OCR A Level Biology: Exchange and Transport (H420) – Revision Notes

These notes condense Module 3 (sections 3.1.1 to 3.1.3) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026). Use the study guide for full explanations, then test yourself with the practice questions. The course hub and printable checklist show where this fits, and the free diagnostics help you spot weak topics. Each outcome is covered, but the practical outcomes (dissection, drawing, slides) are the briefest.

3.1.1 Exchange surfaces

Why they are needed. SA:V = surface area ÷ volume. It falls as size rises, while metabolic activity raises demand. Single-celled organisms can use the body surface; larger or more active animals need a specialised surface plus ventilation.

Efficient exchange surface:

Feature	Example
Large surface area	Root hair cells; folded lamellae
Thin layer, short distance	Alveolar wall
Maintained gradient (blood flow, ventilation)	Gills, alveoli

Mammalian system, by region:

Tissue	Where	Function
Cartilage	Trachea (C-shaped rings), bronchi	Keeps airway open
Ciliated epithelium	Trachea to bronchioles	Sweeps mucus upward
Goblet cells	Trachea, bronchi	Make mucus to trap particles
Smooth muscle	Trachea to bronchioles	Alters airway diameter
Elastic fibres	All, including alveoli	Recoil, helping expiration

The alveolus itself has no cartilage, cilia, goblet cells or smooth muscle: its wall is a thin squamous layer with elastic fibres.

Ventilation (mammal).

Stage	Muscles	Ribs and diaphragm	Volume and pressure
Inspiration	External intercostals and diaphragm contract	Ribs up and out; diaphragm flattens	Volume up, pressure down
Expiration	External intercostals and diaphragm relax; internal intercostals can contract in forced breathing	Ribs down and in; diaphragm domes	Volume down, pressure up

Lung volumes. Tidal volume is the volume per resting breath. Vital capacity is the maximum volume that can be exhaled after a maximal inhalation. Tidal volume is only part of vital capacity, so exercise can raise both tidal volume and breathing rate. Ventilation rate = tidal volume × breathing rate. A spirometer trace gives oxygen uptake from the fall in the trace over time, once carbon dioxide is removed.

Bony fish. Lowering the buccal cavity floor raises volume, so water enters. Raising it forces water over the gills as the operculum opens. Each filament carries lamellae. Blood flows opposite to water (countercurrent), so the gradient is kept across the full length.

Insects. Air enters through spiracles into tracheae and tracheoles. Gas exchange takes place at tracheal fluid at the tracheole ends. Thoracic and abdominal movements change body volume to ventilate.

Practical outcomes (briefest). Fish gill or insect trachea dissection and drawing; stained microscope slides of exchange surfaces: draw only what you see, with clean lines and no shading.

3.1.2 Transport in animals

Need. Greater size, higher metabolic rate and low SA:V mean diffusion is too slow.

Circulatory systems.

Animal	Type	Key point
Insect	Open	Fluid bathes tissues; low pressure
Fish	Closed, single	Blood passes the heart once per circuit
Mammal	Closed, double	Heart pumps twice per circuit, so pressure is restored after the lungs

Vessels.

Vessel	Wall tissues	Notes
Artery	Thick; elastic fibres, smooth muscle, collagen	Withstands high pressure
Arteriole	More smooth muscle	Controls flow
Capillary	Endothelium only	Short distance
Venule, vein	Thin wall; valves in veins	Return blood at low pressure

Tissue fluid. Hydrostatic pressure forces fluid out at the arteriole end. Oncotic pressure from plasma proteins pulls water back. At the venule end the hydrostatic pressure has fallen, so net movement is inwards. Tissue fluid lacks most large proteins and red cells. Lymph is similar but has more lymphocytes and fewer nutrients, and is returned to the blood by lymph vessels.

Heart. External structure: atria, ventricles, coronary arteries. Internal: septum, atrioventricular valves, semilunar valves, thicker left ventricle wall. Dissection: record sizes and cut the wall to reveal the valves.

Cardiac cycle.

Phase	Valves	Pressure
Atrial systole	AV open	Atria above ventricles
Ventricular systole	AV shut, semilunar open	Ventricle above aorta
Diastole	Semilunar shut, AV open	Atria fill

Cardiac output = heart rate × stroke volume.

Coordination. SAN starts excitation across atria, AVN delays it, purkyne tissue carries it through the ventricles. Cardiac muscle is myogenic.

ECG. Normal traces are regular. Tachycardia is faster; bradycardia slower. Fibrillation is irregular. An ectopic beat comes early. Rate = 60 ÷ time between R waves.

Gas transport. Haemoglobin binds oxygen reversibly. Carbon dioxide forms carbonic acid by carbonic anhydrase, then hydrogencarbonate and H⁺. Haemoglobin binds H⁺ (haemoglobinic acid). Hydrogencarbonate leaves and chloride enters (chloride shift).

Dissociation curves. Fetal haemoglobin has a higher affinity and its curve lies to the left of adult. More carbon dioxide shifts the curve right, the Bohr effect.

3.1.3 Transport in plants

Need. Large and complex plants have low SA:V and cells far from sources.

Vascular tissue (herbaceous dicot).

Tissue	Cells	Key features
Xylem	Vessels	Dead, lignified, no end walls
Phloem	Sieve tube elements, companion cells	Living, sieve plates; companions support

Stem: bundles in a ring. Root: central core. Leaf: midrib bundle. Practical outcomes: stained sections and stem dissection.

Transpiration. Water vapour loss through open stomata, a consequence of gas exchange. Faster with higher temperature, light, wind and lower humidity. A potometer estimates uptake: volume = πr² × distance, divided by time.

Water movement. Root hair → cortex by apoplast (cell walls) or symplast (cytoplasm and plasmodesmata) → xylem. Evaporation from leaf cells lowers water potential; cohesion pulls the column up under tension; adhesion helps along walls; this is the transpiration stream. Water moves from higher (less negative) to lower (more negative) water potential. ψ = ψs + ψp.

Adaptations.

Type	Example	Features
Xerophyte	Cactus; marram grass	Thick cuticle, rolled leaves, sunken stomata, hairs, spines, reduced leaves
Hydrophyte	Water lily	Air spaces, stomata on upper surface, thin cuticle

Translocation. Energy-requiring transport of sucrose from sources (leaves) to sinks (roots, meristems). Active loading: companion cells pump H⁺ out using ATP; H⁺ returns with sucrose via co-transport; sieve tube water potential falls, water enters, hydrostatic pressure rises, sap flows. At the sink, sucrose is removed and used or stored.

Methods in steps

Explaining an exchange surface. (1) Name the feature. (2) State what it does to distance, area or gradient. (3) Say how that changes the rate of diffusion. (4) Repeat for each feature the question asks about.

Reading an ECG. (1) Find the time between two successive R waves, in seconds. (2) Divide 60 by it. (3) Compare with the usual resting range of about 60 to 100 beats per minute. (4) Look at the spacing: regular, irregular, or one early beat.

Tissue fluid questions. (1) Subtract oncotic from hydrostatic pressure. (2) A positive answer means fluid leaves the capillary; a negative one means it returns. (3) Link the change in hydrostatic pressure along the capillary to the change in direction. (4) Say what happens to excess fluid: it drains into lymph vessels.

Potometer calculations. (1) Convert all lengths to the same unit. (2) Volume = πr² × distance moved. (3) Divide by the time taken. (4) State that the apparatus measures uptake, which only estimates transpiration.

Must-know distinctions

Compare	First	Second
Open and closed circulation	Fluid bathes tissues directly	Blood stays in vessels
Single and double circulation	One pass through the heart	Two passes, so pressure is restored
Tissue fluid and plasma	Few large proteins	Contains plasma proteins
Hydrostatic and oncotic pressure	Pushes fluid out	Pulls water in
Apoplast and symplast	Walls and spaces	Cytoplasm through plasmodesmata
Xylem and phloem	Water and ions, upward, dead cells	Assimilates, either direction, living cells
Xerophyte and hydrophyte	Reduces loss of water	Floats and exchanges gases through upper surface
Source and sink	Where sucrose is loaded	Where sucrose is removed

Countercurrent reminder

Imagine water that loses oxygen from 80% to 40% saturation along a lamella. Blood moving the other way enters at 30% and leaves at 70%. At every point along the lamella, water carries about 10 percentage points more oxygen than the blood beside it, so diffusion carries on throughout. With parallel flow, the two would meet at an intermediate saturation, about 55%, and net uptake would stop partway along.

Quick self-test

1. A cube has sides of 0.5 mm. Calculate its SA:V, then for 5 mm.
2. Heart rate 72, stroke volume 70 cm³: find cardiac output.
3. R waves are 1.2 s apart: find the rate and name it.
4. Tidal volume 0.5 dm³, 14 breaths per minute: find ventilation rate.
5. Hydrostatic pressure is 4.0 kPa, oncotic 3.0 kPa: state the net pressure out.
6. A cell has ψs of −900 kPa and ψp of +400 kPa: find ψ.
7. Name the cells that transport sucrose.
8. Which way does more carbon dioxide shift the curve?
9. Name the enzyme in red cells that forms carbonic acid.
10. A bubble moves 36 mm in 5 minutes in a tube of radius 0.5 mm: find uptake per minute.

Answers

1. 12 mm⁻¹ and 1.2 mm⁻¹.
2. 72 × 70 = 5040 cm³ per minute.
3. 60 ÷ 1.2 = 50 beats per minute: bradycardia.
4. 7.0 dm³ per minute.
5. 1.0 kPa.
6. −500 kPa.
7. Sieve tube elements, helped by companion cells.
8. Right.
9. Carbonic anhydrase.
10. π × 0.5² × 36 = 28.3 mm³, so 5.65 mm³ per minute.

Where marks are usually lost

• Naming a feature without explaining the effect on diffusion.
• Mixing up the external and internal intercostal muscles.
• Saying “air is sucked in” instead of describing pressure falling.
• Forgetting that fish blood flows opposite to water.
• Not stating the direction of net movement in tissue fluid.
• Mixing up which cardiac valves are shut in each phase.
• Giving heart rate without using the time between R waves.
• Forgetting that xylem is dead and phloem is living.
• Quoting water movement from “high to low” without the sign.

Frequently asked questions

What do the external and internal intercostal muscles do? During normal inspiration the external intercostal muscles contract to raise the ribcage, while the internal intercostal muscles are used in forced expiration. Mixing up the two is a frequent error in gas exchange answers.

How should inspiration be described without saying air is “sucked in”? Describe the volume of the thorax increasing, so the pressure inside falls below atmospheric pressure, and air then moves into the lungs down the pressure gradient.

What is the countercurrent system in fish gills? In fish gills, blood flows in the opposite direction to the water passing over the lamellae. This keeps a diffusion gradient along the whole length of the lamella, so more oxygen is taken up than with parallel flow.

How do you calculate heart rate from an ECG? Measure the time between two R waves, which is the time for one heartbeat, and divide 60 seconds by it to give beats per minute.

What is the main difference between xylem and phloem? Xylem vessels are dead tissue that carry water and mineral ions upwards from the roots, while phloem sieve tubes are living tissue that transport assimilates such as sucrose in both directions. These notes cover Module 3 of OCR A Level Biology A (H420).

Official syllabus

OCR Advanced GCE in Biology A (H420), Version 4.1 (April 2026), for first assessment in 2025, published by OCR.