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F214 - Respiration
1. Outline why plants, animals and microorganisms need to respire, with
reference to active transport and metabolic reactions.
Respiration is the process in which energy, that is stored in complex organic molecules
in living cells, is used to make ATP. ATP is necessary to drive all the necessary metabolic
process inside the body, such as:
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Active transport - much of an organism’s energy is used for this
Secretion- large molecules made in some cells are released by exocytosis
Endocytosis- bulk movement of larger molecules into the cell
Metabolic reactions- synthesis of large molecules from smaller ones e.g. proteins
from Amino Acids, steroids from cholesterol, cellulose from β-glucose = anabolic
reactions
Catabolic reactions = hydrolysis of larger molecules to produce smaller ones.
Replication of DNA and synthesis of organelles before a cell divides
Movement of Bacterial flagella, Eukaryotic cilia and undulipodia.
Muscle contractions
Activation of chemicals e.g. phosphorylation of glucose
2. Describe, with the aid of diagrams, the structure of ATP
ATP is a phosphorylated nucleotide found in
both prokaryotic and eukaryotic cells. Each
molecule consists of adenosine (adenine and
ribose sugar) plus 3 phosphate/phosphoryl
groups.
ATP can be hydrolysed to ADP and inorganic
phosphate (Pi). This releases immediate
energy to cells, in adequate amounts, for
biological processes.
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 (CoA).
Coenzymes aid enzymes in oxidation and reduction reactions of respiration:
> NAD combines with hydrogen atoms in oxidation, taking them to the mitochondrial
membrane where they later split into hydrogen ions and electrons for the election
transport chain and the production of ATP in oxidative phosphorylation. When NAD
accepts 2 hydrogen ions it is reduced. NAD is important to most stages of respiration.
>CoA carries acetate groups from the link reaction, or those made from fatty/amino
acids onto the Krebs cycle.
F214 - Respiration
5. State that Glycolysis occurs in the cytoplasm of cells
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.
1) An ATP molecule is hydrolysed and the phosphate lost attaches to a
glucose molecule at C6
2) = Glucose-6-Phosphate, which becomes fructose 6 phosphate
3) Another ATP is hydrolysed, and the phosphate attaches to C-1 4) The hexose sugar is activated by the energy release from the hydrolysed
ATP molecules, preventing it from leaving the cell. This becomes Hexose1,6-bisphosphate 5) Hexose-1,6-bisphosphate is split into 2 molecules of Triose Phosphate (3
carbon sugar molecules)
6) 2 hydrogen atoms are removed from each Triose Phosphate (the
substrate). This involves dehydrogenase enzymes. 7) NAD combines with the removed hydrogen atoms to form reduced NAD 8) At this stage 2 molecules of ATP are formed (substrate level
phosphorylation).
9) 4 enzyme-catalysed reactions convert each triose phosphate molecule to a
3C molecule of pyruvate. 10) 2 more molecules of ATP are formed so there is a net gain of 2 ATP. ATP is used in the phosphorylation stages and produced in oxidation and the
production on pyruvate. NAD is reduced during oxidation.
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
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Allows protons to pass through inner membrane
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The ETC contain 100s of oxidoreductase enzymes - involved in oxidation
and reduction reactions. Some of the electron carriers also have a coenzyme that pumps protons from the matrix to the intermembrane space.
F214 - Respiration
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
The link reaction converts pyruvate (produced during glycolysis) into acetate (a 2
carbon compound) by decarboxylation and dehydrogenation, catalysed by enzymes.
> Pyruvate dehydrogenase removes hydrogen atoms from pyruvate.
> Pyruvate decarboxylation removes a carboxyl group from pyruvate.
> NAD accepts the hydrogen atoms to become reduced NAD and carries the hydrogen to
the mitochondrial matrix for the production of ATP in oxidative phosphorylation.
11. Explain that acetate is combined with coenzyme A to be carried to the next
stage
Coenzyme A accepts acetate to become acetyl coenzyme A. The function of coenzyme A
is to carry acetate to the Krebs cycle.
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)
1) Acetate is released from CoA and joins to oxaloacetate to
form citrate.
2) Citrate is decarboxlyated (carboxyl group becomes CO2) and
dehydrogenated (H is then accepted by NAD to form reduced
NAD and taken to the electron transport chain) = forms a 5C
compound.
3) The 5C compound is decarboxylated and dehydrogenated to
form a 4C compound and another molecule of reduced NAD.
4) The 4C compound is changed into another 4C compound,
and a molecule of ADP is phosphorylated to form ATP.
5) The second 4C compound is changed into a third 4C
compound. A pair of hydrogen atoms are removed and
accepted by FAD, reducing FAD.
There is one cycle for each molecule of acetate (which is
made from one molecule of pyruvate) Therefore the cycle
occurs twice for each molecule of glucose. Therefore 6x
Reduced NAD, 2x Reduced FAD, 4x CO2 and 2x ATP is
produced by Krebs for each molecule of glucose.
6) The third 4C compound is further dehydrogenated and
regenerates oxaloacetate. Another molecule of NAD is reduced.
F214 - Respiration
14. Explain that during the Krebs cycle, decarboxylation and dehydrogenation
occur, NAD and FAD are reduced and substrate level phosphorylation
occurs
> Decarboxylation and dehydrogenation occurs in 2), 3) and 6)
> NAD is reduced 3 times, when dehydrogenation occurs in 2), 3) and 6)
> FAD is reduced when a pair of hydrogen atoms are removed in 5)
> Substrate level phosphorylation occurs when ADP is phosphorylated in 4)
15. Outline the process of oxidative phosphorylation, with reference to the roles
of electron carriers, oxygen and the mitochondrial cristae
Reduced NAD and reduced FAD need to be oxidised again in order for reactions to
continue, this occurs during oxidative phosphorylation and transfers energy to ATP.
Oxidative phosphorylation is known as the formation of ATP by the addition of a
phosphate group to ADP, in the presence of oxygen, and occurs as follows:
Reduced NAD/FAD provide hydrogen atoms and, when oxidised, protons and electrons.
These electrons pass along the electron transport chain in the inner mitochondrial. This
provides the energy for the active transport of protons, pumping from the matrix into
the intermembrane space before they flow through ATP synthase enzymes and drive the
rotation part of the enzyme to join ADP and Pi to make ATP.
The electrons are passed from the final electron carrier to oxygen, which is the final
electron acceptor and can be reduced to water with hydrogen: 4H+ + 4e- + O2  2H2O
16. Outline the process of chemiosmosis, with reference to the electron
transport chain, proton gradients and ATP synthase.
This is the process by which protons are pumped
only through protein complexes in the ETC (in the
inner membrane), increasing the concentration in the
intermembrane space to create a proton gradient and
a potential difference across the inner membrane.
Protons can only diffuse through channels in ATP
synthase across the gradient. This proton gradient
and potential difference are a ‘proton-motive force’,
which provides energy to synthesise ATP in the
chloroplasts, mitochondria or bacteria.
In chloroplasts: ATP is made when protons move
from the thylakoid space, through the thylakoid membrane, into the stroma.
In mitochondria: ATP is made when protons move from the intermembrane space,
through the inner mitochondrial membrane/cristae, into the matrix.
F214 - Respiration
17. State that oxygen is the final electron acceptor in aerobic respiration
18. Evaluate the experimental evidence for the theory of chemiosmosis
Peter Mitchell realised that the movement of ions across an electrochemical membrane
potential could provide the energy needed to produce ATP. His theory was confirmed by
the discovery of ATP synthase, a membrane-bound protein that uses the potential energy of
the electrochemical gradient to make ATP.
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The pH in the intermembrane space is lower than in the mitochondrial matrix
and is lower in the thylakoid spaces than in the stroma. Protons can lower the
pH of a solution, thus showing that protons are of higher concentration in the
intermembrane spaces.
When isolated chloroplasts are illuminated, the medium in which they are
suspended becomes alkaline - as we would predict if protons were being
removed from the medium and pumped into the thylakoids.
Isolated grana (stacks of thylakoids) kept in an acid medium can make ATP
when transferred into an alkaline solution in the dark and given ADP and
phosphate. The acid solution creates a proton gradient, which protons move
down, into the thylakoid spaces. The alkaline solution provides an environment
like in the stroma (of low proton concentration), meaning that there is now a
proton gradient from the thylakoid space to the stroma. Here, ATP is formed
from added phosphate and ADP by diffusing from the thylakoid space through
ATP Synthase, in the thylakoid membrane, into the stroma.
19. Explain why the theoretical maximum yield of ATP per molecule of glucose
is rarely, if ever, achieved in aerobic respiration
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Some protons leak across the mitochondrial membrane, meaning there are
fewer protons to generate the proton-motive force.
Some ATP is used to actively transport pyruvate into the mitochondria Some ATP is used to bring Hydrogen from reduced NAD made during glycolysis,
into the mitochondria from the cytoplasm.
20. Explain why anaerobic respiration produces a much lower yield of ATP than
aerobic respiration
Anaerobic respiration is the release of energy from substrates, such as glucose, in the
absence of oxygen. In anaerobic respiration, glycolysis is the only process that occurs.
The electron transport chain cannot occur, as there is no oxygen to act as the final
electron acceptor. Thus NAD is all reduced, meaning the Krebs cycle stops, which in turn
prevents the link reaction from occurring.
Anaerobic respiration takes the pyruvate, and by reducing it, frees up the NAD, so
glycolysis can continue, producing two molecules of ATP per glucose molecule respired.
This is a much lower yield of ATP compared to what aerobic respiration can produce.
F214 - Respiration
21. Compare and contrast anaerobic respiration in mammals and in yeast
MAMMALS
Pyruvate combines with a hydrogen atom,
which is provided by reduced NAD. This
reaction forms lactate and oxidised NAD in
the muscles and some tissues
YEAST
Pyruvate converts into ethanal by
decarboxylation, producing CO2. Ethanal
combines with hydrogen from reduced
NAD, forming ethanol
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Net gain of ATP per molecule of
glucose is 2 ATP
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Net gain of ATP per molecule of
glucose is 2 ATP
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Reduced NAD is oxidised, pyruvate
is the hydrogen acceptor
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Reduced NAD is oxidised, ethanal
is the hydrogen acceptor
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Decarboxylation isn’t involved
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Decarboxylation is involved
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Lactate is recycled in the liver to
glucose/glycogen, which preserves
the energy and can make pyruvate
again. So, the reaction is reversible.
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Yeast and plants cannot metabolise
ethanol into another substance.
Therefore this form of anaerobic
respiration is irreversible.
22. Define the term respiratory substrate
An organic substance that can be used for respiration.
23. Explain the difference in relative energy values of carbohydrate, lipid and
protein respiratory substrates.
The higher the number of hydrogen atoms per mole, the higher the relative energy
value, as more NAD molecules can be reduced and used in the Electron Transport Chain.
Lipids have the most number of hydrogen atoms per mole, followed by proteins, and
then carbohydrates.
About twice as much ATP can be made from the complete oxidation of one gram of lipid
compared with one gram of either carbohydrate or protein.