Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
270
ATP
ATP as energy ‘currency’
Look back at the structure of ATP, shown in Figure 6.4,
page 113.
When a phosphate group is removed from ATP,
adenosine diphosphate (ADP) is formed and 30.5 kJmol −1 of
energy is released. Removal of a second phosphate produces
adenosine monophosphate (AMP), and 30.5 kJ mol −1 of
energy is again released. Removal of the last phosphate,
leaving adenosine, releases only 14.2 kJ mol −1 (Figure 12.4).
In the past, the bonds attaching the two outer phosphate
groups have been called high-energy bonds, because
more energy is released when they are broken than
when the last phosphate is removed. This description is
misleading and should be avoided, since the energy does
not come simply from breaking those bonds, but rather
from changes in chemical potential energy of all parts of
the system.
ATP + H 2 O
ADP + H 2 O AMP + H 2 O adenosine
30.5 kJ mol –1 30.5 kJ mol –1 14.2 kJ mol –1
P i P i P i
Figure 12.4 Hydrolysis of ATP (P i
is inorganic phosphate,
H 3
PO 4
).
These reactions are all reversible. It is the
interconversion of ATP and ADP that is all-important in
providing energy for the cell:
ATP + H 2
O ADP + H 3
PO 4
± 30.5 kJ
The rate of interconversion, or turnover, is enormous. It is
estimated that a resting human uses about 40 kg of ATP
in 24 hours, but at any one time contains only about 5 g of
ATP. During strenuous exercise, ATP breakdown may be
as much as 0.5 kg per minute.
The cell’s energy-yielding reactions are linked to
ATP synthesis. The ATP is then used by the cell in
all forms of work. ATP is the universal intermediary
molecule between energy-yielding and energy-requiring
reactions used in a cell, whatever the type of cell. In other
words, ATP is the ‘energy currency’ of the cell. The cell
‘trades’ in ATP, rather than making use of a number of
different intermediates. ATP is a highly suitable molecule
for this role. Not only is it readily hydrolysed to release
energy, it is also small and water-soluble. This allows it to
be easily transported around the cell.
Energy transfers are inefficient. Some energy is
converted to thermal energy whenever energy is
transferred. At the different stages in a multi-step
reaction such as respiration, the energy made available
may not perfectly correspond with the energy needed
to synthesise ATP. Any excess energy is converted to
thermal energy. Also, many energy-requiring reactions
in cells use less energy than that released by hydrolysis of
ATP to ADP. Again, any extra energy will be released as
thermal energy.
Be careful to distinguish between molecules used
as energy currency and as energy storage. An energy
currency molecule acts as the immediate donor of energy
to the cell’s energy-requiring reactions. An energy
storage molecule is a short-term (glucose or sucrose)
or long-term (glycogen, starch or triglyceride) store of
chemical potential energy.
Synthesis of ATP
Energy for ATP synthesis can become available in two
ways. In respiration, energy released by reorganising
chemical bonds (chemical potential energy) during
glycolysis and the Krebs cycle (pages 272–273) is used to
make some ATP. However, most ATP in cells is generated
using electrical potential energy. This energy is from the
transfer of electrons by electron carriers in mitochondria
and chloroplasts. It is stored as a difference in proton
(hydrogen ion) concentration across some phospholipid
membranes in mitochondria and chloroplasts, which
are essentially impermeable to protons. Protons are then
allowed to flow down their concentration gradient (by
facilitated diffusion) through a protein that spans the
phospholipid bilayer. Part of this protein acts as an enzyme
that synthesises ATP and is called ATP synthase. The
transfer of three protons allows the production of one ATP
molecule, provided that ADP and an inorganic phosphate
group (P i
) are available inside the organelle. This process
occurs in both mitochondria (page 275) and chloroplasts
(page 288) and is summarised in Figure 12.5. The process
was first proposed by Peter Mitchell in 1961 and is called
chemiosmosis.
Note that a hydrogen atom consists of one proton and
one electron. The loss of an electron forms a hydrogen
ion, which is a single proton.
H
H + + e −