The big idea: Water crosses membranes on its own, towards the side with more dissolved solute. This is osmosis.
The points to remember
- Osmosis is the net movement of water across a partially permeable membrane, towards the higher solute concentration.
- Water molecules move at random, crossing the membrane in both directions.
- The membrane is impermeable to the solutes, so they cannot cross to even out the difference.
- Fewer water molecules are free to cross from the side with more solute, so more water crosses towards it than away from it.
- Hypotonic: lower solute concentration than the cell, water enters. Hypertonic: higher, water leaves. Isotonic: equal, no net movement.
Get the direction right: Water moves towards the more concentrated solution, from hypotonic to hypertonic. Name the solute, not 'the concentration', when you explain it.
Real example: red blood cells in pure water: Red blood cells placed in pure water take in water by osmosis, swell and burst. In blood plasma, which is isotonic, they keep their shape.
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The big idea: Water is polar, so it crosses the bilayer only slowly. Aquaporins are channel proteins that let water through much faster.
The points to remember
- An aquaporin is an integral channel protein with a narrow pore through the membrane.
- Water molecules pass through the pore in single file; ions are held back.
- Aquaporins increase the permeability of the membrane to water, so osmosis is faster.
- It is still osmosis: passive, no ATP; water follows the difference in solute concentration.
- Each aquaporin carries water quite slowly, so cells that move a lot of water have many: kidney collecting ducts, root cells, gland cells that secrete fluid.
Remember it as: Aquaporin: a water-only door, one molecule at a time.
Real example: the discovery: In the early 1990s, Peter Agre's team put the gene message (mRNA) for a membrane protein into frog egg cells, which normally let little water in. In water the eggs swelled and burst: the protein was a water channel. He shared the 2003 Nobel Prize in Chemistry.
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The big idea: Data questions on osmosis show a change in volume or mass over time. Read the direction first, then compare the lines, then explain.
How to answer
- Direction: the volume rises, so the net movement of water is into the cells.
- Compare with a comparative word: the mRNA eggs swell faster and more, and burst; the controls hardly change.
- Explain: the solution is hypotonic, so water moves in by osmosis towards the higher solute concentration inside.
- Role of aquaporins: they increase the membrane's permeability to water, so osmosis is faster.
Numbers alone do not compare: Write 'larger' or 'faster', not just the two values. And say 'more permeable' or 'faster', not only 'aquaporins let water in'.
How it is asked: Paper 1B gives a volume or mass graph and asks for the direction and the role of aquaporins. Paper 1A asks what aquaporins are.
IB-style questionExplain[2 marks]
Plant root cells treated with a chemical that blocks aquaporins take up water more slowly. Explain the role of aquaporins in the movement of water across cell membranes.
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