The cell membrane acts as a gatekeeper, allowing some substances to enter the cell, but excluding others. In other words, the membrane is selectively permeable. This selective permeability is an essential feature of the membranes of all living cells, because it provides them with the power to control their internal environments. The cell may use channels, carriers, or pumps to move substances from one side of a membrane to the other.
In this animation, we examine a type of transport across a membrane that requires no energy for a cell to perform. It is called passive transport. In passive transport, a substance moves across a membrane from a region of higher concentration to a region of lower concentration. Active transport (not shown here) is the opposite: it requires energy, moving a substance from a lower concentration to a higher concentration.
In general, only small, nonpolar molecules can pass unaided through a phospholipid bilayer. Molecules move randomly, and can move in both directions across the membrane. However, the net movement of molecules will be from regions of high concentration on one side of the membrane to regions of low concentration on the other side. At equilibrium, equal numbers cross the membrane in both directions.
Some membrane proteins form channels that allow polar or charged substances to cross the membrane. In this example of facilitated diffusion, a channel allows specific ions to pass. These ion channels are gated, and open and close by a variety of mechanisms. Here, a stimulus molecule causes the channel to open.
In facilitated diffusion, like in simple diffusion, substances can move in either direction across the membrane, but the net flow will be from a region of higher concentration on one side of the membrane to a region of lower concentration on the other side. Thus, the process is still one of diffusion, but is facilitated by a molecule that creates a passageway in the membrane. This movement down a concentration gradient requires no input energy from the cell.
Some membrane proteins act as carriers that bind a molecule on one side of the membrane, change shape, and then deposit the molecule on the other side of the membrane. In this example of facilitated diffusion, a glucose carrier protein moves glucose down its concentration gradient. This process of diffusion requires no input energy to move the molecules across the membrane.
A cell continually exchanges molecules and ions with its environment. When a cell takes in substances that are in greater concentration outside the cell, the cell does not need to expend energy. These transport processes are called passive transport, and they occur by either simple diffusion (the substance slips through the lipid bilayer on its own) or facilitated diffusion (a channel or carrier provides a passageway). Substances spontaneously flow down their concentration gradients.
In some cases (not shown here), the cell brings in substances that are already in greater concentration inside the cell than outside. This process is called active transport and requires that the cell expend energy.