chemical gradients

The cell membrane is more permeable to non-polar, hydrophobic molecules than to polar, hydrophilic molecules by virtue of the hydrophobic interior of the amphipathic lipids of the bilayer. As a result, some small non-polar molecules such as H2O and CO2 are able to diffuse directly across the cell membrane down a concentration gradient. This osmotic, chemical gradient limits both the rate of diffusion and the maximum concentration of the diffusing molecule in the cytosol or the extracellular fluid (ESF) in the case of waste products.

Cells also utilize energy to generate concentration gradients across cell membranes by means of protein pumps embedded in the cell membrane. Such concentration gradients are locally discharged when ion channels are opened by a conformational change elicited by specific molecules which bind to membrane bound receptor proteins or by receptor-specific neurotransmitter molecules.