The big idea: Cells that exchange a lot raise their SA:V ratio by flattening, by microvilli or by invagination of their membrane.
The points to remember
- Flattening: a thin cell has every part close to the surface, so its SA:V is large.
- Red blood cells are flattened biconcave discs: a large membrane area for gas exchange.
- They have no nucleus, leaving more room for haemoglobin, and are flexible enough to squeeze through capillaries.
- Microvilli: tiny finger-like folds of the plasma membrane, a large area for absorption or reabsorption.
- Invagination: the membrane folded inwards, as at the base of a proximal convoluted tubule cell.
- PCT cells also have many mitochondria (ATP for active transport) and vesicles from endocytosis.
Remember it as: Flatten it, fringe it, fold it in.
Real example: the small intestine: The cells lining the human small intestine are covered in microvilli, which multiply the absorbing surface of each cell roughly twenty times.
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How it is asked: Expect a Paper 2 Describe on red blood cells, or an Outline of the visible features of cells in an electron micrograph, such as a proximal convoluted tubule.
IB-style questionDescribe[4 marks]
Describe the adaptations of proximal convoluted tubule cells for the reabsorption of substances from the filtrate.
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