Concept of Movement of Substances Across a Plasma Membrane

Passive and active movement

Substances cross the plasma membrane in two broad ways. Passive transport needs no energy from the cell and moves particles down a concentration gradient (from high to low concentration). Active transport needs energy (ATP) and moves particles against the gradient (from low to high concentration).

Diffusion and osmosis

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration until they are evenly spread. Small, non-polar molecules such as oxygen diffuse straight through the bilayer. In facilitated diffusion, larger or charged particles diffuse through channel or carrier proteins, still down the gradient and still without energy.

Osmosis is a special case of diffusion. It is the net movement of water molecules through a partially permeable membrane from a dilute solution (high water potential) to a more concentrated solution (low water potential).

Active transport

Active transport uses carrier proteins that act like pumps. They bind a particle, and energy from ATP changes their shape to push the particle across against the gradient. This is how cells absorb useful ions even when the concentration outside is already low. Bulk transport of very large materials happens by endocytosis (the membrane folds inward to take material in) and exocytosis (a vesicle fuses with the membrane to release material out). Both bulk methods also use energy from ATP. Because active transport depends on energy, cells that carry out a lot of it, such as those lining the small intestine and the kidney tubules, contain many mitochondria to supply the ATP needed.