Organisms exchange substances with their environment – A Level AQA Biology revision notes

What this topic covers

Organisms exchange substances with their environment has 4 subtopics:

• Surface area to volume ratio
• Gas exchange
• Digestion and absorption
• Mass transport

Organisms exchange substances with their environment lessons by subtopic

13 lessons, grouped by subtopic.

Surface area to volume ratio

• Surface Area to Volume Ratio: Size Matters: Calculate the surface area to volume ratio for simple geometric shapes and explain how this ratio changes with increasing size; Explain why larger organisms have a lower surface area to volume ratio and describe the physiological consequences of this; Compare the exchange strategies of small and large organisms, including changes to body shape and the development of specialised exchange systems

Gas exchange

• Gas Exchange in Organisms: Explain how gas exchange occurs across the body surface of a single-celled organism and relate this to surface area to volume ratio; Describe the structure and function of the tracheal system in insects, including tracheae, tracheoles, and spiracles; Explain the counter-current principle in fish gill gas exchange and evaluate its efficiency
• The Human Gas Exchange System: Describe the gross structure of the human gas exchange system, naming the alveoli, bronchioles, bronchi, trachea, and lungs; Explain the essential features of the alveolar epithelium that make it an effective gas exchange surface; Explain the mechanism of breathing, including the roles of the diaphragm and the antagonistic interaction between external and internal intercostal muscles
• Lung Disease and Risk Factors: Interpret data relating to the effects of lung disease on gas exchange and ventilation; Analyse the relationship between pollution, smoking, and the incidence of lung disease; Evaluate how experimental evidence led to statutory restrictions on risk factors such as smoking

Digestion and absorption

• Digestive Hydrolysis of Macromolecules: Explain how carbohydrates are hydrolysed by amylases and membrane-bound disaccharidases during digestion; Describe the role of lipase and bile salts in the hydrolysis of lipids; Compare the actions of endopeptidases, exopeptidases, and membrane-bound dipeptidases in protein digestion
• Absorption in the Ileum: Explain the co-transport mechanism by which amino acids and monosaccharides are absorbed across the ileum epithelium; Describe the role of the sodium-potassium pump in maintaining the sodium ion gradient that drives co-transport; Explain how micelles facilitate the absorption of lipid digestion products across the ileum epithelium
• Investigating Digestion Experimentally: Design and carry out investigations into the effect of pH or bile salts on the rate of reaction catalysed by a digestive enzyme; Evaluate experimental variables, controls, and sources of error in enzyme-based digestion investigations; Describe how Visking tubing models the absorption of digestion products across the gut wall

Mass transport

• Xylem Transport: Cohesion-Tension Theory: Describe the structure and function of xylem tissue in the transport of water through stems and leaves; Explain the cohesion-tension theory of water transport, including the roles of transpiration, cohesion, adhesion, and tension; Analyse how the properties of water molecules underpin the cohesion-tension mechanism
• Phloem Transport: Mass Flow Hypothesis: Describe the structure and function of phloem as the tissue responsible for transporting organic substances in plants; Explain the mass flow hypothesis as the proposed mechanism for translocation in phloem; Evaluate the evidence for and against the mass flow hypothesis, including the use of tracers and ringing experiments
• Evidence for Mass Flow: Interpreting Tracer & Ringing Experiments: Examine and interpret evidence from tracer and ringing experiments. Evaluate the evidence for and against the mass flow hypothesis, and recognise correlations and causal relationships in plant transport.
• The Cardiac Cycle & Cardiovascular Disease: Describe the gross structure of the human heart and explain how pressure and volume changes during the cardiac cycle maintain unidirectional blood flow; Analyse and interpret data relating to pressure, volume and valve movements during the cardiac cycle; Compare the structure of arteries, arterioles and veins in relation to their function
• Haemoglobin and Oxygen Transport: Describe the quaternary structure of haemoglobin and explain why it belongs to a group of chemically similar molecules found across many organisms; Explain the loading, transport, and unloading of oxygen in relation to the oxyhaemoglobin dissociation curve, including the cooperative nature of oxygen binding; Explain the Bohr effect and how increased carbon dioxide concentration shifts the dissociation curve
• Capillaries and Tissue Fluid Formation: Describe the structure of capillaries and explain how this structure adapts them as exchange surfaces; Explain the role of capillary beds in facilitating exchange between blood and tissues; Explain the formation of tissue fluid from blood plasma at the arterial end of a capillary

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