Leçon d’essai
Crossing the membrane
Diffusion, osmosis and active transport.
Three ways in
Everything a cell needs has to cross the membrane, and there are only three ways across. Two are free. One costs energy — and knowing which is which tells you what a cell will do when it runs out.
Put a red blood cell in pure water and water rushes in until it bursts. Put it in strong salt solution and water leaves until it shrivels. Neither happens in blood plasma, because plasma is kept at the same water potential as the inside of the cell — which is most of what your kidneys are for.
Vérification des connaissances
A plant root cell takes up nitrate ions from soil where nitrate is already scarcer than inside the root. Which process is it using?
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