3.3 Respiration

Question 1

What are the 4 key stages of aerobic respiration in order and where do they occur?

Answer 1

1. Glycolysis (Cytoplasm)
2. Link reaction (mitochondrial membrane)
3. Krebs cycle (mitochondrial matrix)
4. Oxidative phosphorylation (mitochondrial inner membrane cristae)

Question 2

Why is it that glycolysis can happen in both aerobic and anaerobic respiration?

Answer 2

It happens in aerobic and anaerobic as it does not require energy and it occurs in the cytoplasm so does not need any ATP or oxygen to actively transport it into the mitochondria.

Question 3

Why is it that glycolysis occurs in the cytoplasm and not the mitochondria?

Answer 3

• Glucose cannot pass through the mitochondrial membrane
• Even if glucose could pass through the mitochondria’s membrane, the enzymes for its breakdown are not present there.

Question 4

What are the stages in glycolysis, talk through the steps:

Answer 4

• A glucose molecule (6C) is phosphorylated by the addition of 2 phosphate groups (using up 2 ATP molecules to do this) to make glucose diphosphate (6C). The reason this happens is to make it more reactive and polar.
• The glucose diphosphate (6) splits into two molecules of a triose phosphate, a 3-carbon sugar.
• The two triose phosphate are dehydrogenated i.e. hydrogen is removed from each of them, oxidising them to pyruvate, also a 3 carbon molecule. The hydrogen atoms are transferred to NAD making reduced NAD/NADH+. These steps release enough energy to synthesise four ATP molecules which is formed by substrate level phosphorylation. Thus net gain of 2ATP,2NADH, pyruvate.

Question 5

What is aerobic respiration?

Answer 5

The release of large amounts of energy, made available as ATP, from the breakdown of molecules, with oxygen as the terminal electron acceptor.

Question 6

What is anaerobic respiration?

Answer 6

The breakdown of molecules in the absence of oxygen, releasing relatively little energy, making a small amount of ATP by substrate level phosphorylation.

Question 7

What happens during the link reaction?

Answer 7

1. Pyruvate diffuses from the cytoplasm into the mitochondrial matrix.
2. The pyruvate is dehydrogenated and the hydrogen released is accepted by NAD to from NADH.
3. The pyruvate is decarboxylated i.e. a molecule of carbon dioxide is removed from it. All that remains of the original glucose molecule is a 2-crabon acetate group which combines with coenzyme A, making acetyl coenzyme A, which enters the krebs cycle.

Question 8

What happens during the krebs cycle?

Answer 8

• Acetyl CoA enters the krebs cycle and combines with a 4-carbon acid, to form a 6-carbon compound, and the CoA is regenerated.
• The 6C carbon is dehydrogenated, making reduced NAD, and decarboxylated to make carbon dioxide and a 5 carbon acid.
• The 5 carbon acid is dehydrogenated, making reduced NAD and FAD, and decarboxylated to make carbon dioxide and to regenerate the 4C acid.

Question 9

What does each turn of the krebs cycle produce?

Answer 9

• One ATP produced by substrate level phosphorylation
• Three molecules of reduced NAD
• One molecule of reduced FAD
• Two molecules of carbon dioxide

Question 10

What is the difference between NAD and FAD?

Answer 10

NAD interacts with three proton pumps