All living organisms need a continuous supply of energy to maintain their metabolism.
They must absorb either light energy in photosynthesis or chemical potential energy to
do the work necessary to stay alive.
Such work includes:
• Anabolic reactions: the synthesis of proteins and other large molecules from smaller
ones (e.g. polypeptides from amino acids)
• Active transport of ions and molecules across cell membranes against their
concentration gradient (e.g. the activity of the sodium–potassium pumpneed energy as
against concentration gradient)
• Movement (mechanical work): movement of the whole organism for example muscle
contraction (such as heart beat, breathing movements, walking) or movement within the
organism, e.g. movement of organelles in cells)
• Maintenance of body temperature, particularly in mammals and birds, which must
release thermal energy to maintain the body temperature above that of the
environment.
• Transmission of nerve impulses.
Photosynthesis transfers light energy into chemical potential energy, which can then
be released for work by the process of respiration. Both photosynthesis and respiration
involve an important intermediary molecule in this energy transfer: adenosine
triphosphate (ATP). In many living organisms, most of the energy transferred to ATP is
derived originally from light energy; a few prokaryotes (the chemoautotrophs) are not
dependent on light energy trapped by photosynthesis but use energy from inorganic
chemical reactions instead.
2. The universal energy currency
Processes in cells that require energy are linked to chemical reactions that yield energy
by an intermediary molecule, ATP. Using one type of molecule to transfer energy to
many different energy-requiring processes makes it easier for these processes to be
controlled and coordinated. All organisms use ATP as their energy currency: it is a
universal energy currency. ATP is never stored. Glucose and fatty acids are short-term
energy stores, while glycogen, starch and triglycerides are long-term stores.
When an ATP molecule is hydrolysed, losing one of its phosphate groups, some of this
energy is released and can be used by the cell. In this process, the ATP is converted to
ADP (adenosine d iphosphate). The hydrolysis of one ATP molecule releases a small
‘packet’ of energy that is often just the right size to fuel a particular step in a process. A
glucose molecule, on the other hand, would contain far too much energy, so a lot would
be wasted if cells used glucose molecules as their immediate source of energy
ATP can be synthesised from ADP and an inorganic phosphate group (Pi) using energy,
and hydrolysed to ADP and phosphate to release energy. This interconversion is allimportant in providing energy for a cell. Hydrolysis of the terminal phosphate group of
ATP releases 30.5 kJ mol–1 of energy for cellular work:
Removing the second phosphate, giving AMP, also releases 30.5 kJ mol–1 of energy,
but removing the last phosphate yields only 14.2 kJ mol–1. The energy released comes
not simply from these bonds, but from the chemical potential energy of the molecule as
a whole.
Each cell has only a tiny quantity of ATP in it at any one time. The cell does not import
ATP, adenosine diphosphate (ADP) or adenosine monophosphate (AMP). With few
exceptions, each cell must produce its own ATP by respiration and recycle it very
rapidly. Because it is a small, watersoluble molecule, it is easily moved from where it is
made in a cell to where it is needed.
3. The roles
• binding to a protein molecule, changing its shape and causing it to fold differently, to
produce movement, e.g. muscle contraction
• binding to an enzyme molecule, allowing an energy-requiring reaction to be catalysed
• transferring a phosphate group to an enzyme, making the enzyme active
• transferring a phosphate group to an unreactive substrate molecule so that it can react
in a specific way, e.g. in glycolysis and the Calvin cycle
• transferring AMP to an unreactive substrate molecule, producing a reactive
intermediate compound, e.g. amino acids before binding to tRNA during protein
synthesis
• binding to a trans-membrane protein so that active transport can take place across the
membrane.
Metabolism
The total of all the biochemical reactions needed for an organism to stay alive is its
metabolism.
metabolism = anabolism + catabolism
Anabolism is the building up of more complex molecules from simpler ones, for
example the synthesis of nucleic acids and carbohydrates. Enzymes are needed for
these syntheses of the complex molecules needed for growth. Anabolic reactions are
energy-consuming.
Catabolism is the enzymic breakdown of complex molecules to simpler ones. It is the
opposite of anabolism. The catabolic reactions of respiration yield energy.
