Figure 612

The primary active transport of sodium and potassium ions in opposite directions by the Na,K-ATPase in plasma membranes is responsible for the low sodium and high potassium intracellular concentrations. For each ATP hydrolyzed, three sodium ions are moved out of a cell, and two potassium ions are moved in.

relative to their respective extracellular concentrations (Figure 6-12). For each molecule of ATP that is hydrolyzed, this transporter moves three sodium ions out of a cell and two potassium ions in. This results in the net transfer of positive charge to the outside of the cell, and thus this transport process is not electrically neutral, a point to which we will return in Chapter 8.

Ca-ATPase is found in the plasma membrane and several organelle membranes, including the membranes of the endoplasmic reticulum. In the plasma membrane, the direction of active calcium transport is from cytosol to extracellular fluid. In organelle membranes, it is from cytosol into the organelle lumen. Thus active transport of calcium out of the cytosol, via Ca-ATPase, is one reason that the cytosol of most cells has a very low calcium concentration, about 10~7 mol/L compared with an extracellular calcium concentration of l0~3 mol/L, 10,000 times greater (a second reason will be given below).

H-ATPase is in the plasma membrane and several organelle membranes, including the inner mitochon-drial and lysosomal membranes. In the plasma membrane, the H-ATPase moves hydrogen ions out of cells.

H,K-ATPase is in the plasma membranes of the acid-secreting cells in the stomach and kidneys, where it pumps one hydrogen ion out of the cell and moves one potassium in for each molecule of ATP hydrolyzed. (This pump is thus electrically neutral in contrast to the other three ATPases.)

Secondary Active Transport Secondary active transport is distinguished from primary active transport by its use of an ion concentration gradient across a membrane as the energy source. The flow of ions from a higher concentration (higher energy state) to a lower concentration (lower energy state) provides energy for the uphill movement of the actively transported solute.

In addition to having a binding site for the actively transported solute, the transport protein in a secondary active-transport system also has a binding site for an ion (Figure 6-13). This ion is usually sodium, but

Intracellular fluid

Transporter protein

Intracellular fluid

Transporter protein

High I Solute to be extracellular |Na+ ' actively sodium — transported concentration

High I Solute to be extracellular |Na+ ' actively sodium — transported concentration

Low intracellular sodium concentration

Extracellular fluid

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