Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
282
Summary
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Organisms must do work to stay alive. The energy
input necessary for this work is either light, for
photosynthesis, or the chemical potential energy of
organic molecules. Work includes anabolic reactions,
active transport and movement.
Some organisms, such as mammals and birds, use
thermal energy released from metabolic reactions to
maintain their body temperature. Reactions that release
energy must be harnessed to energy-requiring reactions.
This involves an intermediary molecule, ATP. ATP can
be synthesised from ADP and phosphate using energy,
and hydrolysed to ADP and phosphate to release energy.
ATP therefore acts as an energy currency in all living
organisms.
Respiration is the sequence of enzyme-controlled steps
by which an organic molecule, usually glucose, is broken
down so that its chemical potential energy can be used
to make the energy currency, ATP. In aerobic respiration,
the sequence involves four main stages: glycolysis,
the link reaction, the Krebs cycle and oxidative
phosphorylation.
In glycolysis, glucose is first phosphorylated and then
split into two triose phosphate molecules. These
are further oxidised to pyruvate, giving a small yield
of ATP and reduced NAD. Glycolysis occurs in the
cell cytoplasm. When oxygen is available (aerobic
respiration), the pyruvate passes to the matrix of a
mitochondrion. In a mitochondrion, in the link reaction,
pyruvate is decarboxylated and dehydrogenated and
the remaining 2C acetyl unit combined with coenzyme A
to give acetyl coenzyme A.
The acetyl coenzyme A enters the Krebs cycle in the
mitochondrial matrix and donates the acetyl unit
to oxaloacetate (4C) to make citrate (6C). The Krebs
cycle decarboxylates and dehydrogenates citrate to
oxaloacetate in a series of small steps. The oxaloacetate
can then react with another acetyl coenzyme A from the
link reaction.
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Dehydrogenation provides hydrogen atoms, which are
accepted by the carriers NAD and FAD. These pass to the
inner membrane of the mitochondrial envelope, where
they are split into protons and electrons.
In the process of oxidative phosphorylation, the
electrons are passed along a series of carriers. Some of
the energy released in oxidative phosphorylation is used
to move protons from the mitochondrial matrix to the
intermembrane space. The movement of electrons sets
up a gradient of protons across the inner membrane
of the mitochondrial envelope. Protons pass back
into the matrix, moving down their concentration
gradient through protein channels in the inner
membrane. An enzyme, ATP synthase, is associated
with each of the proton channels. ATP synthase uses
the electrical potential energy of the proton gradient
to phosphorylate ADP to ATP. At the end of the carrier
chain, electrons and protons are recombined and
reduce oxygen to water.
In the absence of oxygen as a hydrogen acceptor (in
alcoholic and lactic fermentations), a small yield of ATP
is made through glycolysis, then dumping hydrogen into
other pathways in the cytoplasm which produce ethanol
or lactate. The lactate pathway can be reversed in
mammals when oxygen becomes available. The oxygen
needed to remove the lactate produced during lactic
fermentation is called the oxygen debt.
The energy values of respiratory substrates depend on
the number of hydrogen atoms per molecule. Lipids
have a higher energy density than carbohydrates or
proteins. The respiratory quotient (RQ) is the ratio of the
volume of oxygen absorbed and the volume of carbon
dioxide given off in respiration. The RQ reveals the
nature of the substrate being respired. Carbohydrate
has an RQ of 1.0, lipid 0.7 and protein 0.9. Oxygen
uptake, and hence RQ, can be measured using
a respirometer.