1 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.
2 Reactions that release energy must be harnessed to energy-requiring reactions. Th is
„harnessing‟ involves an intermediary molecule, ATP. Th is 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.
3 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.
4 In aerobic respiration, the sequence involves four main stages: glycolysis, the link
reaction, the Krebs cycle and oxidative phosphorylation.
5 In glycolysis, glucose is fi rst phosphorylated and then split into two triose phosphate
molecules. Th ese are further oxidised to pyruvate, giving a small yield of ATP and
reduced NAD. Glycolysis occurs in the cell cytoplasm.
6 When oxygen is available (aerobic respiration), the pyruvate passes to the matrix of a
mitochondrion. There, in the link reaction, pyruvate is decarboxylated and
dehydrogenated and the remaining 2C acetyl unit combined with coenzyme A to give
acetyl coenzyme A.
7 The acetyl coenzyme A enters the Krebs cycle in the mitochondrial matrix and
donates the acetyl unit to oxaloacetate (4C) to make citrate (6C).
8 The Krebs cycle decarboxylates and dehydrogenates citrate to oxaloacetate in a
series of small steps. Th e oxaloacetate can then react with another acetyl coenzyme A
from the link reaction.
9 Dehydrogenation provides hydrogen atoms, which are accepted by the carriers NAD
and FAD. Th ese pass to the inner membrane of the mitochondrial envelope, where they
are split into protons and electrons.
10 In the process of oxidative phosphorylation, the electrons are passed along a series
of carriers. Some of the energy released in this process is used to move protons from
the mitochondrial matrix to the intermembrane space. This sets up a gradient of protons
across the inner membrane of the mitochondrial envelope. The 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 these channels.
ATP synthase uses the electrical potential energy of the proton gradient to
phosphorylate ADP to ATP.
11 At the end of the carrier chain, electrons and protons are recombined and reduce
oxygen to water.
12 In the absence of oxygen as a hydrogen acceptor (in anaerobic respiration), a small
yield of ATP is made by 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
anaerobic respiration is called the oxygen debt.
13 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.
14 The respiratory quotient (RQ) is the ratio of the volumes of oxygen absorbed and
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.
15 Oxygen uptake, and hence RQ, can be measured using a respirometer.
1. End-of-chapter questions
1. What does not occur in the conversion of glucose to two molecules of pyruvate?
A hydrolysis of ATP
B phosphorylation of ATP
C phosphorylation of triose (3C) sugar
D reduction of NAD
2 Wheredoes each stage of aerobic respiration occur in a eukaryotic cell?
