Diffusion across the cell's outer membrane: a. never occurs; all molecules cross the membrane within protein carriers. b. occurs without the input of energy. c. always results with the molecules moving from areas of low concentration to areas of high concentration. d. is also called dynamic equilibrium. e. None of the above.

The correct answer is: b.

Explanation:

Diffusion can be defined as the process by which molecules tend to move from a region where the concentration of the molecules is high to a region where the concentration of the molecules is low. This process occurs spontaneously across the outer membrane of the cell. It does not require the expenditure of energy because the molecules are moving down or along the concentration gradient, that is, from a region where the concentration of molecule is high to the region where the concentration of molecule is low. The process of diffusion occurs through the lipid bilayer of the plasma membrane, like diffusion of oxygen and carbon dioxide gases, and do not require the carrier proteins spanning across the plasma membrane for carrying out the process.