Respiration - The Student Room

1. Respiration is a series of enzyme-catalysed reactions which release chemical energy from organic
molecules in order to synthesise ATP.
2. The four main stages of the breakdown of a glucose molecule to carbon dioxide and water are: Glycolysis;
link reaction; Krebs cycle; electron transport chain.
3. Glycolysis is the phosphorylation of glucose, the splitting of the 6C hexose phosphate formed into two 3C
triose phosphate molecules and the oxidation of each of these to 3C pyruvate with a small yield of ATP and
reduced NAD.
Glucose molecule is first PHOSPHORYLATED by ATP. This
then splits into two 3CARBON sugars. Each Sugar is then
converted into PYRUVIC ACID by the removal of the
phosphate group to form 4 ATP and 2 NADH.
The Breakdown of one molecule of Glucose in Glycolysis
produces 2 Pyruvate molecules, 2 NADH and 2 ATP.
4. Glycolysis takes place in the cytoplasm and does not require oxygen.
Glycolysis only occurs in the CYTOPLASM, and does not need OXYGEN.
5. The link reaction involves the conversion of pyruvate to acetate as a result of the loss of carbon dioxide
followed by the removal of hydrogen by the reduction of NAD; the acetyl group then combines with coenzyme A. The link reaction takes place in the matrix of the mitochondrion.
The Link Reaction occurs in the MATRIX of the mitochondria. In
sufficient AEROBIC conditions the Pyruvate enters a
mitochondrion where it LOSES a molecule of CO2 and combines
with a molecule called COENZYME A to form ACETYLCOENZYME
6. Each acetyl co-enzyme A enters the Krebs cycle, the co-enzyme A is regenerated and the acetate
fragment is picked up by a 4C acid, to produce a 6C acid.
Krebs cycle occurs in the MATRIX of the Mitochondria also. The Acetyl Co A reacts with a 4C molecule
OXALOACETATE in the Krebs cycle to form a 6C compound, CITRATE. This is converted back to oxaloacetate in
a series of reactions.
7. The Krebs cycle is a series of decarboxylations (removal of carbon) and dehydrogenations (removal of
hydrogen) where the acetate fragment from the glucose molecule is completely broken down and the 4C is
regenerated via 6C and 5C intermediates.
Two CO2 molecules are removed, pairs of H+ are removed (dehydrogenations) to reduce 3 molecules of NAD,
1 molecule of FAD and 1 molecule of ATP.
8. The function of the Krebs cycle is a means of liberating energy from carbon bonds to provide ATP and
reduced NAD (and FAD) , with the release of carbon dioxide.
The Krebs cycle goes around TWICE – 2 Pyruvate molecules
produced from one ATP molecule. The products of 2 cycles of
the Krebs cycle are 2 ATP, 4 CO2, 6NADH and 2 FADH.
The NADH and FADH are used to store energy.
9. Reduced NAD (and FAD) deliver the hydrogen to the electron transport system in the inner mitochondrial
membrane so acting as triggers for this system.
In the Electron Transport Chain, NADH and FADH2 donate an ELECTRON to the ETC, Acting as a Trigger. The
electron passes from one complex to the next, releasing small amounts of energy.
10. Candidates should be able to describe the formation of ATP via the proton pump mechanism.
The energy released by the passing of the electron is used to PUMP PROTONS into the intermembrane space
of the Mitochondria. A high concentration of H+ ions in this space causes the diffusion of them back down
their concentration gradient via STALKED ATP SYNTHETASE PARTICLES. Theory of CHEMIOSMOSIS.
11. Candidates should be able to define the terms aerobic and anaerobic respiration.
Aerobic Respiration – Oxygen is present
Anaerobic Respiration – Oxygen is NOT present
12. Candidates should understand the role of oxygen as the final electron acceptor of the electron transport
chain and explain the formation of water.
The last molecule in the ETC is OXYGEN, which acts as the FINAL ELECTRON ACCEPTER, which combines with
Protons and Electrons to form Water.
13. Without oxygen the reduced NAD (and FAD) cannot be reoxidised and therefore made available to pick
up more hydrogen, so under anaerobic conditions the link reaction and the Krebs cycle cannot take place.
Due to Oxygen being the final electron acceptor, this means that Oxygen MUST be present for the Krebs cycle
to occur as NADH and FADH2 cannot be reoxidised.
14. Under anaerobic conditions glycolysis can take place as the NAD transfers the hydrogen to pyruvate to
form lactate in animals and ethanol and carbon dioxide in plants, but there is only a very small yield of ATP.
In the absence of Oxygen, only GLYCOLYSIS can occur as Pyruvate becomes the HYDROGEN accepter,
converting Pyruvate to LACTATE in ANIMALS. This process (Anaerobic Respiration) yields 2 ATP in Animals -2%
Energy Release. In Plants, and microorganisms, Pyruvate is converted directly to ETHANOL and CO2.
15. Candidates should be able to state how many molecules of ATP are produced in each of the four stages
of respiration and therefore the total for the complete oxidation of a glucose molecule.
Up to 38 molecules of ATP are produced from each molecule of Glucose.
Glycolysis produces 8 ATP, Link Reaction 6 ATP and Krebs Cycle 24 ATP
NADH is worth 3 ATP / FADH2 is worth 2 ATP
Stage of Respiration
Co Enzyme
Substrate Level/Oxidative
18 ATP
16. Not all the energy of the glucose molecule is captured in ATP and there is a loss of energy as heat
In the conversion of ADP + Pi – ATP some energy is lost as heat energy
17. Under certain circumstances fats and proteins may be used as respiratory substrates.
18. Glycerol is converted to a 3-carbon sugar which enters the Krebs cycle via triose phosphate.
One gram of Fat releases more energy that one gram of Carbohydrate.
19. Long fatty acid chains molecules are split into 2C fragments which enter the pathways as acetyl coenzyme A.
Very large numbers of ATP molecules are built up, the precise number depending on the length of the
hydrocarbon chain of the fatty acid. This is because the HYDROGEN IONS ARE PICKED UP BY NAD and fed into
the ETC.
20. The protein is broken down into its constituent amino acids which are deaminated with the removal of
the NH2 group. This leaves an organic acid that can be fed into the Krebs cycle.
Protein is only used when the individual is suffering from Starvation. The amino group once deaminated is
converted into urea and excreted, and the residue is converted to either ACETYL CoA , PYRUVIC ACID, or some
other Krebs Cycle intermediate and Oxidised. Protein is only RARELY used up.
21. Candidates should be able to describe similarities in mitochondrial and chloroplast membranes in terms
of providing a proton gradient: proton pumps, ATP synthetase, electrochemical gradient, with high energy
electrons fuelling the pumps.
Similarities between Mitochondria and Chloroplasts are present in the ETC of Mitochondria, and in the Light
Dependant Reactions of Photosynthesis. Proton Pumps, ATP Synthetase, Electrochemical Gradients and high
energy electrons.