Active transport is the movement of molecules across cell membranes that requires energy expenditure by the cell. This is different from passive transport where molecules move without the addition of energy. This energy is usually provided in the form of adenosine triphosphate (ATP). Cells rely on active transport mechanisms when molecules are moved against their concentration gradient, while passive transport occurs along the concentration gradient. This means that active transport can move substances from areas of low concentration to areas of high concentration.
The active transport mechanism commonly involves the movement of substance that are unable to freely cross the cell membrane but are important for cell function. These molecules typically have a small mass, such as ions and amino acids. However, other mechanisms can also involve the transport of larger molecules, such as glucose.
In some kinds of active transport, specific carrier proteins undergo phosphorylation by ATP hydrolysis. When the carrier protein binds its target, the ATP transfers a phosphate to the carrier protein, changing the shape of the carrier protein. The changed shape gives the target molecule access to the other side of the membrane, and the target molecule is then released. This is the general mechanism used to transport many amino acids and ions across the membrane. In cotransport, the movement of one substance with its gradient releases energy used to move another substance against its gradient in a coupled reaction. This process is used to move many larger molecules, such as sugars.
Concentration gradients influence the diffusion of substances based on a difference in concentration inside and outside the cell membrane. During active transport, ions cross the cell membrane, creating an electrical gradient. Typically the inside of the cell is negatively charged when compared to the extracellular fluid. Because of this difference in charge, a voltage, or membrane potential, exists across the cell membrane. Voltage refers to the electrical potential energy due to a separation of opposite charges.In addition to the electrical gradient, an ion gradient is established during active transport. There are fewer positive ions, or positively charged molecules, inside the cell compared to the extracellular space. Thus, the membrane potential favors the movement of positively charged ions into the cell. Negatively charged ions move out of the cell. Ion diffusion across the plasma membrane is driven by both concentration and charge differences on each side of the membrane, called the electrochemical gradient. This gradient determines the direction in which an ion moves during active transport across a membrane. Three factors affect the electrochemical potential that is generated across a membrane when an ion is moving: (1) an ion concentration difference on either side of the cell membrane, (2) the ion charge, and (3) the membrane potential on both sides of the cell membrane.
Understanding the Sodium-Potassium Pump
Adenosine triphosphate (ATP), the biological unit of energy, which consists of an adenosine (an adenine group and a ribose sugar) and three phosphate groups, drives the function of specialized proteins involved in active transport, called a pump. These active transport pumps come in different varieties. Cells may have proton pumps, calcium pumps, or sodium-potassium pumps. Most transport pumps move positively charged ions such as Ca2+, Na+, and K+ across the cell membrane.
Because sodium (Na+) and potassium (K+) ions are constantly moving in and out of a cell, the cell relies on the use of a sodium-potassium pump (Na+/K+ pump) to maintain the ideal concentration of sodium and potassium in living cells. This is important for control of cell volume, pH, and nutrient balance of the cell. The pump also helps generate voltages across the cell membrane. These membrane voltages are crucial for the function of some cells, such as nerve cells.
The transport protein involved in the Na+/K+ pump has binding sites for three Na+ ions and two K+ ions. The protein also has a binding site for ATP. Although the function of the pump is cyclical, the discussion will begin with the Na+/K+ pump open to the inside of the cell. In this shape, it has a high affinity for Na+ ions, so three will bind.
The binding of sodium ions to the carrier protein triggers ATP hydrolysis. Hydrolysis involves the breakdown of ATP to adenosine diphosphate (ADP) and inorganic phosphate (a salt of phosphoric acid). The phosphate stays bound to the protein pump, and the energy released by hydrolysis is used to power the pump.
Energy from hydrolysis causes the pump to physically change its shape, opening toward the outside of the cell rather than inside the cell. At this point the pump has a low affinity for Na+ ions, so the three bound Na+ ions are released outside the cell.
When open to the extracellular space, the pump has a high affinity for K+ ions , so two K+ ions will bind to the pump. Binding changes the shape of the protein again and triggers the removal of the phosphate group that is attached to the pump.The pump has changed back to its original form and opens toward the cell's interior. The pump no longer has a strong affinity for the K+ ions, causing the two attached K+ ions to dislodge from the pump. These K+ ions are released inside the cell. The cycle will then continue when three Na+ ions bind again to the protein.