A2 - AQA Biology (Unit 4) - Respiration

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Respiration Notes
1. Overview
There are two forms of Respiration; Aerobic and Anaerobic.
Aerobic Respiration needs Oxygen and produces Carbon Dioxide, Water and ATP. 32
molecules of ATP are produced per Glucose molecule.
Anaerobic Respiration (also known as fermentation) takes place when Oxygen is absent.
In this case Oxygen, Lactate (animals) or Ethanol (plants) and very few molecules of ATP
are produced.
2. Stages of Respiration
1) Glycolysis ­ the splitting of a 6-carbon glucose molecule into 2 x 3-carbon pyruvate
molecules. (Proteins, Lipids and other Carbohydrates can also be used) (Anaerobic
2) Link Reaction ­ the conversion of the 3-carbon pyruvate molecule into CO2 and a
2-carbon molecule called acetylcoenzyme A.
3) Kreb's Cycle ­ the introduction of acetylcoenzyme A into a cycle of
oxidation-reduction reactions that yield some ATP and a large number of Electrons.
4) Oxidative Phosphorylation ­ The Final stage of Respiration, when the majority of ATP
molecules are produced.
3. Anaerobic Respiration
In plants, during Glycolysis, if there is no oxygen present then once Pyruvate is formed a
Carbon is lost as Co2 and a Hydrogen and electron are used to reduce NAD to NADH. This
changes Pyruvate into the 2C molecule Ethanal. A NADH molecule is then oxidised to change
the ethanal into ethanol. The NAD molecule that is left from the oxidised NADH molecule is
then recycled at the beginning of Glycolysis to allow the plant to continue to make a small
amount of ATP in the oxygen starved environment.
In animals, once Pyruvate is formed in Glycolysis, a NADH molecule is used to change
Pyruvate into lactate (lactic acid). Much like the plant's NAD molecule, this is then recycled to
be used again in Glycolysis.
4. Glycolysis
Glycolysis involves splitting one molecule of Glucose (with 6-carbons) into 2 smaller
molecules of Pyruvate (3-carbons).
This process is the only stage of Respiration that happens in the Cell Cytoplasm.

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Glycolysis is the first stage of both anaerobic and aerobic respiration. It doesn't however
need Oxygen, which makes it an anaerobic process.
6-Carbon 3-Carbon
Produces NADH Stage 4
Generates 2 net ATP molecules (4 ATP made ­ 2 ATP used = 2 ATP generated)
5. The Link Reaction
Second stage of Respiration.
Pyruvate is decarboxylated (1C is removed in the form of CO2)
NAD is reduced (collects a hydrogen from pyruvate, changing pyruvate into acetate)
NAD is a coenzyme found in the Cristae.…read more

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Kreb's Cycle (a.k.a Citric Acid cycle)
The 2C molecule, Acetyl CoA, from the Link Reaction combines with a 4C molecule to
produce a 6C molecule (Citrate, a.k.a. Citric Acid).
This 6C molecule loses CO2 and H+'s to give a 4C molecule and a single molecule of ATP
produced as a result of substrate-level phosphorylation.
The 4 C molecule can now combine with another Acetyl CoA to start the cycle again.…read more

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Oxidative Phosphorylation
NAD and FAD are coenzymes that carry high energy electrons.
The e- are used in REDOX reactions (reduction and oxidation)
As the NADH and FADH are oxidised (H+ removed) and ATP is made as a result = oxidative
Cyanide is a non-reversible inhibitor of enzymes and inhibits the final electron carrier in the
Oxidative Phosphorylation stage of Respiration.…read more


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