A-Level Biology · Topic 3
Exchange surfaces and transport systems: every key term you need (+ practice quiz)
16 flashcard terms for A-Level Biology Topic 3, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
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Surface area to volume ratio Falls as an organism gets larger, so big organisms cannot rely on diffusion across the body surface and need specialised exchange systems.
Fick's law Rate of diffusion is proportional to surface area multiplied by concentration difference, divided by diffusion distance.
Alveoli Millions of thin-walled air sacs giving a huge moist surface area, one cell thick and densely capillaried to maintain a steep gradient.
Counter-current exchange In fish gills, blood and water flow in opposite directions, so a concentration gradient is maintained along the whole lamella.
Spiracle A valve-controlled pore in an insect's exoskeleton. Opens for gas exchange and closes to limit water loss.
Xerophyte adaptations Sunken stomata, rolled leaves, thick waxy cuticle and hairs all trap humid air, reducing the water potential gradient driving transpiration.
Haemoglobin A globular protein with four polypeptide chains, each binding one oxygen molecule. Loading and unloading depend on partial pressure.
Cooperative binding Binding the first oxygen changes haemoglobin's shape, making the next easier to bind. This gives the dissociation curve its S shape.
Bohr effect Raised carbon dioxide lowers blood pH and shifts the dissociation curve right, so haemoglobin releases oxygen more readily in active tissue.
Cardiac cycle Atrial systole, ventricular systole, then diastole. Valves open and close purely because of pressure differences between chambers.
Atheroma A fatty plaque in an artery wall. It narrows the lumen, raises blood pressure and can rupture to trigger a thrombus.
Tissue fluid formation High hydrostatic pressure at the arteriole end forces fluid out; low water potential from plasma proteins draws most back at the venule end.
Cohesion-tension theory Transpiration pulls water up the xylem as a continuous column, held together by hydrogen bonding between water molecules.
Potometer Measures water uptake by a shoot as a proxy for transpiration rate. It slightly overestimates, since some water is used in photosynthesis.
Mass flow hypothesis Sucrose loaded at the source lowers water potential, water enters from the xylem, and the resulting pressure drives phloem sap to the sink.
Ringing experiment Removing a ring of bark makes sugars accumulate above the cut, showing that organic solutes travel in phloem rather than xylem.
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