When ATP is hydrolyzed in a test tube, the release of free energy
merely heats the surrounding water. In an organism, this
same generation of heat can sometimes be beneficial. For instance,
the process of shivering uses ATP hydrolysis during
muscle contraction to generate heat and warm the body. In
most cases in the cell, however, the generation of heat alone
would be an inefficient (and potentially dangerous) use of a
valuable energy resource. Instead, the cell's proteins harness
the energy released during ATP hydrolysis in several ways to
perform the three types ofcellular work-chemical, transport,
and mechanical.
For example, with the help of specific enzymes, the cell is
able to use the energy released by ATP hydrolysis directly to
drive chemical reactions that, by themselves, are endergonic.
If the 6.G of an endergonic reaction is less than the amount of
energy released by ATP hydrolysis, then the two reactions can
be coupled so that, overall, the coupled reactions are exergonic
(Figure 8.10). This usually involves the transfer of a phosphate
group from ATP to some other molecule, such as the reactant.
The recipient of the phosphate group is then said to be
phosphorylated. The key to coupling exergonic and endergonic
reactions is the formation of this phosphorylated intermediate,
which is more reactive (less stable) than the original
unphosphorylated molecule.
Transport and mechanical work in the cell are also nearly
always powered by the hydrolysis of ATP. In these cases, ATP
hydrolysis leads to a change in a protein's shape and often its
ability to bind another molecule. Sometimes this occurs via a
phosphorylated intermediate, as seen for the transport protein
in Figure 8.11a.ln most instances ofmechanical work involving
motor proteins "walking" along cytoskeletal elements
(Figure 8.11 b), a cycle occurs in which ATP is first bound
noncovalently to the motor protein. Next, ATP is hydrolyzed,
releasing ADP and ®;; another ATP molecule can then bind.
At each stage, the motor protein changes its shape and ability
merely heats the surrounding water. In an organism, this
same generation of heat can sometimes be beneficial. For instance,
the process of shivering uses ATP hydrolysis during
muscle contraction to generate heat and warm the body. In
most cases in the cell, however, the generation of heat alone
would be an inefficient (and potentially dangerous) use of a
valuable energy resource. Instead, the cell's proteins harness
the energy released during ATP hydrolysis in several ways to
perform the three types ofcellular work-chemical, transport,
and mechanical.
For example, with the help of specific enzymes, the cell is
able to use the energy released by ATP hydrolysis directly to
drive chemical reactions that, by themselves, are endergonic.
If the 6.G of an endergonic reaction is less than the amount of
energy released by ATP hydrolysis, then the two reactions can
be coupled so that, overall, the coupled reactions are exergonic
(Figure 8.10). This usually involves the transfer of a phosphate
group from ATP to some other molecule, such as the reactant.
The recipient of the phosphate group is then said to be
phosphorylated. The key to coupling exergonic and endergonic
reactions is the formation of this phosphorylated intermediate,
which is more reactive (less stable) than the original
unphosphorylated molecule.
Transport and mechanical work in the cell are also nearly
always powered by the hydrolysis of ATP. In these cases, ATP
hydrolysis leads to a change in a protein's shape and often its
ability to bind another molecule. Sometimes this occurs via a
phosphorylated intermediate, as seen for the transport protein
in Figure 8.11a.ln most instances ofmechanical work involving
motor proteins "walking" along cytoskeletal elements
(Figure 8.11 b), a cycle occurs in which ATP is first bound
noncovalently to the motor protein. Next, ATP is hydrolyzed,
releasing ADP and ®;; another ATP molecule can then bind.
At each stage, the motor protein changes its shape and ability
Cambpbell,Neil A..2008.BIOLOGY Eight edition. Pearson Education,
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