Cellular Respiration
Harvesting Energy
Cells and Energy
The chemical energy in glucose and other organic
compounds is not used directly by cells.
Cells carry out a series of reactions that release chemical
energy from glucose and transfer it to ATP. The
energy is then available for use by cells.
The series of energy releasing reactions that break down
organic compounds of food, releasing chemical energy and
transferring it to ATP, is known as cellular respiration (or
sometimes, just respiration).
Cellular respiration occurs all the time in the cells of all
living things — plants, animals, fungi, protists and
bacteria.
Rates of Respiration
Living things use energy all the time, but at varying
rates.
Since energy for an organism’s use is supplied by cellular
respiration, the rate of cellular respiration also varies
depending on the state of the organism.
When animals hibernate or go into a state of torpor, their
rates of energy use fall and so their rates of cellular
respiration drop to a fraction of normal.
Example: Brine shrimp, also called sea monkeys, form cysts when they
dry out. Although they can stay in this form for years, they are alive and
respire with the lowest metabolic rate ever measured. The amount of
energy used is estimated to be about 1/40 000th of a kilojoule per year
per milligram of shrimp.
Energy from glucose
Process of energy transfer from glucose to ATP is not 100
per cent efficient.
About 40 per cent of the chemical energy present in
glucose is transferred to ATP and the remaining 60 per
cent appears as heat energy.
The heat energy produced by living cells cannot be used to
drive energy-requiring activities, such as muscle contraction
or transport against a concentration gradient.
Instead heat energy is used to maintain the core body
temperature of animals such mammals and birds within a
narrow range. Insulating layers of fat, fur or feathers traps
the heat energy released from cellular respiration.
Energy from glucose
C6H12O6 + 6O2
6H2O + 6CO2 + Energy
Three stages of glycolysis
Glycolysis
– Occurs in cytosol
Citric acid cycle
– Occurs in matrix of mitochondria
– Also known as Kreb’s Cycle
Electron transport
– Occurs in cristae of mitochondria
NADH carries electrons to ETC
glycolysis
prep
Krebs
ATP
Electron
Transport
chain
Glycolysis
Splits a glucose
molecule into
2 - 3 Carbon
molecules called
PYRUVATE.
products: ATP, NADH and pyruvate
Preparation for the Citric Acid Cycle
C
The pyruvate loses a
carbon leaving the 2
carbon molecule
C
Acetyl CoA
CO2
products: CO2,
Acetyl CoA and NADH
The Citric Acid Cycle
products:
CO2, ATP, NADH, FADH
Electron Transport
H+ H+
NAD
H+
H+
outer membrane
H+
matrix
H+H+
H+
H+
inner membrane
or cristae
During electron transport, electrons from ‘loaded’ acceptors (NADH
and FADH2) are brought to the inner membranes of the mitochondria.
The electrons are passed back and forth across the membrane from
one cytochrome to another.
During this process their energy is gradually decreased and used
to transport H+ through the membrane.
Oxygen is the final electron acceptor and it joins with the H+ to
produce H2O.
If there is no oxygen, the electron chain cannot continue
because there is no way to release electrons .
products:
H2O, ATP
Outcome of the three stages
In cells of your heart, liver and kidneys, two additional
molecules of ATP are generated to give a total of 38 ATP.
This is because the NADH produced during glycolysis in
those cells enters the respiratory chain earlier than NADH
produced in other kinds of cell.
Reaction for Cellular Respiration
Strictly speaking, ‘cellular respiration’ refers to
the aerobic breakdown of glucose to drive the
production of ATP; that is, the pathways that
evolved when oxygen became available and
which occur in mitochondria in eukaryotic cells.
The general simplified formula for the complete
aerobic breakdown of glucose is:
What happens when there is no
oxygen?
If oxygen is not available, glycolysis is
followed by fermentation and no more energy
in the glucose molecule will be harvested—no
further ATP is produced.
This process is referred to as anaerobic
respiration.
Pyruvate is converted via an anaerobic pathway
to either lactic acid (in most animals) or alcohol
and carbon dioxide (in most plants, and in
microorganisms such as yeast and bacteria).
Fermentation is necessary as it prevents the
accumulation of pyruvate and thus allows
glycolysis to continue.
Anaerobic respiration in mammals
In the absence of oxygen, an enzyme present in human
muscle tissue converts pyruvate to lactate (lactic acid)
molecules.
The total energy yield for anaerobic respiration is two
ATP per glucose molecule.
If strenuous exercise continues, lactate builds up in the
muscles, the pH falls and pain and muscle fatigue occur.
When strenuous exercise stops, the oxygen supply to the
muscles is adequate for normal needs and anaerobic
respiration stops.
Accumulated lactate in muscle tissue is converted back
to pyruvate and enters the Krebs cycle.
Value of anaerobic pathway in
mammals
Aerobic respiration
produces almost 20 times
the number of ATP molecules
than are produced by
glycolysis. .
Cells of animals and plants
rely on the anaerobic pathway
only if there is not enough
oxygen available to continue
aerobic respiration,
In some cases, the rapid
rate of release of energy in
glycolysis can be vital.
In short sprints, it is energy
derived from glycolysis that
gets athletes across the
finishing line.
Alcoholic Fermentation
During fermentation by yeast, pyruvate is broken down to
carbon dioxide and ethanol (an alcohol).
The amounts of ethanol and carbon dioxide produced vary with
different yeasts and different environmental conditions.
In wine-making, grapes are crushed to release the juice which
contains sugars. Yeasts are added to this fluid, fermentation
occurs which produces alcohol. When the alcohol concentration
reaches about 12 per cent (v/v), this kills the yeast cells and
fermentation stops.
Beer is made by fermenting sprouting barley grains using
brewers’ yeast. Hops are added to give colour, taste and aroma.
Spirits are produced by fermenting various products, such
as fruit juice (brandy), molasses (rum), barley grains (whisky).
Spirits are distilled to increase the alcohol content in the
final product to about 40 per cent (v/v).
Comparison of anaerobic and
aerobic respiration
Other substrates for respiration
The products of digestion of fats
(fatty acids and glycerol) and the
products of digestion of proteins
(amino acids) can also enter the
pathways of cellular respiration at
various points.
When starved of food for a long
period, even the proteins in muscles
and other body tissues will be broken
down to provide the energy
necessary to survive.
During starvation in people, up to 97
per cent of fat tissue, 31 per cent of
skeletal muscle and 27 per cent of
blood can be lost. The brain, heart
and diaphragm are not affected
Fats provide more energy per gram
(39 kJ) than either carbohydrates or
proteins (about 17 kJ each).
Applying our understanding of
cellular respiration to medicine
Hyberbaric chambers
Hyperbaric oxygen chambers are being used to deliver a higher
concentration of oxygen to tissues than normally exists to help
heal injuries.
A person being treated in the chamber is placed in a room in
which air pressure can be increased. One hundred per cent
oxygen is then delivered to the person through a hood or mask.
This ensures that a higher level of oxygen is in the bloodstream
of the person involved compared with normal situations in which a
person breathes air containing 21 per cent oxygen.
High levels of oxygen in the blood ensure that the level of oxygen
is not the limiting factor for a cell.
Aerobic respiration can occur at maximum rate and a maximum
yield of ATP can be expected. This means that the cell has
sufficient energy to boost the repair requirements that may exist
after accident or surgery.
Applying our understanding of
cellular respiration to medicine
PET Images to asses damage to heart muscles
The heart has a high oxygen demand for ATP production by
aerobic respiration.
Blockage of a coronary artery (e.g. a clot) will interrupt the
oxygen supply to a region of the heart, interfere with heart
function and damage heart tissue.
If the region of heart muscle affected by the blockage is still
alive, the damage may be reversed. In such cases, bypass
surgery may be done to restore the blood supply to the region.
If the affected region of heart muscle has died, this damage is
permanent. In such cases, surgery to restore the blood supply
to the affected area is of no use and exposes the patient to
unnecessary risk.
It is important to be able to distinguish whether the damaged
area consists of living or dead heart tissue.
A non-invasive technique, known as positron emission
tomography (PET), can make this distinction.
PET can obtain an image of glucose uptake and use by the
heart. When damaged areas of the heart are still alive, they
can take up and use glucose. Dead areas of heart tissue
neither take up nor use glucose.
Link between cellular respiration
and photosynthesis
Carbon dioxide and water are the waste
products of respiration.
These are the basic materials that a plant
uses for photosynthesis.
Photosynthesis is an endergonic (energyrequiring) reaction.
Cellular respiration is an exergonic (energyreleasing) reaction.
Respiration
C6H12O6 + 6O2 → 6CO2 + 6H2O + 36−38 ATP
Glucose + oxygen → carbon dioxide + water + energy
Photosynthesis
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2
carbon dioxide + water + light → glucose + oxygen