MediumBiologyHigh School (11–12)Cell Biology › Cell Structure and OrganellesCell Biology › Transport Across the Cell MembraneConceptual
Riya sprinkles salt on cucumber slices for a salad…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Riya sprinkles salt on cucumber slices for a salad. Ten minutes later, the plate holds a pool of water and the slices are limp. (a) Where did the water come from? (b) Name the process. (c) Did the cells spend ATP on it? (d) Which way would water move if the slices were dropped into pure water instead?
Need a reminder? The key ideaShow
The cell membrane is a lipid bilayer: polar heads outwards, hydrophobic tails inwards, mostly phosphoglycerides, with cholesterol. It also holds proteins (integral, buried; peripheral, on the surface) and carbohydrates. The human red blood cell membrane is about 52 per cent protein and 40 per cent lipid. Fluid mosaic model (Singer and Nicolson, 1972): a quasi-fluid lipid in which proteins move sideways. Passive transport: diffusion and osmosis, no energy. Active transport: against the gradient, using ATP; the Na+/K+ pump moves 3 Na+ out and 2 K+ in for each ATP.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The water. The salt makes the fluid outside the cells far more concentrated than the inside. Water leaves the cells and collects on the plate.
Step 2. The process. The movement of water across the membrane by diffusion is osmosis.
Step 3. Energy. No. Osmosis is passive transport; it uses no ATP.
Step 4. Pure water. Then the inside of the cells is more concentrated, so water moves into the cells, and the slices become firm.
Answer: (a) from inside the cucumber cells; (b) osmosis; (c) no, it is passive; (d) into the cells.
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