Cell membranes are made of two “sheets” of fatty molecules known as a lipid bilayer, which contains many proteins on the surface of the bilayer and inserted into it. These proteins are mobile in the...

Cell membranes are made of two “sheets” of fatty molecules known as a lipid bilayer, which contains many proteins on the surface of the bilayer and inserted into it. These proteins are mobile in the bilayer, and some function as carriers to move other molecules into or out of the cell. The reason the proteins can move has to do with the “fluidity” of the bilayer, which arises because the lipids are strongly attracted to one another but are not covalently bound. Different kinds of lipids attract each other differently, so the fluidity of the membranes can change as the lipid composition changes. Zheng and coworkers* hypothesized that changes in membrane fluidity would change carrier-mediated transport of molecules so that as the membrane became less fluid, transport rate would slow. To investigate this hypothesis, they took advantage of the fact that increasing cholesterol content in a membrane decreases its fluidity, and they measured the accumulation of the amino acid, leucine, over time in cells with either no cholesterol or one of three amounts of cholesterol (the data are in Table D-9, Appendix D). Is there evidence that increasing the cholesterol content of the cell membrane slows the rate of leucine transport?

Table D-9