Concept 5.3: Active Transport Moves Solutes against Their Concentration Gradients

In many biological situations, there is a different concentration of a particular ion or small molecule inside compared with outside a cell. In these cases, the concentration imbalance is maintained by a protein in the cell membrane that moves the substance against its concentration gradient. This is called active transport, and because it is acting “against the normal flow,” it requires the expenditure of energy. Often the energy source is the nucleotide adenosine triphosphate (ATP). In eukaryotes, ATP is produced in the mitochondria and plastids, and it has chemical energy stored in its terminal phosphate bond. This energy is released when ATP is converted to adenosine diphosphate (ADP) in a hydrolysis reaction that breaks the bond between the terminal phosphate and the rest of the molecule.

The differences between diffusion and active transport are summarized in TABLE 5.1. In many cases of simple and facilitated diffusion, ions or molecules can move down their concentration gradients in either direction across the cell membrane. In contrast, active transport is directional, and moves a substance either into or out of a cell or organelle, depending on the transport protein’s function. As in facilitated diffusion, there is usually a specific carrier protein for each substance that is transported.

Different energy sources distinguish different active transport systems

There are two basic types of active transport:

• Primary active transport involves the direct hydrolysis of ATP, which provides the energy required for transport.
• Secondary active transport does not use ATP directly. Instead, its energy is supplied by an ion concentration gradient or an electrical gradient, established by primary active transport. This transport system uses the energy of ATP indirectly to set up the gradient.

In primary active transport, energy released by the hydrolysis of ATP drives the movement of specific ions against their concentration gradients. For example, the concentration of potassium ions (K⁺) inside a cell is often much higher than the concentration in the fluid bathing the cell. However, the concentration of sodium ions (Na⁺) is often much higher outside the cell. A protein in the cell membrane pumps Na⁺ out of the cell and K⁺ into the cell against these concentration gradients, ensuring that the gradients are maintained. This sodium–potassium (Na⁺−K⁺) pump is an integral membrane glycoprotein that is found in all animal cells. It breaks down a molecule of ATP to ADP and a free phosphate ion (P_i) and uses the released energy to bring two K⁺ ions into the cell, and export three Na⁺ ions (FIGURE 5.7).

In secondary active transport, the movement of a substance against its concentration gradient is accomplished using energy “regained” by letting ions move across the membrane down their concentration gradients. For example, once the Na⁺−K⁺ pump establishes a concentration gradient of sodium ions, the passive diffusion of some Na⁺ back into a cell can provide energy for the secondary active transport of glucose into the cell. This occurs when glucose is absorbed into the bloodstream from the digestive tract. Secondary active transport is usually accomplished by a single protein that moves both the ion and the actively transported molecule across the membrane. In some cases, the ion and the transported molecule move in opposite directions, whereas in others they move in the same direction (as for glucose and Na⁺ in the digestive tract). Secondary active transport aids in the uptake of amino acids and sugars, which are essential raw materials for cell maintenance and growth.

We have examined a number of passive and active ways by which ions and small molecules can enter and leave cells. But what about large molecules such as proteins? Many proteins are so large that they diffuse very slowly, and their bulk makes it difficult for them to pass through the phospholipid bilayer. It takes a completely different mechanism to move intact large molecules across membranes.