12 Energy and respiration
Key terms
ADP (adenosine diphosphate): a nucleotide, present in every living cell, made of adenosine and two
phosphate groups linked in series, and important in energy transfer reactions of metabolism
aerobic respiration: respiration requiring oxygen, involving oxidation of glucose to carbon dioxide (CO 2)
and water (H2O)
anabolism: the building up of complex molecules from smaller ones
anaerobic respiration: respiration in the absence of oxygen, involving breakdown of glucose to lactate or
ethanol
ATP (adenosine triphosphate): a nucleotide, present in every living cell, formed in photosynthesis and
respiration from ADP and Pi, and functioning in metabolism as a common intermediate between energyrequiring and energy-yielding reactions
ATP synthase: (also called ATPase) an enzyme that catalyses the synthesis of ATP from ADP and Pi in
mitochondria (and chloroplasts)
autotrophic: capable of synthesising organic molecules from simple inorganic substances
calorimeter: an instrument to measure heat production which allows a calculation of the energy value of a
substance
chemiosmosis: the process by which energy is transferred in the production of ATP, from the flow of
+
protons (H ) through ATP synthase
citric acid cycle: see Krebs cycle
coenzyme A: a coenzyme of the link reaction that acts as a carrier in the oxidation of pyruvate to citrate
compensation point: the point where respiration and photosynthesis are balanced
cristae: folds in the inner membrane of mitochondria that project into the matrix
deamination: a chemical reaction that removes the amino group (–NH2) from an amino acid
decarboxylation: a chemical reaction that removes a carboxyl group from a compound and releases carbon
dioxide
dehydrogenation: a chemical reaction that removes hydrogen from a compound
electron transport chain: carriers that transfer electrons along a redox chain, permitting ATP to be
synthesised in the process
endergonic reaction: a metabolic reaction requiring energy input
exergonic reaction: a metabolic reaction releasing energy
facilitated diffusion: diffusion across a membrane facilitated by molecules in the membrane (without the
expenditure of metabolic energy)
fermentation: anaerobic respiration in which glucose is broken down in the absence of oxygen
free energy: when molecules are broken, the part of the potential chemical energy in the molecules that is
available to do useful work
glycerate-3-phosphate: a 3-carbon sugar; an intermediate of the Calvin cycle of photosynthesis, and of
glycolysis
glycolysis: the first stage of respiration in cells in which glucose is phosphorylated and then oxidised to
form pyruvate; this occurs without a need for oxygen and produces a small yield of ATP and reduced NAD
heterotrophic: incapable of synthesising organic molecules from simple inorganic substances so obtains
organic molecules from other organisms
hexose: a monosaccharide containing six carbon atoms, e.g. glucose, fructose
intermediate: a metabolite formed as part of a metabolic pathway which is neither the starting substance
nor the end product of the pathway
joule: the SI unit of energy
Krebs cycle: a stage of respiration in cells in which citrate is converted to oxaloacetate in a series of small
steps; oxygen is required and carbon dioxide is given off and NAD (or FAD) is reduced
link reaction: the reaction linking glycolysis and the Krebs cycle; it occurs inside the mitochondrion and
pyruvate is converted to acetyl coenzyme A
lysis: breakdown, of cells or molecules
matrix: ground substance of connective tissue; or the innermost part of a mitochondrion, where it is the site
of the link reaction and the Krebs cycle of cellular respiration
metabolism: integrated network of all the biochemical reactions of life
metabolite: a chemical substance involved in metabolism
mitochondrion (plural: mitochrondria): an organelle in eukaryotic cells, site of many steps of aerobic
respiration
NAD (nicotinamide adenine dinucleotide): a coenzyme involved in redox reactions, it acts as a hydrogen
acceptor; it is reduced in the link reaction and the Krebs cycle and then converted back to NAD in the
electron transport chain
nucleotide: phosphate ester of a nucleoside, i.e. an organic base combined with a pentose sugar and
phosphate (Pi)
nutrient: any substance used or required by an organism as food
oxidative phosphorylation: the synthesis of ATP from ADP and Pi using energy from oxidation reactions
in aerobic respiration
phosphate (Pi): phosphate ions, as involved in metabolism
potential energy: stored energy
pyruvate: a three-carbon organic compound, a product of glycolysis
redox reaction: a reaction in which reduction and oxidation happen simultaneously
respiration: the cellular process involving ATP, the universal energy currency of cells, by which sugars and
other substances are broken down to release chemical energy for other cellular processes
respiratory quotient (RQ): the volume of carbon dioxide produced divided by the volume of oxygen used
during respiration; it can also be determined theoretically by calculation
substrate: a molecule that is the starting point for a biochemical reaction, that forms a complex with a
specific enzyme
sugars: compounds with the general formula Cx(H2O)y, where x is approximately equal to y, and containing
an aldehyde or a ketone group
Topic summary
• Respiration is a cellular process in which energy is transferred from nutrients, such as glucose, to the
cellular machinery. Energy is required to do useful work, such as the transport of metabolites across
membranes, the driving of anabolic reactions and to cause movements in organisms.
• Aerobic respiration involves the complete oxidation of glucose to carbon dioxide and water with the
release of a large amount of energy. In addition to hexose sugar, fats and proteins may be used as respiratory
substrates. Anaerobic respiration involves the partial oxidation of glucose with the release of only a small
amount of energy.
• ATP is the universal energy currency molecule by which energy is transferred to do useful work. ATP is a
soluble molecule, formed in the mitochondria but able to move into the cytosol by facilitated diffusion. It
diffuses freely about cells.
• The stages of aerobic respiration are:
• glycolysis, in which glucose is converted to pyruvate
• the link reaction, in which pyruvate is converted to acetyl coenzyme A
• the Krebs cycle, in which acetyl coenzyme A is metabolised, carbon dioxide is given off and NAD (or
FAD) is reduced
• the electron transport chain and oxidative phosphorylation, in which reduced NAD (and reduced FAD)
are oxidised and water is formed. Most of the ATP is produced during this stage.
• Glycolysis occurs in the cytoplasm, but the Krebs cycle is located in the matrix of mitochondria and
oxidative phosphorylation occurs on the inner mitochondrial membrane including the cristae.
• In anaerobic respiration, the products are either lactate (in lactic acid fermentation, typically found in
vertebrate muscle) or ethanol and carbon dioxide (in alcoholic fermentation, found in yeast and in plants
under anaerobic conditions).
• Anaerobic respiration is wasteful of respiratory substrate. It yields only a tiny quantity of ATP, when
compared with the yield of ATP from aerobic respiration of the same quantity of respiratory substrate. The
waste products, ethanol or lactate, contain much unused chemical energy. They are both energy-rich
molecules.
• The rate of aerobic respiration can be measured manometrically, in a respirometer. The respiratory
quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen used, in a
given time. The respiratory quotient is an indicator of the respiratory substrate.
• Plants show adaptations to different environments. The adaptations to swamp conditions by rice include
extensive aerenchyma tissue, with continuous, interconnecting air spaces throughout stems, leaves and roots,
and physiological tolerance of ethanol by root cells.
Data handling
The metabolism of glucose in aerobic and anaerobic conditions
The table below shows the results of an experiment in which a solution of glucose was inoculated with a
small sample of yeast, as air was bubbled through. The variety of yeast used could respire both aerobically
and anaerobically. The resulting growth of the yeast culture (detected by changing optical density in samples
of the medium) was monitored by means of a colorimeter. The changing concentrations of glucose and
ethanol were monitored by chemical means at the same times.
a) Using the Excel spreadsheet facility, plot a graph of these results. Label your graph clearly.
b) Describe the phases you detect in the growth rate of the yeast colony during the period of the experiment?
How often does the rate of growth change, for example?
c) Growth of the yeast culture continued throughout the experiment. What was the respiratory substrate used by
yeast
i) during the first 15 hours
ii) subsequently?
d) Outline the alternative pathways by which glucose may be metabolised by this variety of yeast.
e) What evidence suggests that more than one respiratory path provided energy for the growth of yeast during
the first 10 hours of the experiment?
f) It has been shown that yeast cells, when supplied with air, use glucose more slowly than when air is
excluded. Explain why you would expect this, by reference to the alternative pathways of glucose
breakdown.
Do and understand
Testing a hypothesis by experiment
The rate of respiration (the uptake of oxygen and/or the output of carbon dioxide) of dormant or inactive
organisms is low, but is high in actively growing and physically active ones. So, is the rate of respiration
directly controlled by the processes of cells that require energy?
If so, by what mechanism could they be linked?
Look at Figure 12.3 on page 237. It illustrates a possible link point between cell processes that require
energy and the respiratory pathway.
Working with isolated mitochondria
Respiration has been investigated in isolated mitochondria extracted from respiring cell. These were
incubated in a buffer solution in the presence of intermediates of respiration in an instrument known as an
oxygen electrode. Here, the oxygen uptake by the mitochondria was measured. Look at the graph below.
Oxygen was rapidly taken up by these mitochondria when Krebs cycle acids were added (at B), in the
presence of ADP and phosphate (Pi), added at A, but oxygen uptake had stopped at X. If more ADP and
Pi were added at X, however, oxygen uptake (i.e. respiration) continued.
Alternatively, when a substance known as DNP was added, oxygen uptake was equally vigorous. DNP is
an inhibitor that detaches ATP formation from the steps of oxidative phosphorylation (shown in Figure
12.8).
The control mechanism
There is a only a limited amount of ATP within cells. In active and growing cells ATP is used up as fast as it
is formed and there is an excess of ADP present. In dormant or inactive cells the reverse applies, ADP is
absent and ATP is present in excess.
Since oxidative phosphorylation and ATP formation are linked, in the absence of ADP the whole
respiratory pathway is blocked. By adding DNP, the linkage is broken and respiration can proceed, even in
the absence of a supply of ADP.
So respiration in cells and tissues is regulated by the balance between ATP and ADP; that is, by the
demand for energy by cell processes.
Suggested websites
• www.ncbe.reading.ac.uk/ncbe/protocols/fermentation.html – a guide to practical fermentation
• www.sci.sdsu.edu/movies/actin_myosin.html – an animation about muscle contraction
Further reading
Articles in recent editions of Biological Sciences Review:
• ‘Respiration’, 23.2, pp 20–21 (Nov 2010)
• ‘How skeletal muscles work: from molecules to movement’, 22.4, pp 10–14 (April 2010)