The plasma membrane functions to isolate the inside of the cell from its environment, but it is imperative that such isolation not be complete. A large number of molecules must constantly transit between the inside and outside of the cell, most frequently one-at-a-time, but also in large packages.
The plasma membrane functions as a selectively permiable membrane with exquisite selectivity regarding which molecules cross and which direction they are allowed to travel. In the simplist case, nutrients must be allowed to enter the cell and waste products to exit. Another critical task is to maintain an ionic composition inside the cell that is very different from that outside the cell.
All transport across cell membranes takes place by one of two fundamental processes:
- Passive tranport is driven by the kinetic energy of the molecules being transported or by membrane transporters by facilitate crossing.
- Active transport depends upon the expenditure of cellular energy in the form of ATP hydrolysis.
Another way of looking at the difference is to determine whether the transported solute is move "up" or "down" its concentration gradient - movement against an electrochemical gradient necessitates expenditure of energy and is therefore accomplished by active transport. For each of the two basic processes, several distinct types of transport can be identified.
A molecule or ion that crosses the membrane by moving down a concentration or electrochemical gradient and without expenditure of metabolic energy is said to be transported passively. Another name for this process is diffusion. All molecules and ions are in constant motion and it is the energy of motion - kinetic energy - that drives passive transport. Transport of uncharged species across a membrane is dictated by differences in concentration of that species across the membrane - that is, by the prevailing concentration gradient. For ions and charged molecules, the electrical potential across the membrane also becomes critically important. Together, gradients in concentration and electric potential across the cell membrane constitute the electrochemical gradient that governs passive transport mechanisms.