OCR A Level Biology: Exchange and Transport (H420)

This guide covers Module 3: Exchange and Transport in OCR A Level Biology A (H420), following the site’s existing guides to Foundations in Biology and Development of Practical Skills. Module 3 applies the cell biology and biochemistry foundations from Module 2 to the systems organisms use to exchange substances with their environment and transport them internally.

Where this fits in H420

As organisms increase in size, their surface-area-to-volume ratio falls, and as their activity level (metabolic rate) rises, their demand for oxygen and for removal of carbon dioxide increases. Together these mean diffusion across the body surface alone becomes too slow to meet the organism’s needs – this single principle underlies almost every structure covered in this module, from gas exchange surfaces to the circulatory system.

Syllabus coverage

OCR A LEVEL BIOLOGY A (H420) – MODULE 3: EXCHANGE AND TRANSPORT

• Exchange surfaces: the general features of an efficient exchange surface, and why size and activity level make specialised exchange surfaces necessary in larger, more active organisms
• Gas exchange systems across different organism types, including the mammalian gas exchange system, gas exchange in bony fish, and gas exchange in insects
• Transport in animals: the structure of the heart and blood vessels, and the cardiac cycle
• Haemoglobin and gas transport: the structure of haemoglobin, oxygen dissociation curves, and the transport and unloading of oxygen and carbon dioxide
• Transport in plants: the structure and function of xylem and phloem, and the mechanisms of water and assimilate movement through each

How to approach it

This module rewards building a consistent mental model – structure determines function, and function is explained by the demands of exchange or transport at scale – rather than memorising each organ system as an unconnected list of facts. Practising “explain why” questions (why is a fish’s gill structured this way, why does haemoglobin’s affinity for oxygen change under certain conditions) develops this skill directly.

Official syllabus

OCR A Level Biology A (H420) specification, for first assessment in 2017 – ocr.org.uk.

Haemoglobin and the oxygen dissociation curve

Haemoglobin is a conjugated protein made of four polypeptide chains (two alpha and two beta in adult human haemoglobin), each bound to a haem group containing an iron ion capable of binding one oxygen molecule – meaning a single haemoglobin molecule can carry up to four oxygen molecules at once. The oxygen dissociation curve, an S-shaped (sigmoid) curve, shows how haemoglobin’s saturation with oxygen changes with the surrounding partial pressure of oxygen: the curve’s shape reflects cooperative binding, where binding the first oxygen molecule makes it easier for haemoglobin to bind further molecules. A shift of this curve to the right (the Bohr effect), caused by increased carbon dioxide concentration, reflects haemoglobin releasing oxygen more readily in actively respiring tissue, where it is needed most.

Mass transport: why diffusion alone is not enough

Mass transport refers to the bulk movement of substances driven by pressure differences, rather than relying on diffusion alone – the circulatory system in animals and the flow of water through xylem and sugars through phloem in plants are both examples. Insects, which lack a closed circulatory system that carries oxygen, instead rely partly on mass transport of air through a tracheal system, aided by abdominal muscle movements that actively ventilate the tracheae in larger or more active insects, supplementing simple diffusion of gases along the tracheal tubes.

Worked example: linking structure to function in the alveolus

The routine below is an original model written for this resource, not a reproduction of any official past paper or mark scheme.

Question style: "Explain how the structure of an alveolus is
adapted for efficient gas exchange."

Step 1 - name a specific structural feature:
e.g. the alveolus wall is one cell thick (squamous epithelium).

Step 2 - link the feature to a specific property it creates:
e.g. this creates a short diffusion pathway for oxygen and
carbon dioxide.

Step 3 - explain the functional consequence:
e.g. a shorter diffusion pathway increases the rate of gas
exchange, since diffusion rate is inversely related to distance.

Step 4 - repeat for further features (large surface area from
many alveoli, rich capillary network maintaining a diffusion
gradient, moist surface for gases to dissolve into) rather than
stopping at one feature alone.

Step 4 matters because “explain” questions in this module typically carry marks for multiple structure- function links, not just one – a single well-explained feature will not earn full marks if the question asks for several.

Common mistakes

Describing a structural feature without explaining the specific functional consequence it produces (naming “thin walls” without explaining why a shorter diffusion pathway matters). Confusing the roles of xylem (water and mineral transport, one direction, from roots to leaves) and phloem (assimilate transport, both directions, following source-to-sink demand). Describing the oxygen dissociation curve’s shape without explaining what causes a shift to the left or right. Treating all gas exchange systems (mammalian, fish, insect) as structurally identical, when each is adapted to that organism’s specific size, activity level, and environment.

Quick revision checklist

• Practise structure-to-function explanations for each exchange surface named in the syllabus, not just the alveolus.
• Be able to explain the shape of the oxygen dissociation curve and the cause of a leftward or rightward shift.
• Distinguish xylem and phloem clearly by direction of flow and the substances each transports.
• Compare gas exchange mechanisms across mammals, fish, and insects, linking each to that organism’s specific demands.
• Revise the surface-area-to-volume principle that motivates the whole module, since many exam questions return to it explicitly.