Download 4.4.1 Respiration

OCR A2 UNIT F214 RESPIRATION
Specification:
1. Outline why plants, animals and microorganisms need to respire, with
reference to active transport and metabolic reactions;
2. Describe, with the aid of diagrams, the structure of ATP;
3. State that ATP provides the immediate source of energy for biological
processes;
4. Explain the importance of coenzymes in respiration, with reference to NAD
and coenzyme A;
5. State that glycolysis takes place in the cytoplasm;
6. Outline the process of glycolysis beginning with the phosphorylation of
glucose to hexose bisphosphate, splitting of hexose bisphosphate into two
triose phosphate molecules and further oxidation to pyruvate, producing a
small yield of ATP and reduced NAD;
7. State that, during aerobic respiration in animals, pyruvate is actively
transported into mitochondria;
8. Explain, with the aid of diagrams and electron micrographs, how the
structure of mitochondria enables them to carry out their functions;
9. State that the link reaction takes place in the mitochondrial matrix;
10. Outline the link reaction, with reference to decarboxylation of pyruvate to
acetate and the reduction of NAD;
11. Explain that acetate is combined with coenzyme A to be carried to the
next stage;
12. State that the Krebs cycle takes place in the mitochondrial matrix;
13. Outline the Krebs cycle, with reference to the formation of citrate from
acetate and oxaloacetate and the reconversion of citrate to oxaloacetate
(names of intermediate compounds are not required);
14. Explain that during the Krebs cycle, decarboxylation and dehydrogenation
occur, NAD and FAD are reduced and substrate level phosphorylation
occurs;
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15. Outline the process of oxidative phosphorylation, with reference to the
roles of electron carriers, oxygen and the mitochondrial cristae;
16. Outline the process of chemiosmosis, with reference to the electron
transport chain, proton gradients and ATPsynthase;
17. State that oxygen is the final electron acceptor in aerobic respiration;
18. Evaluate the experimental evidence for the theory of chemiosmosis;
19. Explain why the theoretical maximum yield of ATP per molecule of
glucose is rarely, if ever, achieved in aerobic respiration;
20. Explain why anaerobic respiration produces a much lower yield of ATP
than aerobic respiration;
21. Compare and contrast anaerobic respiration in mammals and in yeast;
22. Define the term respiratory substrate;
23. Explain the difference in relative energy values of carbohydrate, lipid and
protein respiratory substrates.
Definition of Cell Respiration
Cell respiration is the process whereby energy, stored in complex organic
molecules*, is transferred to ATP in living cells.
ATP provides the immediate source of energy in cells, for biological processes.
*The complex organic molecules referred to above are the respiratory substrates
and include carbohydrates, lipids and proteins
Summary of Energy Transfer between Organisms

Photoautotrophs use sunlight energy to synthesise complex organic
molecules from simple inorganic molecules and ions in photosynthesis

Photoautotrophs are the producers in many food chains – plants, some
protoctists and some bacteria

Heterotrophs are the consumers or decomposers in food chains –
animals, fungi and most bacteria
.
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
Heterotrophs obtain their complex organic molecules and energy, by
feeding on photoautotrophs or other heterotrophs that have fed on
photoautotrophs. Heterotrophs have to feed on other organisms

All organisms must respire to transfer the chemical potential energy of
carbohydrates, lipids and proteins to ATP

The summary below also shows that some of the potential chemical
energy in respired organic molecules is converted to thermal energy. This
is important to maintain cell temperatures suitable for enzyme reactions
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Metabolism and Metabolic Reactions in Cells
Metabolism refers to all the chemical reactions that occur in cells
Most metabolic reactions occur within metabolic pathways
A metabolic pathway has the following features:

A sequence of chemical reactions where the product of one reaction
is the substrate for the next.

Each reaction is catalysed by a particular enzyme.
Enzyme a
Enzyme b
Enzyme c
A  B  C  D
Two Types of Metabolic Pathway - Catabolic and Anabolic
CATABOLIC REACTIONS
ANABOLIC REACTIONS
Involve the break down of larger
molecules into smaller molecules
Involve the synthesis of larger
molecules from smaller molecules
Release energy
Require energy
Examples include:
 Breakdown of glucose to
pyruvate in glycolysis in
respiration
 Digestion of starch to maltose
Examples include:
 Protein synthesis from amino
acids in all living cells
 The synthesis of carbohydrate,
protein and lipid molecules in
photosynthesis
Advantages of metabolic pathways include:

Greater control over the release of energy from catabolic reactions,
preventing cell damage

Intermediate products may be useful themselves, or be substrates for
other pathways

The final product may act as an inhibitor of an earlier enzyme in the
pathway leading to end product inhibition, an important method of
regulating the pathway
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Why Organisms Need Energy and ATP
Energy in the form of ATP is required to drive metabolic reactions in all living
cells. These metabolic reactions include the following:

Active transport of molecules and ions across plasma membranes,
against a concentration gradient

Secretion of large molecules by exocytosis

Endocytosis

Anabolic reactions as detailed in the table on page 4

DNA replication and organelle synthesis in interphase

Movement such as movement of bacterial flagella, eukaryotic cilia and
flagella, muscle contraction and microtubule motors that move organelles
within cells

Phosphorylation of metabolites to activate them – such as
phosphorylation of glucose in glycolysis so that it can be broken down to
release energy

Some energy released from reactions is in the form of thermal energy
and this is important to maintain cell temperatures suitable for enzyme
controlled reactions
ATP (Adenosine Triphosphate)
Structure of ATP
ATP is a mononucleotide consisting of the nitrogenous base adenine, the
pentose sugar ribose, (adenine + ribose = adenosine) and three phosphate
groups (rather than the usual one in a nucleotide).
Number the carbon atoms of ribose in the diagram above
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Draw and label the components of a molecule of ATP
Features of ATP

ATP is one of the main products of aerobic and anaerobic respiration in
cells

ATP has a universal role as an energy source in living cells. ATP is
often referred to as the universal energy currency of a cell

ATP is an immediate source of energy in a cell

When hydrolysed, a small amount of energy is released that will not
damage the cell

ATP can release energy very quickly in the cell as required

In cells, the release of energy by hydrolysis of ATP is coupled with a
metabolic reaction that requires energy eg protein synthesis

To release energy, ATP is broken down by hydrolysis into ADP
(adenosine diphosphate) and inorganic phosphate (Pi). This reaction is
exergonic (releases energy) and is catalysed by the enzyme ATPase
(ATP synthase)
ATP + H2O  ADP + Pi + energy (30.6kJmol-1)

In this reaction, 30.6kJ of energy is released per mole of ATP. This small
amount of energy is often sufficient for an energy requiring process

This reaction is reversible, the reverse reaction is also catalysed by
ATPase (ATPsynthase). ATP is synthesised from ADP and Pi in
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respiration (oxidative phosphorylation and substrate level phosphorylation
reactions)

More energy is released if the ADP product is hydrolysed further
ADP + H2O  AMP + Pi + energy (30.6kJmol-1)
AMP stands for adenosine monophosphate

ATP is an efficient energy donor molecule because the covalent bonds
between the phosphate groups are unstable – the phosphate groups
themselves are negatively charged and repel each other

ATP is a small water-soluble molecule and easily diffuses within the cell to
the site of use

ATP remains in cells in low concentrations and is not normally transported
from one cell to another (although companion cells in phloem tissue do
transport ATP to the phloem sieve tube elements).

ATP can only be stored for a few minutes and therefore, it must be
continuously produced as it provides an immediate source of energy in the
cell. This means that all cells must continually carry out cell respiration
Coenzymes in Respiration

Coenzymes are molecules that are required for the activity of some
enzymes

Two vital coenzymes in respiration are needed for oxidationreduction reactions. These molecules are NAD and FAD

Many reactions in respiration, involve oxidation of substrates by
removal of electrons and protons (hydrogen atoms).

If the oxidized substrate is to remain in this form, the electrons/hydrogen
atoms must be removed from the substrate. This can be done by passing
the electrons and protons (hydrogen atoms) to an electron carrier
molecule such as NAD or FAD

To summarise, the oxidation of a substrate in respiration is coupled to the
reduction of NAD or FAD, as two hydrogen atoms are removed from the
substrate and transferred to the coenzyme NAD or FAD
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
NAD and FAD are called coenzymes because they are needed for the
activity of dehydrogenase enzymes that catalyse oxidation/reduction
reactions
NAD (nicotinamide adenine dinucleotide)

NAD is a di-nucleotide made of two linked nucleotides

Both nucleotides of NAD contain ribose as the pentose sugar and one
phosphate group

One nucleotide has adenine as the nitrogenous base; the other contains
nicotinamide (derived from vitamin B3). Nicotinamide is a ring structure
that can accept hydrogen atoms

When a molecule of NAD has accepted two hydrogen atoms it is reduced.
When it donates the two electrons and protons, it is re-oxidised
Reaction to show the reduction of NAD+
NAD+ +
2H+
+
oxidized
2e-  NADH + H+

reduced
This reaction is reversible and cataysed by an oxidoreductase enzyme called
dehydrogenase.
Note that NADH + H+ may also be written as reduced NAD
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FAD (flavine adenine dinucleotide)

FAD is a di-nucleotide containing ribose, two phosphate groups, adenine
and flavine (a derivative of the B vitamin riboflavine)
Reaction to show the reduction of FAD
FAD
+
2H+
+
oxidised
2e-  FADH2
 reduced

This reaction is reversible and catalysed by a dehydrogenase enzyme

FADH2 may also be written as reduced FAD
Coenzyme A (CoA)

Coenzyme A is not a coenzyme linked to oxidation-reduction reactions

The function of CoA is to transfer ethanoate (acetate) groups from
pyruvate, fatty acids and some amino acids into the Krebs cycle in
respiration. Coenzyme A is used in the link reaction
A Summary of Aerobic and Anaerobic Respiration
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Locations of Stages of Aerobic and Anaerobic Respiration
STAGE
Glycolysis
Link reaction
Krebs cycle
Oxidative
phosphorylation
Anaerobic
respiration
LOCATION IN
CELL
Cytoplasm
Mitochondrial
matrix
Mitochondrial
matrix
Inner
mitochondrial
membrane/cristae
Cytoplasm
Glycolysis
Location - Cytoplasm of the cell
Glycolysis is a metabolic pathway involving ten enzyme controlled reactions. For
the A level examination, you only need to know the pathway in outline
Glycolysis is the first pathway of aerobic respiration. It also occurs in anaerobic
respiration in which it is the sole source of ATP
Summary:
one molecule of Glucose (6C) is converted to two molecules of pyruvate (3C)
There are four main events that take place in glycolysis:
1. Phosphorylation of Glucose to Hexose bis-phosphate

The addition of a phosphate group to glucose forms glucose phosphate
(also referred to as hexose phosphate)

The source of phosphate is ATP, which is hydrolysed to ADP and Pi
Energy is also released for the phosphorylation reaction, during ATP
hydrolysis

A second phosphorylation of the hexose phosphate, also using ATP,
produces hexose bis-phosphate (bis means that the hexose sugar has 2
phosphate groups on different carbon atoms)
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Reasons for phosphorylation of glucose

Phosphorylation makes the glucose more reactive

Phosphorylation also prevents the glucose from leaving the cell
(glucose phosphate is too large to leave the cell)
Summary of the reaction
GLUCOSE
HEXOSE BIS-PHOSPHATE
Complete the detail of each reaction summary, including the number of carbon
atoms within each substrate and product
2. Splitting of Hexose bis-phosphate

Each hexose bis-phosphate molecule is split into two triose phosphate
(3C) molecules. Triose phosphate may be abbreviated to TP
Summary of the reaction
HEXOSE BIS-PHOSPHATE
TRIOSE PHOSPHATE
3. Oxidation of Triose Phosphate and ATP formation by Substrate Level
Phosphorylation

Each triose phosphate molecule loses 2 hydrogen atoms and becomes
oxidized and dehydrogenated

The oxidation of triose phosphate is catalysed by a dehydrogenase
enzyme.

NAD+ accepts the 2 hydrogen atoms from triose phosphate and becomes
reduced to NADH +H+
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
Two molecules of reduced NAD are produced from the 2TP  GP
conversions

The product of triose phosphate dehydrogenation is (3C) Glycerate-3phosphate (glycerate phosphate may be abbreviated to GP)

During the conversion of TP to GP, enough energy is released to directly
phosphorylate ADP to ATP. ATPase/ATP synthase catalyses this
reaction

Two molecules of ATP are produced from the 2 TP  2 GP
conversions. This method of ATP synthesis is called substrate level
phosphorylation
Summary of the reaction
TRIOSE PHOSPHATE
GLYCERATE PHOSPHATE
4. Conversion of Glycerate-3-Phosphate to Pyruvate

Glycerate phosphate is finally converted to pyruvate

During this conversion, there is again, sufficient energy released to
phosphorylate ADP to ATP by substrate level phosphorylation,
catalysed by ATPase/ATP synthase

2 molecules of ATP are produced when 2 molecules of GP are
converted to 2 molecules of pyruvate
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Summary of glycolysis
GLYCERATE PHOSPHATE
PYRUVATE
ATP Yield from One Molecule of Glucose in Glycolysis
4 molecules of ATP produced from substrate level phosphorylation
2 molecules of ATP used up in the initial phosphorylation steps
Overall yield of ATP from 1 molecule of glucose in glycolysis:
4 - 2 = 2 ATP molecules per molecule of glucose
Products of Glycolysis from one molecule of glucose:
2 x pyruvate (3C)
2 x NADH +H+
Net gain of 2 x ATP
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Summary of Glycolysis
Number of molecules
Number of carbon atoms
GLUCOSE
HEXOSE BIS-PHOSPHATE
TRIOSE PHOSPHATE
GLYCERATE PHOSPHATE
PYRUVATE
Mitochondrial Structure

Mitochondria are usually 1.0µm in diameter and up to 5µm long. More
active cells will have larger mitochondria and more of them

All mitochondria have inner and outer mitochondrial membranes.
Together they form the mitochondrial envelope

The outer membrane is smooth and has a similar phospholipid bilayer
structure to other organelle membranes, with channel and carrier proteins
and some enzymes

The inner membrane is folded into cristae. These increase the membrane
surface area. Electron carriers and the stalked particles with ATP
synthase molecules are located within the inner membrane
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
The inter-membrane space is formed between the outer and inner
mitochondrial membranes

The matrix is surrounded by the inner membrane. The matrix is gel-like
and contains a loop of mitochondrial DNA and small (70S) ribosomes.
Mitochondrial Structure to Function

Mitochondria are the site of aerobic respiration in eukaryotic cells,
specifically, the site of the Link reaction, Krebs cycle and oxidative
phosphorylation

The mitochondrial membranes compartmentalize the reactions that occur
in aerobic respiration and confine the enzymes involved within a small
area

The smooth outer mitochondrial membrane is partially permeable and
has carrier proteins for the active transport of pyruvate into the
mitochondrion, from the cytoplasm
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
The matrix is the site of the link reaction and Krebs cycle. For these
reactions, the matrix contains the substrates (eg oxaloacetate), NAD,
FAD, coenzyme A and the enzymes required

The loop of DNA has genes that code for proteins. The 70S ribosomes
are the site of synthesis of proteins such as ATP synthase, proteins acting
as electron carriers and enzymes involved in the link reaction and Krebs
cycle

The inner mitochondrial membrane is the site of oxidative
phosphorylation by which ATP is synthesised

The electron transport chain carriers and ATP synthase (in the
stalked particles) are within the inner mitochondrial membrane. The
stalked particles have a protein channel that allows protons to pass
through them by chemiosmosis. This proton flow is linked to ATP
synthesis

Apart from the protein channels in the stalked particles, the inner
membrane is impermeable to small ions such as protons. This allows the
protons to accumulate in the inter-membrane space, building up a proton
gradient between the space and the matrix. This is the source of potential
energy used to synthesise ATP

The increased surface area of the cristae increases the number of
electron carriers and the number of ATP synthase enzymes, for increased
ATP synthesis
16
X 200,000
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The Link Reaction
Location - The matrix of the mitochondrion.

Pyruvate (produced in the cytoplasm from glycolysis) must first pass into
the mitochondrion matrix. It passes across the mitochondrial
membranes, by active transport

The link reaction requires coenzyme A (CoA) to transfer a 2 carbon
acetate group from pyruvate into the Krebs cycle reactions
Summary of the link reaction
2 Pyruvate + 2 CoA + 2 NAD+  2 acetyl CoA + 2 CO2 + 2 NADH + H+
The link reaction involves
1. Pyruvate losing CO2 by de-carboxylation, catalysed by a
decarboxylase enzyme
2. Pyruvate transferring an acetyl group to coenzyme A to form acetylCoA
3. Pyruvate losing 2 hydrogen atoms by dehydrogenation, catalysed by a
dehygrogenase. The 2 hydrogen atoms are accepted by NAD+ ,
producing reduced NAD+ /NADH+H+
4. Pyruvate is oxidized and dehydrogenated in the link reaction
5. The link reaction takes place twice for each molecule of glucose as two
molecules of pyruvate are produced from one molecule of glucose.
The overall final products of the Link Reaction, from one molecule of glucose,
are as follows:
2 x Acetyl Coenzyme A
2 x CO2
2 x NADH + H+
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Krebs Cycle
Location: the matrix of the mitochondrion
Summary of the Krebs Cycle reactions

Oxaloacetate (4C) combines with acetate (2C) from acetyl CoA to form
citrate (6C)

The cycle involves a series of enzyme controlled reactions that convert
citrate (6C) back to oxaloacetate (4C), so that it may be re-used in the
cycle

During the reactions, CO2 is produced as a waste product

Several molecules of reduced coenzymes are produced in
oxidation/reduction reactions, involving the transfer of hydrogen atoms
from intermediates in the cycle, to NAD+ or FAD

A little ATP is also produced directly by substrate level phosphorylation
Three main processes involved in one cycle
1. Removal of carbon dioxide from intermediates by decarboxylation
Decarboxylation occurs twice in each cycle, once when a 6C molecule is
being converted to a 5C molecule and again when a 5C molecule is
converted to a 4C molecule. Two molecules of CO2 are formed per turn of
the cycle.
These decarboxylation reactions are catalysed by decarboxylase enzymes
2. Oxidation/Dehydrogenation
Removal of 2 hydrogen atoms from an intermediate occurs four times in
each cycle producing three molecules of NADH/H+ and one molecule of
FADH2.
The intermediate is oxidised/dehydrogenated and NAD+ and
FAD are reduced
3. Phosphorylation of ADP
This happens once in each cycle producing one molecule of ATP directly,
by substrate level phosphorylation. The conversion of one intermediate to
another releases sufficient energy for ATP synthesis from ADP and Pi.
19
Krebs Cycle occurs Twice per Molecule of Glucose
Remember that the cycle is performed twice for each molecule of glucose as
two pyruvate molecules, and therefore, two acetyl CoA, molecules are
produced from each molecule of glucose.
Final products of Krebs cycle, for one molecule of glucose, are as follows:
4 x CO2
6 x NADH + H+
2 x FADH2
2 x ATP (directly)
Summary of Krebs Cycle
2C acetyl-CoA
CoA
acetate
4C oxaloacetate
6C citrate
4C intermediate
5C intermediate
4C intermediate
4C intermediate
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Oxidative Phosphorylation
Location: Cristae/inner mitochondrial membrane
Aims of Oxidative Phosphosphorylation

To synthesise ATP (most of the ATP produced in aerobic respiration is
synthesised in this process)

To re-oxidise NADH + H+ and FADH2 so that the processes of
glycolysis, link reaction and Krebs cycle may continue
Processes occurring in oxidative phosphorylation

In the process of re-oxidising NADH + H+ and FADH2, electrons are
passed from NADH + H+ and FADH2 to a series of electron carriers in
the electron transport chain (ETC). These electron carriers are located
in the inner membrane of the mitochondrion and are enzyme complexes

The first electron carrier to accept electrons from reduced NAD is a protein
complex called complex I. This complex contains reduced NAD
dehydrogenase enzyme.

At complex I, reduced NAD releases 2 protons and 2 electrons. The
protons are released into solution in the matrix and the electrons are
donated to complex I which becomes reduced.

The electrons are transferred along a series of complexes in the inner
mitochondrial membrane. Each complex becomes reduced as it accepts
the electrons and then re-oxidised as it transfers the electrons to the next
carrier in the chain

The final electron acceptor in the chain is oxygen, which is reduced to
water. Both electrons and protons are required to reduce water, as
shown below
O2 + 4H+ + 4e-  2H2O
(or ½ O2 + 2H+ + 2e H2O)

The transfer of electrons along the electron carriers releases free energy,
since each carrier is at a lower energy level than the previous carrier. This
energy is used to synthesise ATP from ADP and Pi

As re-oxidation of NADH + H+ and FADH2 only takes place in the
presence of oxygen, and is coupled with phosphorylation of ADP, the
processes occurring on the cristae are known as oxidative
phosphorylation
21
A Simplified Version of the Electron Transport Chain
ATP Synthesis in Oxidative Phosphorylation and the Chemiosmosis Theory

The energy released from the electrons as they are transferred along the
electron transport chain, is used to pump the protons (H+) out of the matrix
and into the inter-membranal space. This is not active transport since
ATP is not required. The energy required for the proton pumps is
released during electron flow in the electron transport chain

A high concentration of protons accumulate in the inter-membranal space,
creating an electrochemical gradient of protons between the intermembranal space and the matrix
22

There are (protein) proton channels in the stalked particles, located in
the cristae membrane, that allow diffusion of these protons from the intermembranal space back into the matrix, down a proton gradient

The stalked particles have ATP synthase located in their structure.
When protons diffuse through the protein channel of the stalked particles
back into the matrix, the kinetic energy released due to the proton motive
force is used to generate ATP from ADP and Pi

The diffusion of protons through the stalked particles is called
chemiosmosis (although nothing to do with water potentials!)
Error!
Yield of ATP from Oxidative Phosphorylation

Re-oxidation of one molecule of NADH/H+ releases 2.6 ATP molecules

Re-oxidation of one molecule of FADH2 releases 1.6 ATP molecules (this
is because FADH2 transfers its electrons to an electron transport carrier
further down the chain than NADH + H+ - check the top diagram on page
20
23
Overall Yield of ATP from One Molecule of Glucose in Aerobic Respiration
Name of Stage
Source of ATP
Glycolysis
Substrate level
phosphorylation
Substrate level
phosphorylation
2 x NADH + H+
2 x NADH + H+
6 x NADH + H+
2 x FADH2
Krebs Cycle
Glycolysis
Link Reaction
Krebs Cycle
Krebs Cycle
Number of ATP molecules
per glucose molecule
2
2
5.2
5.2
15.6
3.2
Total is approx. 33 ATP
molecules
Why is the theoretical yield of ATP from aerobic respiration rarely
achieved?
According to the table above, approximately 33 ATP molecules should be
synthesised from each glucose molecule in aerobic respiration.
However, this total of ATPs is rarely achieved because:

Some protons leak across the outer mitochondrial membrane, reducing
the proton concentration for generating the proton motive force

Some ATP is used to actively transport pyruvate into the mitochondrion
from the cytoplasm

Some ATP is used to bring hydrogen from reduced NAD made in
glycolysis, into the mitochondrial matrix (the inner membrane is
impermeable to reduced NAD)
Efficiency of Aerobic Respiration using Glucose as the Respiratory
Substrate

Aerobic respiration is a relatively efficient process for releasing energy
from a fuel, such as glucose

Less than 40% of the energy in glucose is used to synthesise ATP

The remaining energy is lost as heat, which in mammals warms the cells
and the blood. Much heat is lost eventually to the atmosphere.
24
Evaluation of the Evidence for Chemiosmosis
The chemiosmotic theory was devised by Peter Mitchell in 1961 and at first,his
theory was treated with scepticism.
Now there is much evidence in support and he was awarded the Nobel Prize for
Chemistry in 1978
Scientists now know:

That the stalked particle is ATP synthase

That some of the complexes in the electron transport chain have
coenzymes that can use the energy released from electron transport to
pump protons across the inner membrane into the inter-membrane space

When mitochondria are placed into a solution of higher water potential, the
outer membrane ruptures releasing the contents of the inter-membranal
space. Electron transport in these mitochondria did not produce ATP
suggesting that the space was important for ATP synthesis

ATP synthesis does not occur in the presence of oligomycin, an antibiotic
that blocks the flow of protons through ATP synthase

ATP is not synthesised if the head of the stalked particle is removed.
25

In intact mitochondria, the potential difference across the inner membrane
is -200mV, being more negative on the matrix side of the membrane
(lower concentration of positive ions)

Also, in intact mitochondria, the pH of the inter-membrane space is lower
than that of the matrix

The outer mitochondrial membrane is permeable to protons. If isolated
mitochondria are supplied with ADP and inorganic phosphate and placed
in a solution of pH 8, no ATP is produced. If however, these mitochondria
are placed in an acidic solution, ATP is produced. This indicates that the
protons must accumulate in the inter-membrane space
Anaerobic Respiration

Anaerobic respiration is respiration in the absence of oxygen
Consequences of a lack of oxygen

The electron transport chain cannot function as there is no oxygen to
act as the final electron acceptor

NADH + H+ and FADH2 produced from the link reaction or Krebs cycle,
cannot be re-oxidized, without the ETC

The link reaction and Krebs cycle cannot take place, without a supply of
NAD+ and FAD

Oxidative phosphorylation cannot occur and therefore. ATP cannot be
synthesized in this process
26

The only stage that does generate ATP, in anaerobic respiration, is
glycolysis, although far less ATP ( 2 ATP molecules per molecule of
glucose) is produced in this pathway on its own

However, for glycolysis to continue, NAD+ must be regenerated from
NADH + H+ and this occurs by another mechanism, not involving the ETC
.
Two alternative mechanisms for the re-oxidation of NADH + H+ are used by
living organisms:
Anaerobic Respiration in Yeast and Green Plants
Site of anaerobic respiration: in the cytoplasm

First of all, glycolysis occurs to produce pyruvate and NADH + H+

Pyruvate is then decarboxylated (CO2 is removed) to produce ethanal.
This reaction is catalysed by pyruvate decarboxylase

The ethanal is then reduced to ethanol, using 2 hydrogen atoms from
NADH + H+. this reaction is catalysed by ethanol dehydrogenase

This means that the NADH + H+ is re- oxidized to NAD+ that can be reused in glycolysis, so that glycolysis can continue

The energy yield from glycolysis and hence anaerobic respiration is much
lower than from aerobic respiration. 2 ATP molecules are released per
molecule of glucose substrate. Some energy remains in the ethanol
product

Ethanol is toxic and if allowed to build up will eventually kill the yeast/plant
cells

Anaerobic fermentation in yeast cells is an important process exploited in
the production of beer and wine. It is therefore also known as alcoholic
fermentation.
Complete the equation below
PYRUVATE
ETHANAL
27
ETHANOL
Note the error in the flow diagram above
Anaerobic respiration in Animals (in muscle)
Site of anaerobic respiration: the cytoplasm

Again, glycolysis continues in the absence of sufficient oxygen, to
produce pyruvate and NADH + H+ in the muscle cells

Pyruvate is reduced directly by the transfer of 2 hydrogen atoms from
NADH + H+. the product is lactate and NAD+ is regenerated

The regenerated NAD+ can then be re-used in glycolysis
Complete the equation below
PYRUVATE
LACTATE
28

Lactate will build up in muscles during heavy exercise, causing muscle
fatigue, as lactate is toxic

However the reaction described above is reversible and the lactate can
be metabolized when more oxygen becomes available.

The lactate is transported to the liver where it is oxidized back to
pyruvate, which can then be aerobically respired. Liver cells can tolerate
lactate/low pH and have the enzymes to metabolise lactate, unlike muscle
cells. Also, the conversion of lactate requires oxygen which is deficient in
muscle cells during anaerobic respiration

Alternatively the pyruvate can be converted to glycogen and stored. in
liver cells

The oxygen needed to fully oxidize the lactate produced by anaerobic
respiration is called the oxygendebt.
REMEMBER THAT ANAEROBIC RESPIRATION ALWAYS STARTS WITH
GLYCOLYSIS. IF YOU ARE ASKED TO DESCRIBE ANAEROBIC
RESPIRATION IN MAMMALS OR YEAST, YOU MUST DESCRIBE THE
DETAILS OF GLYCOLYSIS FIRST
29
Comparison of Anaerobic respiration in Yeast and Mammals
YEAST
MAMMALS
Hydrogen acceptor
Is carbon dioxide
produced?
Is ATP produced?
Is NAD re-oxidised?
End products
Enzymes involved
Respiratory Substrates
A respiratory substrate is an organic molecule that can be used for respiration
Energy Values of Different Respiratory Substrates

Most ATP is produced in oxidative phosphorylation as protons flow
through ATP synthase

It follows that the more hydrogen atoms a substrate has per molecule, the
more protons will flow and the more ATP will be produced in respiration

It also follows that a substrate containing more hydrogen atoms per mole
will require more oxygen per mole of substrate for respiration
Carbohydrate

Glucose is the main respiratory substrate in mammalian cells. Brain cells
can only respire glucose

Animal cells store glycogen and plant cells store starch. Both
polysaccharides can be hydrolysed to glucose as a respiratory substrate

Fructose and galactose, both hexose sugars are converted to glucose for
respiration

The theoretical energy yield from glucose is 2870kJmol-1
30

Therefore, the theoretical ATP yield is 94mol per mol of glucose (the
hydrolysis of ATP to ADP + Pi produces 30.6kJmol-1)

The actual ATP yield is around 30 moles of ATP; an efficiency of 32%

The remaining energy is lost as heat but this is important to maintain
enzyme activity
Protein

Excess amino acids are deaminated in the liver. The keto acids produced
can be metabolised to glycogen or fat for storage

When an individual is fasting or undergoing vigorous exercise, amino
acids from muscle tissue can be respired

Some amino acids can be converted to pyruvate, some to acetate and
some enter the Krebs cycle directly

One mol of protein contains slightly more hydrogen atoms than one mol of
glucose. Therefore slightly more ATP is produced per mol of protein
Lipids

Lipids are important respiratory substrates particularly in muscle

Glycerol from triglycerides can be converted to glucose and then respired
31

Fatty acids have hydrocarbon chains, some with many hydrogen atoms

These hydrogen atoms are a source of many protons for chemiosmosis,
so fatty acids produce many ATP molecules

Each fatty acid combines with CoA. This requires energy from the
hydrolysis of ATP to AMP + 2Pi

The fatty acid-CoA complex enters the mitochondrial matrix where 2C
acetate groups are successively removed from the fatty acid, and
combined with CoA. This pathway is called β-oxidation

The acetyl CoA complexes enter Krebs cycle

The β-oxidation pathway produces reduced NAD and reduced FAD (one
molecule of each for each acetate group released). A long chain fatty acid
therefore produces many reduced NAD and reduced FAD molecules both
in β-oxidation and Krebs cycle
Energy Values of Different Respiratory Substrates
Respiratory Substrate
Carbohydrate
Lipid
Protein
Energy Content kJg-1
15.8
39.4
17.0
32
33
Measuring Rates of Respiration using Respirometers
The rate of respiration of small organisms can be determined by measuring the
volume of oxygen taken in over a known time, using a respirometer.
One type of respirometer is shown below
The respirometer consists of:

Two identical chambers – one containing the respiring organisms
(woodlice in this experiment) and the other a control chamber containing
an equal volume of non-respiring material such as glass beads. the glass
beads must be put into the control chamber (A) otherwise A would contain
more air than B

The two chambers are connected by a U-shaped manometer tube
containing a coloured fluid

The point of the control chamber is to ensure that any
temperature/pressure fluctuations affect both chambers equally and
therefore cancel each other out
34

An equal volume of a substance that absorbs CO2 – such as soda lime, is
added to each chamber
Use of this respirometer to measure the effects of temperature on the rate
of respiration
Method:

The apparatus is left in the water bath for 10min to allow it to reach the
required temperature. Screw clips A and B are left open during this time
to allow expanded air to escape

After 10 min the screw clips A and B are closed. The level of the fluid in
the manometer tube is equal on both sides

The woodlice respire, using up O2 and releasing CO2

The CO2 released from the respiring woodlice in chamber B is absorbed
by the soda lime

Since the woodlice are removing O2 from the air in chamber B, there is a
reduction in B’s air volume

Since chamber B is air-tight, there is a reduction in air pressure in
chamber B

The air pressure in chamber A is now greater than that in chamber B, so
air moves from chamber A to chamber B causing the fluid in the U-tube to
move towards tube B

The distance moved by the manometer fluid in a given time is measured

Clip A is opened and the syringe plunger above tube A is raised to equate
the air pressures in both A and B (fluid is at equal height ib both sides of
the manometer tube)

The method is repeated twice more at the same temperature to collect
replicate results

Both clips A and B are opened to allow oxygen to diffuse back into
chamber B

The procedure is repeated at a different temperature
35
Calculating the volume of oxygen used up per unit time
Volume = πr2h
Where r is the internal radius of the manometer tube and h is the distance moved
by the fluid per unit time
What effect will temperature have on respiration rate?

As temperature increases, respiration rate will increase. This is because
at higher temperatures, enzyme and substrate molecules have increased
kinetic energy and energetically favourable collisions are more likely

The important enzymes affected by temperature in respiration are the
decarboxylases, dehydrogenases and ATP synthase

At higher temperatures, enzymes may be denatured and respiration rate
will decrease
A simpler respirometer used in the laboratory
36

The simple respirometer shown on page 35 contains pea seeds previously
soaked in water to start their germination

When soaked seeds germinate, they start to grow a root and a shoot.
Growth requires repeated mitosis and cell division, DNA replication and
protein synthesis. All of these processes require more ATP – hence
soaked seeds have an increased respiration rate

A control could be set up with an equivalent volume of glass beads or dry
pea seeds.

Dry pea seeds will have a very slow growth rate (or none at all) and
reduced/no respiration because water is required as a medium for
metabolic reactions – water allows enzymes and substrates to move and
collide so that reactions can occur
Calculating Respiratory Quotients
This is not strictly on the syllabus but is an application of using respirometers.
Therefore, it is worthwhile you being familiar with the concept of respiratory
quotients
The respiratory quotient (RQ) is a measure of the ratio of carbon dioxide given
out by a respiring organism to the oxygen consumed over a given time period:
RQ
=
volume of CO2 given out / volume of O2 taken in

Different RQ values indicate the type of respiratory substrate

Glucose gives an RQ of 1.0.

Lipids contain more carbon and less oxygen and therefore give out more
CO2. Their RQs are lower than carbohydrates (approx. 0.7)

Proteins give variable RQ values depending upon the types of amino
acids they contain. Most proteins give an RQ value of approx.. 0.9

Most respiring organisms have an RQ of between 0.8 to 0.9. This
suggests that protein is the main respiratory substrate but this would only
be the case in starvation. Therefore, values of 0.8-0.9 suggest a mixture
of lipids and carbohydrates in the diet as respiratory substrates
37
Studying respiration using redox indicators

A redox indicator is one that changes colour when it accepts electrons and
becomes reduced

One example is TTC (2,3,5-triphenyl-tetrazolium chloride) which is
colourless when oxidised and pink when reduced

TTC will diffuse into cells and accept electrons from the electron transport
chain if the cells are actively respiring

The speed of the colour change is an indication of the speed of metabolic
activity. A faster change in colour indicates more reduced NAD
production in glycolysis, link reaction and Krebs cycle

There will then be more electron transfer to TTC and a faster TTC colour
change to pink
Examination tip
OCR has a habit of including applied questions in their papers, often based
on data from an experiment.
In your answers you must include A level factual information so think hard
about the scientific background to the data and include this information
38