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Transport can be free, meaning unaided diffusion, or mediated, meaning that something (usually a protein or proteins) helped it across. How can you tell the difference? Look at the relationship between the rate of movement insideand the concentration outside.

Rate

Rate

[outside]

[outside]

Which is which? Free movement is not saturable; the rate increases proportionally with the outside concentration, because there is a very large region (the whole cell membrane) over which it can cross. Mediated transport is saturable, because there are a finitenumber of protein "helpers" in the membrane. B. Active vs Passive 1. Active transport requires ATP Transport can also be considered passiveor active. Passive transport always moves downa concentration gradient; Active transport can move up a concentration gradient. This is like moving water up a hill, what does that tell you? -that you need to add energy. Facilitated transport can be active or passive; free diffusion can only be passive. Q. Where does energy come from in a cell? ATP hydrolysis