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Cellular Respiration
Higher Human Biology
Unit 1 – Section 7
Cellular Respiration
Learning Intentions
By the end of this section you should be able to:
Identify ATP as the high energy compound which transfers energy, and is produced when
ADP combines with phosphate in phosphorylation, building up energy, and releases energy
when broken down into ADP and phosphate once more.
State that cells use high energy electrons to pump hydrogen ions across a membrane,
which activates the enzyme ATP synthase, and helps to produce ATP.
Describe the process of glycolysis including the energy investment stage and the energy
pay off phase, and the use of phosphofructokinase in progressing the glycolysis pathway.
Describe the sequence of events that occur after pyruvate production, and onto the citric
acid cycle, in the presence of oxygen including the production of acetyl coenzyme A, citric
acid and oxaloacetate.
State the number of carbon atoms present in each of the main carbon compounds in
respiration, and the use of dehydrogenase enzymes to remove hydrogen ions which are
then passed to the coenzyme molecules FAD and NAD.
Describe NAD and FAD as carrier molecules, which carry the high energy electrons to the
electron transport chain.
What is a Cellular Respiration?
• This is a series of metabolic pathways
that bring about the release of energy
from a foodstuff and the regeneration
of the high-energy compound adenosine
triphosphate (ATP).
• Adenosine Triphosphate
• This is made up of three inorganic
phosphate (Pi) and an adenosine molecule.
• Molecule able to provide energy
• Adenosine Triphosphate
• This terminal bond breaks by enzyme action to
release energy
• Adenosine diphosphate (ADP) and an inorganic
phosphate is produced
ATP and energy transfer
ATP molecules are the energy currency of the
The bond between
the end phosphate
must be broken by enzymes in order
to release the energy.
Adenosine triphosphate
The enzyme breaking the
bond results in adenosine
diphosphate (ADP) and
inorganic phosphate (Pi)
Energy released
However, this reaction is reversible – where energy
is required to regenerate ATP. This is called
breakdown releasing energy
building up requiring energy
ADP + Pi
ATP acts as a link between catabolic, energy-releasing
reactions, and anabolic, energy-consuming, reactions.
ATP transfer diagram
CO2 +
Glucose +
energy transfer
ADP + Pi
Some bacterial cells require 2 million molecules of ATP
per second
In fact, about 400g is produced and used / hour but only
50g is ever present within the body
This is possible due to the rapid turnover of ATP
molecules – where ATP is breaking down and being
This is the process, controlled by enzymes in which
phosphate groups are added to a molecule
ADP + Pi  ATP
Phosphorylation can also occur when a phosphate and
energy are transferred from ATP to the molecules in a
metabolic pathway, making them more reactive
Often this stage has to occur for the next reaction to
take place. The reactant is now phosphorylated and
energised which allows the pathway to proceed.
In respiration, some reactants must undergo
phosphorylation in what is called the investment phase
Use/Role of ATP
ATP is important because it acts as a link by
which chemical energy is transferred from one
type of reaction to another
ATP breakdown promotes the transfer of energy
to new chemical bonds
e.g. peptide bonds joining amino acids together
ATP is necessary for muscle contraction, protein
synthesis, and active transport
Synthesis of ATP
When glucose is broken down it
releases energy to synthesise
ATP from ADP + Pi
A flow of high energy electrons,
from the respiratory pathway,
transfer their energy to proteins
in the membrane. This provides
the energy used to pump
hydrogen ions across the inner
mitochondrial membrane, by
active transport. It helps
maintain a higher concentration
of hydrogen ions on one side of
the membrane
Molecules of the enzyme
ATPsynthase are embedded in
the mitochondrial membrane
The return flow of hydrogen
ions from a region of higher
concentration to a region of
lower concentration makes the
ATPsynthase molecules rotate
and catalyse the synthesis of
ATP from ADP and Pi
The return flow of
hydrogen ions makes the
ATPsynthase molecules
rotate and catalyse the
synthesis of ATP from
ADP and Pi
• Process by which energy is released
from foods by oxidation.
• It involves the regeneration of ATP
which is a high energy compound.
• Consists of 3 stages
• Takes place in the cytoplasm of the cell.
• It consists of a series of enzyme
controlled reactions
• Does not require oxygen.
• Glucose (6-C) is broken down into two
molecules of Pyruvic Acid (3-C).
• Net gain of 2 ATP.
• Hydrogen released binds to a coenzyme, NAD by reduction to form
• The reactions in the first half of the chain
make up the energy investment phase
where 2ATP are used up.
• The reactions in the second half of the
chain make up an energy payoff phase
where 4ATP molecules are produced per
molecule of glucose. H+ ions are also
released from the substrate by the
dehydrogenase enzyme. These are passed
to NAD.
• This gives a net gain of 2ATP
Glycolysis 2
• Krebs Cycle takes
place in the matrix
of the
• Electron Transfer
System takes place
on the cristae.
Krebs Cycle
• Takes place in the matrix of the mitochondria.
• Requires oxygen.
• 3-C Pyruvic acid/Pyruvate converted to CO2 and
Acetyl CoA (2-C). H+ ions reduce the NAD to
• Acetyl CoA enters Krebs and combines with a
4-C (oxaloacetate) compound to form a 6-C
compound (citrate).
• The Coenzyme A returns to combine with more
Krebs Cycle cont…
• This Citrate (6-C) compound is converted
back to a oxaloacetate (4-C) compound by
a series of enzyme controlled reactions.
This will again combine with acetyl
coenzyme A to begin the cycle again.
• During the cycle, carbon is released in the
form of carbon dioxide and hydrogen H+
ions are released and become bound to
NAD, forming NADH.
Electron Transfer System
• Also known as cytochrome system or
hydrogen transfer system.
• Takes place on the cristae of the
mitochondria on groups of protein molecules.
• The reduced co-enzymes (NADH and FADH2)
from the glycolytic and citric acid pathways
transfer the hydrogen to a chain of carriers.
• High energy electrons are also
released and pass to the electron
transport chains.
• The electrons begin in a high energy state
and as they release their energy it is used
to pump hydrogen ions across the
membrane from the inner cavity (matrix)
to the intermembrane space. This helps
maintain a high H+ ion concentration.
• The return journey of the H+ ions from
the intermembrane to the matrix is via
ATP synthase. This drives the synthesis of
ATP from ADP and Pi.
• This transfer releases 3 ATP molecules.
• When the electrons come to the end of the
chain, they combine with oxygen , the final
• Oxygen also combines with two hydrogen ions
to form water.
Electron Transfer cont…
• 36 molecules of ATP are produced
from the electron transfer system.
• The hydrogen combines with oxygen
to form water.
• 38 molecules of ATP produced in total
• 2 from glycolysis and 36 from electron
transfer system.
Anaerobic Respiration
• The process where a little amounts of
energy are derived from the partial
breakdown of sugar.
• It takes place in the absence of oxygen.
• Only glycolysis occurs.
• 2ATP molecules are produced.
Anaerobic Respiration
• Due to the oxygen debt, pyruvic acid is
converted to lactic acid.
• When the oxygen debt is repaid, the
lactic acid is converted back to pyruvic
acid which then enters the aerobic
Anaerobic Respiration
Oxygen debt builds
Pyruvic Acid
Lactic Acid
(2 X 3C)
(2 X 3C)
Oxygen debt repaid
Aerobic V Anaerobic
Number of ATP
molecules per
glucose molecule
Products of
reaction (other
than ATP)
dioxide and
Location in cell mitochondrion
Lactic acid
Substrates for Respiration
• Starch and glycogen are made up of chains of glucose. This means that they can
act as respiratory substrates since they can be broken down into glucose.
• Sucrose and maltose – these can be converted to glucose or intermediates
These are broken down into glycerol and fatty acids
Glycerol – converted to a glycolytic intermediate
Fatty Acids – these are metabolised into metabolic fragments that enter the
pathways as acetyl coenzyme A.
These are broken down into amino acids by enzymes. These undergo deamination
forming urea and respiratory intermediates.
Regulation of the
Respiratory Pathway
The third step in this process is
catalysed by phosphofructokinase.
This is an irreversible step so is
therefore a key regulatory point.
When the cell has too much ATP
the high concentration inhibits
phosphofructokinase and slows
down glycolysis.
When the concentration of ATP
decreases the enzyme is no longer
inhibited so glycolysis speeds up.
Phosphofructokinase is also
inhibited by high concentrations
of citrate.
Feedback Inhibition
This regulates and
glycolysis and the
Krebs Cycle
pathways. This is
important because:
• Build up on
intermediate is
• ATP is only
produced when it is
• Resources are
Oxidation + Reduction
• Oxidation is the removal of electrons
(hydrogen) from a substance.
• Reduction is the addition of electrons
(hydrogen) to a substance.
• OIL RIG!!!!!!