A-Level Biology: Exchange and Transport Lesson-by-Lesson Breakdown

Lesson 1: Surface area to volume ratio and the need for exchange systems

Focus: Surface area to volume ratio; why larger organisms need exchange surfaces and transport systems.

Students should learn:

• how surface area to volume ratio changes as size increases
• why diffusion alone is often insufficient in large multicellular organisms
• why larger organisms develop specialised exchange surfaces and mass transport systems
• the link between surface area to volume ratio and metabolic demand

Suggested practical:

• agar block diffusion model
• calculating surface area to volume ratios for different shapes

Good outcome: Students can explain why body size affects exchange efficiency.

Lesson 2: Gas exchange surfaces in different organisms

Focus: Adaptations of exchange surfaces; comparing organisms.

Students should learn:

• gas exchange across the surface of a single-celled organism
• gas exchange in insects through spiracles, tracheae, and tracheoles
• gas exchange in fish gills, including the counter-current principle
• gas exchange in dicotyledonous leaves, including mesophyll and stomata
• the general features of efficient exchange surfaces: large area, short diffusion distance, and suitable gradients

Good outcome: Students can compare exchange surfaces in animals and plants.

Lesson 3: Human gas exchange and ventilation

Focus: Structure of the human gas exchange system; breathing and ventilation.

Students should learn:

• the gross structure of the human gas exchange system: trachea, bronchi, bronchioles, alveoli, lungs
• the features of the alveolar epithelium
• how ventilation works
• the role of the diaphragm
• the antagonistic action of external and internal intercostal muscles
• how pressure changes in the thoracic cavity bring about breathing

Suggested practical:

• ventilation measurements
• spirometer-style data interpretation

Good outcome: Students can explain how the lungs and breathing mechanism enable gas exchange.

Lesson 4: Digestion and absorption

Focus: Digestion of large molecules; absorption in the small intestine.

Students should learn:

• that large biological molecules must be broken down before absorption
• digestion of carbohydrates by amylases and disaccharidases
• digestion of lipids by lipase and the role of bile salts
• digestion of proteins by endopeptidases, exopeptidases, and dipeptidases
• absorption in the ileum
• co-transport of monosaccharides and amino acids
• the role of micelles in lipid absorption

Suggested practical:

• Visking tubing absorption models
• investigation of enzyme activity linked to digestion

Good outcome: Students can explain how digestion and absorption support exchange with the internal environment.

Lesson 5: Membrane transport and diffusion principles

Focus: Diffusion and transport across membranes; links to exchange surfaces.

Students should learn:

• diffusion
• facilitated diffusion
• active transport
• osmosis
• endocytosis and exocytosis in Edexcel’s framing
• that Edexcel also applies Fick’s Law of Diffusion to exchange surfaces
• how concentration gradients and membrane properties affect movement of substances

Good outcome: Students can explain the mechanisms by which substances cross membranes at exchange surfaces.

Lesson 6: Mass transport in animals I — blood, haemoglobin, and oxygen transport

Focus: Blood as a transport medium; oxygen transport.

Students should learn:

• the role of red blood cells and haemoglobin
• loading and unloading of oxygen
• the oxyhaemoglobin dissociation curve
• cooperative binding
• the Bohr effect
• that different haemoglobins can suit different environments

Good outcome: Students can explain how haemoglobin makes oxygen transport efficient.

Lesson 7: Mass transport in animals II — heart, vessels, and tissue fluid

Focus: Circulatory system; heart structure and blood vessels; tissue fluid.

Students should learn:

• the general pattern of mammalian circulation
• the gross structure of the heart
• pressure and volume changes during the cardiac cycle
• the structure-function relationships of arteries, arterioles, veins, and capillaries
• formation of tissue fluid
• how tissue fluid returns to the circulatory system
• that Edexcel also frames this through why animals need mass transport and through heart/vessel structure

Suggested practical:

• pulse rate or heart rate investigation
• dissection of an animal mass transport organ or system

Good outcome: Students can explain how the circulatory system supports exchange across the body.

Lesson 8: Transport in plants I — xylem and transpiration

Focus: Water transport in plants; xylem structure and function.

Students should learn:

• that xylem transports water through stems and leaves
• the cohesion-tension theory
• how xylem structure supports transport
• that Edexcel highlights the role of cellulose microfibrils and secondary thickening in xylem properties
• how transpiration contributes to water movement

Suggested practical:

• potometer investigation into the effect of environmental variables on transpiration

Good outcome: Students can explain how water moves through plants and why xylem is adapted for this role.

Lesson 9: Transport in plants II — phloem and translocation

Focus: Phloem transport; movement of organic substances.

Students should learn:

• that phloem transports organic substances in plants
• the mass flow hypothesis
• the difference between xylem and phloem
• how evidence from tracer and ringing experiments is used
• that Edexcel explicitly compares xylem and phloem in terms of support and transport

Good outcome: Students can compare plant transport tissues and evaluate evidence for phloem translocation.

Lesson 10: Review, application, and exam practice

Focus: Bringing the topic together; data interpretation and practical application.

Students should practise:

• comparing exchange surfaces in different organisms
• explaining how ventilation and circulation maintain diffusion gradients
• interpreting data on lung disease, pollution, smoking, or cardiovascular risk
• comparing xylem and phloem
• analysing practicals involving potometers, spirometers, diffusion models, and transport investigations
• linking structure to function throughout the topic

Good outcome: Students can apply exchange and transport knowledge to unfamiliar questions rather than only recall facts.