BIOL 3410 Lecture Notes - Lecture 3: Hydrophile, Periplasm, Active Transport
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Multiplying this concentration difference by the permeability coefficient gives the flow of solute in moles per second per square centimeter of membrane. As would be expected, channels transfer solutes at a much greater rate than transporter. Ion channels can exist in either an open or a closed conformation, which is shown here. Channel opening and closing is usually controlled by an external stimulus or by conditions within the cell. Figure 11-4a solutes cross cell membranes by either passive or active transport. Some small nonpolar molecules such as co2 can move passively down their concentration gradient across the lipid bilayer by simple diffusion, without the help of a transport protein. Most solutes, however, require the assistance of a channel or transporter. Passive transport, which allows molecules to move down their concentration gradients, occurs spontaneously; whereas active transport against a concentration gradient requires an input of energy. Figure 11-4b an electrochemical gradient has two components.
