HardBiologyHigh School (11–12)Cell Biology › Cell Structure and OrganellesCell Biology › Transport Across the Cell MembraneNumerical
A nerve cell has 6000 sodium-potassium pumps…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A nerve cell has sodium-potassium pumps. Each one turns times a second. (a) How many ATP molecules do the pumps use in one minute? (b) How many sodium ions do they carry out, and how many potassium ions in? (c) Why is this active transport and not diffusion? (d) What is the net number of positive charges carried out?
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. ATP. Turns , so ATP.
Step 2. The ions. Sodium out . Potassium in .
Step 3. Why active. The ions move against their concentration gradients, and the pump spends ATP. Diffusion moves substances down a gradient with no energy.
Step 4. Net charge. positive charges out, one for each turn.
Answer: (a) ATP; (b) sodium ions out, potassium ions in; (c) they move against the gradient using ATP; (d) .
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