AP Biology · Unit 2 · Topic 2.5
Moving with a concentration gradient is free. Moving against it costs energy. Run both and watch the ATP counter.
Learning objectives 2.5.A Describe the mechanisms that organisms use to maintain solute and water balance. 2.5.B Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.
Narrated walkthrough of the “Move a solute” tab. Captions are on by default.
Two compartments, a membrane between them, and a concentration difference you can set. Drag a particle across the membrane. The tool will tell you whether the move happens, and whether it had to be paid for.
Try it: set the starting difference to 0 so both sides are equal, choose passive, and press Let it run. Nothing happens, because there is no gradient to run down. Then choose active and press it again. A gradient appears out of nothing, and the ATP counter climbs at exactly the same rate.
A macromolecule will not fit through a channel or a transport protein, however much ATP you spend. The cell has to wrap it in membrane instead. Pick a direction and press play.
Notice where the vesicle membrane comes from and where it goes. In endocytosis the new vesicle is made from the plasma membrane; in exocytosis the vesicle membrane becomes part of the plasma membrane. This is the endomembrane system from Topic 2.1 doing its job, and it is why the plasma membrane counts as part of that system.
Read each situation and choose the process it describes.
Answer these in your own words before opening the answers.
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