Final answer:
When substances cross cell membranes by simple diffusion, C. they move from a higher to lower concentration.
Step-by-step explanation:
The movement of substances across cell membranes involves processes such as passive diffusion, facilitated diffusion, active transport, and endocytosis/exocytosis. These mechanisms regulate the entry and exit of ions, molecules, and macromolecules, maintaining cellular homeostasis and supporting various cellular functions.
When substances cross cell membranes by simple diffusion, they move from a higher to lower concentration. Simple diffusion is a passive process of transport where a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. This process does not require any help from channel proteins or squeezing between phospholipid molecules.