[3.5.2] Respiration

Question 1

Why is respiration important?

Answer 1

• Respiration produces ATP (to release energy).
• For active transport, protein synthesis etc.

Question 2

Summarise the stages of aerobic and anaerobic respiration.

Answer 2

Aerobic Respiration

1. Glycolysis - cytoplasm (anaerobic).
2. Link reaction - mitochondrial matrix.
3. Krebs cycle - mitochondrial matrix.
4. Oxidation phosphorylation - inner mitochondrial matrix.

Anaerobic Respiration

1. Glycolysis - cytoplasm.
2. NAD regeneration- cytoplasm.

Question 3

Describe the process of glycolysis.

Answer 3

1. Glucose phosphorylated to glucose phosphate.
   
   • Using inorganic phosphates from 2 ATP.
2. Hydrolysed to 2x triose phosphate.
3. Oxidised to 2x pyruvate.
   
   • 2 NAD reduced.
   • 4 ATP regenerated (net gain of 2).

Question 4

Explain what happens after glycolysis if respiration is anaerobic.

Answer 4

1. Pyruvate converted to lactate (animals & some bacteria) or ethanol (plants & yeast).
2. Oxidising reduced NAD -> NAD regenerated.
3. So glycolysis can continue (which needs NAD) allowing continued production of ATP.

Question 5

Suggest why anaerobic respiration produces less ATP per molecule of glucose than aerobic respiration.

Answer 5

• Only glycolysis invovled which produces little ATP (2 molecules).
• No oxidative phosphorlyation which forms majority of ATP (around 34 molecules).

Question 6

What happens after glycolysis if respiration is aerobic?

Answer 6

• Pyruvate is actively transported into the mitochondrial matrix.

Question 7

Describe the link reaction.

Answer 7

1. Pyruvate oxidised (and decarboxylated) to acetate.
   
   • CO₂ produced.
   • Reduced NAD produced (picks up H).
2. Acetate combined with coenzyme A, forming Acetyl Coenzyme A.

Question 8

Describe the Krebs cycle.

Answer 8

1. Acetyl coenzyme A reacts with a 4C molecule.
   
   • Releasing coenzyme A
   • Producing a 6C molecule that enters the Krebs cycle.
2. In a series of oxidation-reduction reactions, the 4C molecule is regenerated and:
   
   • 2x CO₂ lost.
   • Coenzymes NAD & FAD reduced.
   • Substrate level phosphorylation (direct transfer of Pi from intermediate compound to ADP) which produces ATP.

Question 9

Describe the process of oxidative phosphorylation.

Answer 9

1. Reduced NAD/FAD oxidised to release H atoms -> split into protons (H⁺) and electrons (e⁻).
2. Electrons transferred down electron transfer chain (chain of carriers at decreasing energy levels).
   
   • By redox reactions.
3. Energy released by electrons used in the production of ATP from ADP + Pi (chemiosmotic theory):
   
   • Energy used by electron carriers to actively pump protons from matrix -> intermembrane space.
   • Protons diffuse into matrix down an electrochemical gradient, via ATP synthase (embedded).
   • Releasing energy to synthesise ATP from ADP + Pi.
4. In matrix at the end of electron transfer chain, oxygen is the final electron acceptor (electrons can’t pass along otherwise).
   
   • So protons, electrons and oxygen combine to form water.

Question 10

Give examples of other respiratory substrates.

Answer 10

• Breakdown of products of lipids and amino acids, which enter the Krebs cycle. For example:
  
  • Fatty acids from hydrolysis of lipids -> converted to Acetyl Coenzyme A.
  • Amino acids from hydrolysis of proteins -> converted to intermediates in Krebs cycle.