A raisin swells in water. A lettuce becomes crisp in a bowl of water. A drop of ink spreads without stirring. In this course we learn the two ideas behind all of this, diffusion and osmosis, and we test them with a potato.
3 lessons, about 1 h 20 min3 simulations8 problems6 practice questionsPython in every lesson
Created by StemPy LabDiffusion is the net movement of particles from a place where there are many of them to a place where there are fewer. It needs no energy from the cell. It is faster when it is warmer and when the difference is large, and it works well only over short distances, because the time grows with the square of the distance.
Osmosis is the movement of water through a selectively permeable membrane, from the side with less dissolved substance to the side with more. It decides whether a cell swells, stays the same or shrinks. A plant cell becomes turgid in a dilute solution, held by its wall, and shows plasmolysis in a strong one.
Diffusion and osmosis move substances down a difference in concentration, and need no energy. Some substances must move the other way, from fewer to more. A cell does this by active transport: carrier proteins in the membrane act as pumps, run by energy from respiration. Root cells take up minerals this way, and without oxygen the uptake stops.
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