An amalgamation of the term biological
, is the branch of biology and biochemistry
that is concerned with how organisms extract energy from
their environment and with how energy is used to fuel the
myriad of life’s endergonic processes. Organisms may be
usefully divided into two broad groups with respect to
how they satisfy their need for energy. Autotrophic organisms
convert energy from nonorganic sources such as light
or from the oxidation of inorganic molecules to chemical
energy. As heterotrophic organisms, animals must ingest
and break down complex organic molecules to provide the
energy for life.
Interconversions of forms of energy are commonplace
in the biological world. In photosynthesis, the electromagnetic
energy of light is converted to chemical energy,
largely in the form of carbohydrates, with high overall
efficiency. The energy of light is used to drive oxidation–
reduction reactions that could not take place in the dark.
Light energy also powers the generation of a proton
electrochemical potential across the green photosynthetic
membrane. Thus, electricalwork is an integral part of photosynthesis.
Chemical energy is used in all organisms to
drive the synthesis of large and small molecules, motility
at the microscoPi
c and macroscoPi
c levels, the generation
of electrochemical potentials of ions across cellular
membranes, and even light emission as in fireflies.
Given the diversity in the forms of life, it might be expected
that organisms have evolved many mechanisms to
deal with their need for energy. To some extent this expectation
is the case, especially for organisms that live in
extreme environments. However, the similarities among
organisms in their bioenergetic mechanisms are as, or even
more, striking than the differences. For example, the sugar
glucose is catabolized (broken down) by a pathway that
is the same in the enteric bacterium Escherichia coli
it is in higher organisms. All organisms use adenosine
5´-triphosphate (ATP) as a central intermediate in energy
metabolism. ATP acts in a way as a currency of free energy.
The synthesis ofATP from adenosine 5´-diphosphate
(ADP) and inorganic phosphate (Pi
) is a strongly endergonic
reaction that is coupled to exergonic reactions
such as the breakdown of glucose. ATP hydrolysis in
turn powers many of life’s processes. The central role of
ATP in bioenergetics is illustrated in Fig. 1. Partial structures
of several compounds that play important roles in
metabolism are shown in Fig. 2.
eral compounds that play important roles in Fig.2
In this article, the elements of energy metabolism will
be discussed with emphasis on howorganisms satisfy their
energetic requirements and on how ATP hydrolysis drives
otherwise unfavorable reactions.
|Figure 1 Central role of adenosine 5´-triphosphate (ATP) in metabolism. Catabolic (degradative) metabolism
is exergonic and provides the energy needed for the synthesis of ATP from adenosine 5´-diphosphate (ADP)
and inorganic phosphate (Pi). The exergonic hydrolysis of ATP in turn powers the endergonic processes of
|Figure 2 Some important reactions in metabolism. Shown are
the phosphorylation of ADP to ATP, NAD+, NADH, FAD, FADH2 acetate, CoA, and acetyl CoA. For clarity, just the parts of the
larger molecules that undergo reaction are shown. NAD+, nicotinamide
adenine dinucleotide; NADH, nicotinamide adenine dinucleotide
(reduced form); FAD, flavin adenine dinucleotide; FADH2,
flavin adenine dinucleotide (reduced form); CoA, coenzyme A;
AMP, adenosine monophosphate.