3.3 Aerobic Respiration

Question 1

What is respiration?

Answer 1

• A catabolic process involving a series of enzyme-catalysed reactions in cells in which energy rich respiratory substrates are broken down to release energy as ATP

Question 2

Aerobic respiration

Answer 2

Glucose + Oxygen –> Carbon dioxide + Water + Energy

C₆H₁₂O₆ + 6O₂ –> 6CO₂ + 6H₂O + ATP

Question 3

NAD

Answer 3

Function:
-Hydrogen and electron acceptor molecule
-Acts as a coenzyme by assisting dehydrogenase enzymes by accepting protons and transferring them to other molecules

Structure:
-Two nucleotides both containing ribose sugar
-One nucleotide containing adenine base
-One nucleotide containing nicotinamide ring

Question 4

Stages of Aerobic Respiration

Answer 4

1) Glycolysis
2) The Link Reaction
3) The Krebs Cycle
4) The Electron Transport Chain (ETC)

Question 5

Glycolysis

Answer 5

-Means the breaking of sugar

• Takes place in the cytosol of cells (The liquid part surrounding organelles) as glucose is too big to enter mitochondria and enzymes needed are found here

Question 6

Stages of Glycolysis

Answer 6

1) Activation of glucose (By phosphorylation):
- 2x inorganic phosphate ions from 2 ATP added to glucose (6C)
-Destabilises glucose and makes more reactive
- Forms Glucose diphosphate (6C)

2) Splitting of phosphorylated glucose:
- Glucose diphosphate is unstable and reactive
- It splits to 2x Triose phosphate (3C)

3) Oxidation of Triose Phosphate (TP):
- Each Triose phosphate (3C) loses 2x hydrogen
- 2x Triose phosphate is oxidised to 2x Pyruvate (3C)
-This stage is catalysed by dehydrogenase enzymes and the coenzyme NAD
-NAD is reduced to form NADH/ reduced NAD

4) ATP production (By substrate level phosphorylation):
- 4x ADP are phosphorylated by 2x Pi from Triose Phosphate and 2x Pi from cytosol
- 4x ATP is formed for 2x Pyruvate

Question 7

The Link Reaction

Answer 7

• Occurs in the matrix of the mitochondria
  -Pyruvate is actively transported into mitochondria via carrier proteins and involves the decarboxylation of pyruvate to Acetyl CoA

Question 8

The Link Reaction stages

Answer 8

• Pyruvate (3C) is oxidised/dehydrogenated, catalysed by dehydrogenase enzymes
• NAD acts as an acceptor and is reduced to NADH
• Pyruvate is also decarboxylated (catalysed by decarboxylase enzymes) to Acetate (2C) and CO₂

-Acetate (2C) combines with Coenzyme A (CoA) to form Acetyl CoA

Question 9

The Krebs Cycle

Answer 9

• Occurs in the matrix of the mitochondria
• Involves a series of enzyme catalysed reactions

Question 10

The Krebs Cycle stages

Answer 10

1) Entering the Krebs Cycle:
- The Coenzyme A fragment of Acetyl CoA helps the 2C Acetyl fragment to enter the Krebs Cycle
- The Coenzyme A fragment detaches and is recycled

2) Hydrolysis of Oxaloacetate (4C) to Citrate (6C)
- Oxaloacetate (regenerated during the Krebs Cycle) combines with Acetate (2C) to form Citrate (6C)
- A water molecule is inserted during the bond breaking process

3) Oxidative decarboxylation of Citrate (6C)
- Citrate (6C) is oxidised (catalysed by dehydrogenase enzymes)
- NAD is reduced to NADH
- Citrate also decarboxylated (catalysed by decarboxylase enzymes) and a CO₂ is yielded
- Hydrolysis also occurs at this stage
- α-Ketoglutarate is formed (5C)

4) Oxidative decarboxylation of α-Ketoglutarate (5C)
- α-Ketoglutarate (5C) is oxidised (catalysed by dehydrogenase enzymes)
- NAD is reduced to NADH
- α-Ketoglutarate (5C) is decarboxylated (catalysed by decarboxylase enzymes) to Succinate (4C) and a CO₂ is yielded

5) Substrate level phosphorylation
- A substrate molecule acts as a donor of a phosphate group
- ATP is produced by substrate level phosphorylation

6) Oxidation of Succinate (4C)
- Succinate (4C) is oxidised (catalysed by dehydrogenase enzymes) to Fumarate (4C)
- FAD is reduced to FADH₂

7) Hydrolysis of Fumarate (4C)
- Fumarate (4C) is hydrolysed to Malate (4C)
- High energy bonds are broken and lower energy bonds are formed

8) Oxidation of Malate (4C)
- Malate is oxidised (catalysed by dehydrogenase enzymes) to regenerate Oxaloacetate (4C)
- NAD is reduced to NADH