current principle’s effect (maintaining a gradient along the whole gill) with simple diffusion. Describing co-transport as passive diffusion when it is linked to active sodium-ion transport. Misreading a dissociation-curve shift direction. Confusing xylem’s cohesion-tension mechanism with phloem’s mass flow mechanism, which work in opposite directions in terms of energy input.

Quick revision checklist

• Explain any named exchange surface using the four shared features: surface area, diffusion distance, gradient maintenance, and supply.
• Compare gas exchange in a single cell, an insect, a fish and a mammal, naming the key structures each time.
• Trace the hydrolysis-then-absorption sequence for carbohydrates, lipids and proteins.
• Interpret oxyhaemoglobin dissociation curve shifts, including the Bohr effect, and state their adaptive significance.
• Distinguish the cohesion-tension theory (xylem, water) from the mass flow hypothesis (phloem, organic solutes).