CAIE A Level Biology 9700 · Cell membranes and transport
In a marine alga, iodide ions are taken up by the cells from seawater where the concentration of iodide is lower than inside the cell. Glucose molecules enter the same cells by facilitated diffusion, moving down their concentration gradient.
Explain how active transport of iodide differs from facilitated diffusion of glucose in terms of energy use and the role of membrane proteins.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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