The cell membrane is selectively permeable due to its structure. Thus, the internal environment of the cell is distinct from the external environment of the cell. One biologist hypothesizes that small nonpolar molecules readily pass through the membrane. Another biologist alternatively hypothesizes that these types of molecules require channel and transport proteins that are embedded in the membrane in order to move across the membrane.

Which of the following data would best refute this alternative hypothesis?

Ethanol is found in the cytosol of cells when they are briefly exposed to a ten percent ethanol solution.

A

Cells become oxygen deficient when membrane protein activity is blocked.

B

CO2 and N2 movement in and out of cells is unaffected when membrane protein activity is blocked.

C

Sodium ions cannot move across the cell membrane when membrane protein activity is blocked.

The selective permeability of the biological membranes, like a cell membrane, to small molecules, allows the cell to control and maintain its internal composition.Free diffusion across the lipid bilayer can happen only for small uncharged or nonpolar molecules, such as CO2 and N2. Thus, they can easily cross cell membranes, without the assistance of the transmembrane proteins. Small uncharged polar molecules, such as H2O, also can diffuse through membranes without any help from channel proteins or carrier proteins, which are the two types of transmembrane proteins.

In the case of molecules like glucose, passage across the membrane occurs via the action of specific transmembrane proteins, which act as transporters. These transmembrane proteins allow polar or charged molecules to cross the membrane through a protein pore without interacting with the hydrophobic fatty acid chains of membrane phospholipids.