Respiration

Question 1

Glycolysis

Answer 1

Glycolysis takes place in the cytosol.

Phosphorylation of glucose to fructose-1,6-biphosphate by PFK (phosphofructokinase) using up 2 ATP
Activates glucose (which is energy-rich and unreactive) and commits it to glycolytic pathway
Confers negative charge to glucose to prevent it from diffusing across plasma membrane, maintaining glucose concentration

Lysis of fructose-1,6-biphosphate to form 2 G3P

Oxidation of G3P to 1,3-biphosphoglycerate by dehydrogenation of coenzyme NAD+ to form NADH

Substrate-level phosphorylation to form pyruvate and 2 ATP (4 per molecule glucose)

Question 2

PFK

Answer 2

When ATP levels are high, ATP binds to allosteric site of PFK which induces a change in the 3D conformation of PFK active site such that it is no longer complementary in shape and charge to substrate ADP and Pi, and stabilises inactive conformation of PFK which will have lower infinity for substrate.
Rate of glycolysis is lowered due to end-product inhibition.

Question 3

Anaerobic respiration

Answer 3

Substrate-level phosphorylation of 1,3-biphosphoglycerate to pyruvate, followed by reoxidation of NADH to NAD+ by fermentation to produce ethanol/lactate. NAD+ which can be reused in glycolysis to form more ATP.

In yeast, alcohol fermentation takes place where pyruvate is reduced to ethanol via ethanal while in mammalian cells lactic acid fermentation takes place where pyruvate is reduced to lactic acid.

In yeast, pyruvate undergoes 1 decarboxylation step to produce CO2 while in mammalian cells there is no decarboxylation step.

In yeast, alcohol dehydrogenase reduces pyruvate to ethanol while in mammalian cells lactate dehydrogenase reduces pyruvate to lactic acid.

Question 4

Link Reaction

Answer 4

Pyruvate diffuses into the mitochondrial matrix.
Decarboxylated to produce CO2.
Oxidised by dehydrogenation of NAD+ to NADH
Combines with coenzyme A to form Acetyl CoA

Question 5

Krebs cycle

Answer 5

Krebs cycle takes place in the matrix of the mitochondria.

2C Acetyl CoA formed through link reaction combines with 4C oxaloacetate to form citrate (6C).

Citrate is decarboxylated and dehydrogenated to form 5C a-ketoglutarate and NADH.

Oxaloacetate is regenerated when a-ketoglutarate undergoes
3 dehydrogenation steps where electrons originally from glucose have been now transferred to electron carriers NAD and FAD to form 2 NADH and 1 FADH2.
1 decarboxylation: 1 CO2
1 SLP: 1 ATP

All the carbon in glucose is lost to form carbon dioxide.

Overall 1 molecule of glucose goes through 2 rounds of Krebs cycle yielding ().
(The coenzymes with their reducing power will be transported to ETC where bulk of ATP is generated)

Question 6

Role of O2

Answer 6

Oxygen is the final electron acceptor of the electron transport chain during oxidative phosphorylation where it combines with electrons to form water.

This reoxidises electron transport chain so NADH and FADH2 can continue donating electrons, allowing oxidative phosphorylation and chemiosmosis to continue to generate ATP.

This regenerates NAD+ and FAD+, which can continue to pick up electrons in Krebs cycle, link reaction and glycolysis.

Question 7

Krebs cycle vs Calvin cycle

Answer 7

1. KC takes place in mito matrix while CC takes place in stroma of chloroplast
2. In KC, NAD+ and FAD+ accept electrons to be reduced to NADH and FADH2 respectively while in CC, NADP+ accepts electrons to be reduced to NADPH
3. KC produces CO2 during decarboxylation while CC uses up CO2 in carbon fixation.
4. KC produces ATP during substrate-level phosphorylation while CC uses up ATP to reduce and regenerate RuBP