CIE Categories Archives: 12. Respiration

Summary of Energy and Respiration

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
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?






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Anaerobic respiration – Ethanol and Lactate pathways

Anaerobic respiration is a type of respiration
that does not use oxygen. It is used when there
is not enough oxygen for aerobic respiration.
In the absence of free oxygen:
 Oxidative phosphorylation cannot take
place, as there is nothing to accept the electrons
and protons at the end of the electron transport
 Hydrogen cannot be used up by combining
it with oxygen to give water, so reduced NAD
cannot be recycled to NAD in this way to allow
glycolysis to continue.
 The mitochondrion quickly runs out of NAD
or FAD that can accept hydrogens from the
Krebs cycle reactions. The Krebs cycle and the
link reaction therefore come to a halt.
 Glycolysis, however, can still continue, so long as the pyruvate produced
at the end of it can be removed and the reduced NAD can be converted back to
Two other pathways allow the recycling of reduced NAD formed during

The Cori cycle serves two purposes:
• it ‘rescues’ lactate and prevents the wasteful loss of some of its chemical bond
• it prevents a potentially disastrous fall in plasma pH.
The lactate that is produced in muscles diffuses into the blood and is carried in
solution in the blood plasma to the liver. Here, liver cells convert it back to
pyruvate. This requires oxygen, so extra oxygen is required after exercise has
finished. The extra oxygen is known as the oxygen debt.

Later, when the exercise has finished and oxygen is available again, some of the
pyruvate in the liver cells is oxidised through the link reaction, the Krebs cycle
and the electron transport chain. Some of the pyruvate is reconverted to glucose in the liver cells. The glucose may be released into the blood or converted to
glycogen and stored.

• Both reactions ‘buy time’ by providing hydrogen acceptors so that NAD is
released and glycolyis can continue.
• Both pathways are inefficient and wasteful in that the products (ethanol or
lactate) have chemical bond energy that is untapped.
• The ethanol or lactate produced is toxic and restricts the use of the pathways.
• While the lactate pathway is reversible (by the Cori cycle) in the mammalian
liver, the ethanol pathway is irreversible.
• There is a net gain of only two ATP molecules per glucose molecule (from
glycolysis) during anaerobic respiration.







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Aerobic respiration, Link reaction

The link reaction
In the link reaction, pyruvate enters the matrix of a mitochondrion and is:
• decarboxylated: CO2 is removed from the pyruvate and then diffuses out of
the mitochondrion and out of the cell.
• dehydrogenated: Hydrogen is removed from the pyruvate, and is picked up
by NAD, producing reduced NAD. This converts pyruvate into a 2-
carbon compound.
• combined with coenzyme A to give acetylcoenzyme A (ACoA).

Coenzyme A consists of:
• adenine
• ribose (making a nucleoside together with adenine)
• pantothenic acid (a B vitamin).
Coenzyme A transfers an acetyl group (with 2 carbon atoms) from pyruvate into
the Krebs cycle and plays a central role in respiration. It is present in small
quantities in a cell and is recycled.


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Respiration, Glycolysis

Respiration is the oxidation of energy-containing organic molecules. The energy
released from this process is used to combine ADP with inorganic phosphate to make
All cells obtain useable energy through respiration. Most cells use carbohydrate,
usually glucose, as their fuel. Some cells, such as nerve cells, can only use glucose as
their respiratory substrate, but others can use fatty acids, glycerol and amino acids.
Respiration may be aerobic or anaerobic. In both cases, glucose or another respiratory
substrate is oxidised.
– In aerobic respiration, oxygen is involved, and the substrate is oxidised completely,
releasing much of the energy that it contains.
– In anaerobic respiration, oxygen is not involved, and the substrate is only partially
oxidised. Only a small proportion of the energy it contains is released.

• glycolysis in the cytoplasm (cytosol) of the cell
• the link reaction in the matrix of a mitochondrion
• the Krebs cycle in the matrix of a mitochondrion
• oxidative phosphorylation on the inner mitochondrial membrane.

Glycolysis (the breakdown of glucose) is the first stage of respiration. It takes place in
the cytoplasm and does not require oxygen. It begins with the 6-carbon ring-shaped
structure of a single glucose molecule and ends with 2 molecules of a 3-carbon sugar
called pyruvate and a net gain of 2 ATP. Glycolysis is summarised below.

A glucose (or other hexose sugar) is phosphorylated, using phosphate from 2
molecules of ATP, to give hexose bisphosphate. This phosphorylation converts an
energy-rich but unreactive molecule into one that is much more reactive, the chemical
potential energy of which can be trapped more efficiently.

The hexose bisphosphate is split into 2 triose phosphate molecules.
• Hydrogen atoms and phosphate groups are removed from the triose phosphate (by the
coenzyme NAD). The removal of hydrogens is an oxidation reaction, so triose
phosphate is oxidised to 2 molecules of pyruvate (pyruvic acid). During this step, the
phosphate groups from the triose phosphates are added to ADP to produce a small yield
of ATP.
• Overall, 2 molecules of ATP are used and 4 are made during glycolysis of one glucose
molecule, making a net gain of 2 ATPs per glucose. The pyruvic acid is then converted
to either lactic acid or alcohol and carbon dioxide without the production of any more
The pyruvate formed in glycolysis is still energy-rich. It passes next to the link reaction.
This reaction and all subsequent stages of respiration occur inside a mitochondrion, and
can only occur in the presence of free oxygen. Respiration requiring free oxygen is
aerobic respiration. Pyruvate is transported into the mitochondrial matrix by a membrane
transport protein, which exchanges it for OH– in the matrix.
If the cell cannot catabolize the pyruvate molecules further, it will harvest only 2 ATP
molecules from 1 molecule of glucose. For example, mature mammalian red blood cells
are only capable of glycolysis, which is their sole source of ATP. If glycolysis is
interrupted, these cells would eventually die.






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12.3) Anaerobic respiration

12.3) Anaerobic respiration


Anaerobic respiration: is the term for the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.

  • The word anaerobic means ‘in the absence of oxygen’.
  • Anaerobic respiration happens in muscles during hard exercise
  • glucose → lactic acid
  • C6H12O6 → 2C3H6O3
  • Anaerobic respiration also happens in plant cells and some microorganisms. Anaerobic respiration in yeast is used during brewing and bread-making
  • glucose → ethanol + carbon dioxide
  • C6H12O6 → 2C2H5OH + 2CO2

Anaerobic respiration is much less efficient than aerobic respiration because it releases much less energy per glucose molecule broken down (respired).


  • There is a buildup of lactic acid in the muscles during vigorous exercise.
  • The lactic acid needs to be oxidised to carbon dioxide and water later.
  • This causes an oxygen debt, that needs to be ‘repaid’ after the exercise stops.
  • Lactic acid is removed in the bloodstream.
  • The blood needs to move more quickly during and after exercise to maintain this lactic acid removal process, so the heart rate is rapid.
  • On reaching the liver, some of the lactic acid is oxidised to CO2 and H2O, using up oxygen in the process.
  • After exercise has stopped, a high level of oxygen consumption may persist until the excess of lactic acid has been oxidised.
  • This is characterised by deeper breathing.
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12.2) Aerobic respiration

12.2) Aerobic respiration


Aerobic respiration: is the term for chemical reactions in cells that use oxygen to break down nutrient molecules to releases energy.

  • The word aerobic means that oxygen is needed for this chemical reaction.
  • The food molecules are combined with oxygen.
  • The process is called oxidation and the food is said to be oxidised.
  • glucose + oxygen → carbon dioxide + water + 2830 kJ energy
  • C6H12O6 + 6O2 → 6CO2 + 6H2O
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12.1) Respiration

12.1) Respiration


Most of the processes taking place in cells need energy to make them happen. Examples of energy-consuming processes in living organisms are:

  • The contraction of muscle cells – to create movement of the organism, or peristalsis to move food along the alimentary canal, or contraction of the uterus wall during childbirth.
  • Building up proteins from amino acids.
  • The process of cell division to create more cells,more replace damaged or worn out cells, or to make reproductive cells.
  • The process of active transport, involving the movement of molecules across a cell membrane against a concentration gradient.
  • Growth of an organism through the formation of new cells or a permanent increase in cell size.
  • The conduction of electrical impulses by nerve cells
  • Maintaining a constant body temperature in homoiothermic (warm-blooded) animals to ensure that vital chemical reactions continue at a predictable rate and do not slow down or speed up as the surrounding temperature varies.


Respiration is a chemical process that takes place in cells and involves the action of enzymes.

Can sometimes be called cellular respiration, internal respiration or tissue respiration.

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