Figure 5.1. Lipid bilayer membrane with channel proteins. Sodium and chlo-
ride ions distributed in water cannot pass through the membrane when the
channel proteins are closed (left); when they are open, these channel proteins
allow chloride ions, but not sodium ions, to cross the membrane (right).The energy to power the Na/K pump comes from adenosine
triphosphate (ATP). An ATP molecule consists of an adenine portion
(right)—the same molecular structure that is part of DNA and RNA
—together with a ribose sugar portion (the carbon-oxygen ring
structure, middle) and three phosphate groups linked by covalent
phosphorus-oxygen bonds (left). The phosphorus-oxygen bonds store
a substantial amount of energy, and when the bonds are broken by
specific enzymatic actions within a cell, that energy becomes avail-
able to other cellular processes. Life has capitalized on this property of
the phosphate bond to store energy—from bacteria to plants to fungi
to human brains, phosphate bonds provide a primary currency for
moving energy around inside cells and powering all sorts of cellular
processes. ATP is a widely used representative of this phosphate-bond
energy currency.