Active transport is the pumping of solutes across a biological membrane, against their concentration or electrochemical gradient. The ability of cells to maintain small solutes within the cytoplasm at concentrations higher than that of the surrounding fluid is an essential factor in cell survival. Many animal cells, for example, maintain concentrations of sodium and potassium that are significantly different than that of their surroundings. This process enables cells not only to maintain viable solute levels, but also to pump ions across an electrochemical gradient. This process creates a voltage across the membrane that can be tapped to power cellular work.
To understand active transport, one must first understand passive transport. According to the second law of thermodynamics, without additional energy input, particles will always move from a state of order to a state of disorder. In the case of cellular traffic, this means that small solutes will naturally move from more orderly regions of high concentration to the less orderly regions of low concentration. This is known as diffusion down a concentration gradient. Passive transport is the natural movement of solutes across a membrane down the concentration gradient.
During active transport, the cell must work against the natural diffusion of solutes. To do this, specialized transport proteins are embedded in the cellular membrane. Powered by adenosine triphosphate (ATP,) transport proteins selectively move specific solutes into or out of the cell. A common way ATP powers this work is to donate its terminal phosphate group to the transport protein, triggering a change of shape in the protein molecule. The conformational change causes the protein to move solutes that have bound to its extracellular surface to the cells interior and release them.
An example of this type of protein is the sodium-potassium pump. Most animal cells hold a higher concentration of potassium, and a lower concentration of sodium, than what is found in the extracellular environment. Since sodium ions carry a positive charge and potassium ions carry a negative charge, this imbalance represents not only a concentration gradient, but also an electrochemical gradient. Sodium-potassium pumps move three sodium ions out of the cell for every two potassium ions they bring into it, resulting in a net negative charge on the cell as a whole. The difference of charges on each side of the cellular membrane creates a voltage — the membrane potential — that allows the cell to act as a battery, and power cellular work.
As mentioned, most active transport is powered by the molecule ATP. Sometimes, however, a solute can move into a cell by taking advantage of the diffusion of other substances. As diffusing substances move into the cell along a gradient that has previously been created by active transport, other solutes are able to bind to them and cross the membrane simultaneously. Known as secondary transport or co-transport, this is the form of membrane traffic that is responsible for moving sucrose into plant cells, as well as moving calcium and glucose into animal cells.