Active & Passive Transport

Passive transport is when substances cross the plasma membrane without any energy input from the cell. The substances diffuse from a higher concentration to a lower concentration, down the concentration gradient. With diffusion, molecules move through the membrane with their own kinetic energy, so the speed of diffusion is dependent on the molecule's size and the temperature. Because of the lipid tails of the phospholipids, only small, fat soluble particles can pass through the plasma membrane. Larger molecules and lipid-insoluble molecules pass through channel proteins and transport proteins embedded in the membrane.

Active transport is when substances cross the plasma membrane with energy from the cell in the the form of ATP. Substances cross over the plasma membrane with help from transport proteins. Primary active transport is when ATP is used to power the protein and it changes shape to release the molecule. Secondary transport is when the pump stores energy as it moves a molecule against the concentration gradient then uses the energy when bringing molecules the other way. Vesicular transport is a form of active transport in exocytosis and endocytosis when a membrane sac contains the substance to be expelled, then connects with the plasma membrane and releases it. The fluidity of the lipid bilayer allows the plasma membrane to move in order to let molecules out.