Cells capture, store, and transport free energy in a chemical form.
Adenosine triphosphate (ATP) (Fig. 1-12) functions as the major
carrier of chemical energy in all cells. ATP carries energy between metabolic
pathways by serving as the shared intermediate that couples endergonic reactions
to exergonic ones. The terminal phosphate group of ATP is transferred to a
variety of acceptor molecules, which are thereby activated for further chemical
transformation. The adenosine diphosphate (ADP) that remains after the phosphate
transfer is recycled to become ATP, at the expense of either chemical energy
(during oxidative phosphorylation) or solar energy in
photosynthetic cells (by the process of photophosphorylation).
ATP is the major connecting link (the shared intermediate) between the catabolic
and anabolic networks of enzyme-catalyzed reactions in the cell (Fig. 1-13).
Metabolism Is Regulated to Achieve Balance and Economy
| Not only can living cells simultaneously synthesize thousands of
different kinds of carbohydrate, fat, protein, and nucleic acid molecules and
their simpler subunits, they can also do so in the precise proportions required
by the cell. For example, when rapid cell growth occurs, the precursors of
proteins and nucleic acids must be made in large quantities, whereas in
nongrowing cells the requirement for these precursors is much reduced. Key
enzymes in each metabolic pathway are regulated so that each type of precursor
molecule is produced in a quantity appropriate to the current requirements of
the cell. Consider the pathway shown in Figure 1-14 (see also Fig. 1-11), which
leads to the synthesis of isoleucine (one of the amino acids, the monomeric
subunits of proteins). If a cell begins to produce more isoleucine than is
needed for protein synthesis, the unused isoleucine accumulates. High
concentrations of isoleucine inhibit the catalytic activity of the first enzyme
in the pathway, immediately slowing the production of the amino acid. Such
negative feedback keeps the production and utilization of each
metabolic intermediate in balance.
Living cells also regulate the synthesis of their own catalysts, the enzymes.
Thus a cell can switch off the synthesis of an enzyme required to make a given
product whenever that product is available ready-made in the environment. These
self adjusting and self regulating .properties allow cells to maintain
themselves in a dynamic steady state, despite fluctuations in the external
environment. Living cells are self regulating chemical engines, adjusted for maximum economy. |
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