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.

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You will find more details about how ATP functions as an energy shuttle in cells in Concept 6.1

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.

Membrane Transport Mechanisms

Different energy sources distinguish different active transport systems

There are two basic types of active transport:

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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 (Pi) and uses the released energy to bring two K+ ions into the cell, and export three Na+ ions (FIGURE 5.7).

Figure 5.7: Primary Active Transport: The Sodium-Potassium Pump In active transport, energy is used to move a solute against its concentration gradient. Here, energy from ATP is used to move Na+ and K+ against their concentration gradients.

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.

Go to ANIMATED TUTORIAL 5.3 Active Transport

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CHECKpoint CONCEPT 5.3

  • Why is energy required for active transport?
  • The drug ouabain inhibits the activity of the Na+−K+ pump. A nerve cell is incubated in ouabain. Make a table in which you predict what would happen to the concentrations of Na+ and K+ inside and outside the cell, as a result of the action of ouabain.
  • How would you use experiments to distinguish between the following two ways for glucose to enter a cell: (1) facilitated diffusion via a carrier protein and (2) secondary active transport?

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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.