Page 16 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: FYD Revised Pages
Encyclopedia of Physical Science and Technology EN002H-54 May 17, 2001 20:22

112 Bioenergetics

FIGURE 11 Uses of ATP. The diagram shows some of the major processes in cells that are powered by ATP
hydrolysis.

for the active transport of ions across the plasma mem- plasma membrane potential is on the order of −50 mV.
+
brane is provided by the hydrolysis of ATP to ADP and In addition, the pump keeps the intracellular Na concen-
P i . As much as 75% of cellular ATP may be consumed tration nearly 100-fold lower than that in the serum, and
simply to generate and maintain ion gradients. the intracellular concentration of K , about 30-fold higher
+
The electrogenic ion pump in the plasma membrane of than in serum.
+
+
animal cells is the Na /K -ATPase. As shown in Fig. 12, Indirectly, the Na /K -ATPase provides the energy for
+
+
three Na ions are transported out of the cell and two K + the active transport of amino acids and some carbohy-
+
ions are pumped in for each ATP that is hydrolyzed. Since drates into cells. The plasma membrane contains specific
three positively charged ions are exported, but only two proteins that mediate the transport of these molecules in
imported, the Na /K -ATPase is electrogenic. The trans a manner that is obligatorily linked to the cotransport of
+
+

FIGURE 12 Some ion pumps in the plasma membrane. The Na /K -ATPase of animal cells uses the energy of
+
+
ATP hydrolysis to move three Na + ions out of the cells and two K + ions in, which results in the generation of ion
gradients and a membrane potential. Plant, yeast, and fungal cells do not have a Na /K -ATPase, but instead have
+
+
+
aH -ATPase, as the electrogenic pump. The plasma membrane also contains a Ca 2+ -ATPase that pumps Ca 2+ out
of cells to help keep the intracellular Ca 2+ concentration low.