5.2 Respiration

Question 1

Describe the process of glycolysis

Answer 1

• Glucose is phosphorylated to two molecules of triose phosphates using two molecules of ATP.
• The two molecules of TP are oxidised to pyruvate.
• A single TP molecule produces two molecules of ATP and one molecule of reduced NAD (NADH) in this process.
• Net gain = 2 molecules of ATP and 2 molecules of NADPH

Question 2

What are the two types of anaerobic respiration

Answer 2

• Ethanol fermentation (plants)
• Lactate fermentation (animals)

Question 3

Describe the process of ethanol fermentation

Answer 3

Pyruvate is first converted to ethanal by the enzyme pyruvate decarboxylase.
CO2 is produced as a by-product.
Ethanal is then reduced to ethanol.
This reaction is catalysed by ethanol dehydrogenase.
NADPH is oxidised to NADP.

Question 4

Describe the process of lactate fermentation

Answer 4

Pyruvate is reduced to lactate.
This reaction is catalysed by lactate dehydrogenase.
NADPH is oxidised to NADP

Question 5

Describe the process of the link reaction

Answer 5

• Pyruvate is first actively transported from the cytoplasm across the mitochondrial membrane and into the matrix of the mitochondria.
• In the mitochondrial matrix, pyruvate is decarboxylated and dehydrogenated (oxidised further) to acetate. CO2 is produced as a by-product. NADP is reduced to NADPH.
• Acetate then combines with coenzyme A (CoA) to produce acetyl coenzyme A.
• Net gain = 1 CO2 molecule and 1 NADPH molecule

Question 6

Describe the process of the Krebs cycle

Answer 6

Acetyl coenzyme A acts as a carrier for the two-carbon acetyl group. It reacts with oxaloacetate (a four-carbon molecule) to produce citrate (a six-carbon molecule). CoA goes back to the link reaction.
Citrate is converted to a five-carbon molecule (5C) by decarboxylation and dehydrogenation.
- CO2 is produced as a by-product.
- NADP is reduced to NADPH.

The five-carbon molecule is decarboxylated and dehydrogenated again to a four-carbon compound.
- CO2 is produced.
- NADP is reduced to NADPH.
- ATP is also produced by substrate-level phosphorylation.

This 4C molecule is then dehydrogenated again to produce another molecule of NADH. FAD is also reduced to FADH2. Oxaloacetate is reproduced.

Question 7

Describe the process of oxidative phosphorylation

Answer 7

Oxidative phosphorylation takes place at the inner mitochondrial membrane. The membrane has three electron carriers and ATP synthase. NADH and FADH2 (from the Krebs cycle) are oxidised by the first electron carrier protein in the inner mitochondrial membrane.
This initiates oxidative phosphorylation because NADH and FADH2 release two protons and two electrons each. The electrons are then transferred along the ETC. As the electrons move down the ETC, they lose energy.
This energy pumps the protons from NADH and FADH2 into the intermembrane space.
This creates a proton gradient The protons diffuse down the concentration gradient through the ATP synthase enzyme.
As protons flow through the ATP synthase, energy is released.
This energy converts ADP and inorganic phosphate to ATP.
This process is called chemiosmosis. After the electrons have reached the end of the ETC and protons have flowed through the ATP synthase enzyme, they combine with O2 to form water (H2O).
Oxygen is called the final electron acceptor for this reason.