Feedback inhibition operates when a substance triggers a cellular change that shuts down production of that substance. Hormones are the signaling molecules in enzyme control. B. Gases, nonpolar molecules, and water can move through the lipid bilayer by Diffusion . C. Glucose and other large polar molecules must cross the membrane through transport proteins, in some cases via diffusion and in other cases by Active Transport . Gradients A. A concentration gradient is a difference in the number of molecules or ions of a given substance in two adjoining regions.
Molecules constantly collide and tend to move according to existing concentration gradients.
Gradients in temperature, electric charge, and pressure, can influence movements. B. Passive Transport 1. In passive transport, solutes pass through the cell membrane with assistance from transport proteins in accordance with the concentration gradient. Transport proteins change shape to move substances into and out of the cell. C. Active Transport 1.