Respiration is a process where organic molecules act as fuel and are broken down through a series of steps to release energy in the form of ATP. Glucose breakdown occurs in four stages - glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. In glycolysis, glucose is broken down to pyruvate, generating a small amount of ATP. In the link reaction, pyruvate enters the mitochondria and is converted to acetyl-CoA. The Krebs cycle further breaks down acetyl-CoA, producing carbon dioxide and hydrogen carriers. Finally, oxidative phosphorylation uses the hydrogen carriers to power the electron transport chain and generate most of the ATP through phosphorylation.

1. Respiration as an energy
transfer process
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2. Respiration
• A process in which organic molecules act as a
fuel
• Molecules broken down to release chemical
potential energy which is used to synthesise ATP
• Many cells can only use glucose as their
respiratory substrate but others break down
fatty acid, glycerol and amino acids in
respiration
• Glucose breakdown occur in 4 stages:
– Glycolysis
– Link reaction
– Krebs cycle
– Oxidative phosphorylation
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3. Glycolysis
• Splitting/lysis of glucose
• 6C glucose molecule to 2 molecules of 3C
pyruvate
• ATP needed at first but energy released in
later steps can be used to make ATP
• Net gain of 2 ATP molecules per glucose
molecule broken down
• Takes place in cytoplasm
• Pyruvate enters link reaction in
mitochondria
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5. Link reaction
• Pyruvate passed from cytoplasm into the
mitochondrial matrix (active transport)
• Decarboxylated (CO2 removed), dehydrogenated
and combined with coenzyme A (CoA) to give
acetyl coenzyme A
• Coenzyme A:
– adenine + ribose + pantothenic acid
– acts as a carrier of acetyl groups to Krebs cycle
• Pyruvate + CoA + NAD ↔ acetyl CoA + CO2 + reduced NAD
• Fatty acids from fat metabolism also used to
produce acetyl coenzyme A
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6. Krebs cycle
• aka citric acid cycle/tricarboxylic acid cycle
• Closed pathway of enzyme-controlled reactions:
– Acetyl CoA + oxaloacetate (4C) → citrate (6C)
– Citrate decarboxylated and dehydrogenated to yield
CO2 (waste) and hydrogens (accepted by NAD and
FAD)
– Oxaloacetate is regenerated to combine with another
acetyl CoA
• For each turn of the cycle, 2 CO2 produced, 1
FAD and 3 NAD reduced and 1 ATP generated
• O2 not used
• Most important contribution: release of
hydrogen (used in oxidative phosphorylation to
provide energy to make ATP)
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9. Oxidative phosphorylation
• Energy for the phosphorylation of ADP to ATP
comes from the activity of the electron transport
chain (mitochondrial membranes)
• NADH and FADH2 are passed to the electron
transport chain (ETC)
• Hydrogen H+ and e-
• e- transferred to the first of a series of electron
carriers; H+ remains in solution in mitochondrial
matrix
• e- transferred to O2 (in solution), H+ drawn from
solution to reduce O2 to H2O
• Transfer of e- along series of electron carriers
makes energy available which is used to convert
ADP + Pi to ATP
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11. • Potentially, 3 ATP from each NADH
molecule/2 ATP from each FADH2
molecule
• This yield cannot be realised unless ADP
and Pi are available inside the
mitochondrion
• 25% of total energy yield is used to
transport ADP into the mitochondrion and
ATP into cytoplasm
• Each NADH molecule entering the chain
produces on average 2½ molecules of
ATP and each FADH2 produces 1½
molecules of ATP
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13. Sites of events of respiration in a cell
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15. Anaerobic respiration
• When free oxygen is not present, hydrogen
cannot be disposed of by combination with
oxygen
• ‘dumping’ hydrogen stops ETC and affects
glycolysis
• 2 anaerobic pathways in cytoplasm which solve
the problem:
– Ethanol pathway (yeast and some plant tissues)
– Lactate pathway (microorganisms and mammalian
muscles)
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16. • Ethanol pathway
– Hydrogen from NADH is passed to ethanal (CH3CHO),
releasing NAD and allows glycolysis to continue
– Alcoholic fermentation: pyruvate decarboxylated
to ethanal; ethanal reduced to ethanol (C2H5OH) by
alcohol dehydrogenase
• Lactate pathway
– Pyruvate acts as the hydrogen acceptor and is
converted to lactate by lactate dehydrogenase
– NAD is released and allows glycolysis to continue
• These reactions allow continued production of
ATP even though oxygen is not available as the
hydrogen acceptor
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17. • However, ethanol and lactate are toxic, and so
reactions cannot continue indefinitely
• Pathway leading to ethanol cannot be reversed
and the remaining chemical potential energy of
ethanol is wasted
• Lactate pathway can be reversed in mammals
(carried by blood plasma to the liver and
converted back into pyruvate; 20% oxidised to
CO2 and H2O via aerobic respiration when O2 is
available again; remainder converted to
glycogen)
• O2 needed to allow this removal of lactate is
called oxygen debt
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