The Sodium Potassium Pump

Primary active transport carriers are often referred to as pumps. Although some of these carriers transport only one molecule or ion at a time, others exchange one molecule or ion for another. The most important of the latter type of carrier is the Na+/K+ pump. This carrier protein, which is also an ATPase enzyme that converts ATP to ADP and Pi, actively extrudes three sodium ions (Na+) from the cell as it transports two potassium ions (K+) into the cell. This transport is energy dependent because Na+ is

Interactions Between Cells and the Extracellular Environment 137

Ca2+

Ca2+

Cytoplasm

Physiology Sodium Potassium Pump

High

Ca2+

Carrier proteins (active transport pump)

Binding site

Extracellular fluid

Plasma membrane

Cytoplasm

High

Ca2+

Carrier proteins (active transport pump)

Binding site

Extracellular fluid

Cytoplasm

Extracellular fluid

■ Figure 6.16 A model of active transport. This model (a mental construct, consistent with the scientific evidence) features a hingelike motion of the integral protein subunits.

more highly concentrated outside the cell and K+ is more concentrated within the cell. Both ions, in other words, are moved against their concentration gradients (fig. 6.17).

Most cells have numerous Na+/K+ pumps that are constantly active. For example, there are about 200 Na+/K+ pumps per red blood cell, about 35,000 per white blood cell, and several million per cell in a part of the tubules within the kidney. This represents an enormous expenditure of energy used to maintain a steep gradient of Na+ and K+ across the cell membrane. This steep gradient serves four functions:

1. The steep Na+ gradient is used to provide energy for the "coupled transport" of other molecules.

2. The activity of the Na+/K+ pumps can be adjusted (primarily by thyroid hormones) to regulate the resting calorie expenditure and basal metabolic rate of the body.

3. The gradients for Na+ and K+ concentrations across the plasma membranes of nerve and muscle cells are used to produce electrochemical impulses needed for functions of the nerve and muscles, including the heart muscle.

4. The active extrusion of Na+ is important for osmotic reasons; if the pumps stop, the increased Na+ concentrations within cells promote the osmotic inflow of water, damaging the cells.

Inside cell

Plasma membrane

Inside cell

Sodium Potassium Pump Heart

Outside cell

Outside cell

■ Figure 6.17 The exchange of intracellular Na+ for K+ by the Na+/K+ pump. The active transport carrier itself is an ATPase that breaks down ATP for energy. Dashed arrows indicate the direction of passive transport (diffusion); solid arrows indicate the direction of active transport.

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