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EnglandBiology

3.3 Organisms exchange substances with their environment

5 dot points across 5 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

How are large food molecules digested and then absorbed across the small intestine into the blood?

Digestion in mammals: the action of carbohydrases, lipases and proteases (including membrane-bound disaccharidases and dipeptidases); the role of bile salts in lipid digestion; absorption of the products across the ileum epithelium, including co-transport of glucose and amino acids and the absorption of monoglycerides and fatty acids.
An AQA A-Level Biology answer on digestion and absorption. Covers carbohydrase, lipase and protease action, membrane-bound disaccharidases and dipeptidases, bile salt emulsification and micelles, villi adaptations, and sodium-dependent co-transport of glucose and amino acids.
11 min answer →

How are gas exchange surfaces in single-celled organisms, insects, fish and plants adapted to a low surface area to volume ratio?

Gas exchange in single-celled organisms and across the body surface of insects, gills of fish, and the leaves of dicotyledonous plants; structural and functional adaptations for efficient gas exchange; the limitation of water loss and how it is overcome.
An AQA A-Level Biology answer on gas exchange across single-celled organisms, insect tracheae, fish counter-current gills, and dicot leaves. Covers structural adaptations, the counter-current principle, the stomata water-loss trade-off and xerophytes.
11 min answer →

How does haemoglobin load and unload oxygen, and how does the heart move blood around the body?

Mass transport in animals: the role of haemoglobin in oxygen transport, the oxygen dissociation curve and the Bohr effect; the structure of the heart and the cardiac cycle; the structure of arteries, veins and capillaries in relation to function.
An AQA A-Level Biology answer on mass transport in animals. Explains cooperative oxygen binding by haemoglobin, the sigmoid dissociation curve and the Bohr effect, the structure of the heart, the cardiac cycle and pressure changes, and how vessels are adapted to function.
12 min answer →

How do plants move water up the xylem and sugars through the phloem without a pump?

Mass transport in plants: transport of water in the xylem by the cohesion-tension theory and transpiration; transport of organic substances in the phloem by mass flow (the source-to-sink translocation model) and supporting evidence.
An AQA A-Level Biology answer on mass transport in plants. Explains transpiration and the cohesion-tension theory of water movement in the xylem, the factors affecting transpiration rate, and translocation of sugars in the phloem by the mass flow hypothesis with its supporting evidence.
10 min answer →

Why does body size dictate whether an organism needs a specialised exchange surface?

The relationship between the size of an organism or structure and its surface area to volume ratio, and the consequences for exchange of substances and heat with the environment, including the role of Fick's law.
An exam-focused answer to the AQA A-Level Biology dot point on surface area to volume ratio. Explains why SA:V falls as size rises, how this drives the need for specialised exchange surfaces and mass transport, and how Fick's law quantifies the rate of diffusion.
9 min answer →