P1 Respiration

Question 1

What is the first stage of aerobic and anaerobic respiration and where does it take place?

Answer 1

Glycolysis, in the cytoplasm

Question 2

What is the first step of glycolysis?

Answer 2

• Phosphorylation: two phosphate groups are added to glucose to form hexose bisphosphate (6C).
• These two phosphate groups come from two ATP molecules, which are broken down to ADP.

Question 3

What is the second step of glycolysis?

Answer 3

• Hexose bisphosphate (6C) is broken down into two molecules of triose phosphate (3C).

Question 4

What is the third step of glycolysis?

Answer 4

• Two triose phosphate molecules (3C) are broken down into two pyruvate molecules (3C).
• A breakdown of one triose phosphate molecule releases two molecules of ATP (so the breakdown of two triose phosphate molecules releases 4 ATP molecules).
• During this reaction triose phosphate is oxidised - it looses a hydrogen which binds to NAD to form NADH.

Question 5

During glycolysis, for a single molecule of glucose, what is the overall yield of ATP?

Answer 5

2 ATP molecules

Question 6

Why is glucose converted to hexose bisphosphate (adding two phosphate groups) in the first stage of glycolysis?

Answer 6

• Because hexose bisphosphate cannot travel through the same transport proteins as glucose.
• Glucose enters the cell by facilitated diffusion, however if the concentration of glucose is higher inside the cell than outside, glucose could end up leaving again. But converting glucose to hexose bisphosphate prevents this.

Question 7

What is the second stage of aerobic respiration and where does it take place?

Answer 7

• The link reaction, in the mitochondrial matrix.
• Pyruvate is moved from the cytoplasm to the mitochondrial matrix by co-transport, facilitated by active transport - therefore it requires a transport protein and energy.

Question 8

What is the first step of the link reaction?

Answer 8

• Pyruvate (3C) is converted to acetate (2C), and looses a carbon atom in the form of carbon dioxide, which is then exhaled (decarboxylation).
• Pyruvate is also oxidised, it looses a hydrogen atom which bonds to NAD, forming NADH (NAD is a coenzyme).

Question 9

What is the second step of the link reaction?

Answer 9

• Coenzyme A is added to acetate (2C) to form acetyl-CoA/acetyl coenzyme A (2C).

Question 10

What is the net yield of ATP molecules in the link reaction?

Answer 10

0 ATP molecules

Question 11

What is the third stage of aerobic respiration?

Answer 11

The Krebs cycle (also in the mitochondrial matrix), it is a cycle that produces 3 important molecules.

Question 12

Describe the steps of the Krebs cycle

Answer 12

1. Acetyl CoA (2C) reacts with oxaloacetate (4C) to form citrate (6C), and looses Coenzyme A in this reaction.
2. Citrate (6C) is then converted back to oxaloacetate (4C), the two carbon atoms are lost in the form of 2 carbon dioxide molecules (decarboxylation).
   - During an intermediate reaction ADP reacts with a phosphate to form ATP. This is substrate level phosphorylation (as is ATP production in glycolysis) because it requires a molecule that contains a phosphate group (to donate to ADP) and an enzyme to catalyse the reaction.
   - When citrate is converted to oxaloacetate, it looses H atoms and is oxidised (dehydrogenation), which are transferred to NAD molecules to form NADH. In total 3 NAD molecules react with hydrogen to form 3 NADH molecules.
   - Other hydrogens are transferred to FAD (another coenzyme) to form FADH2.

Question 13

What is the overall yield of ATP in the Krebs cycle?

Answer 13

2 ATP molecules per glucose molecule/ 1 ATP molecule per turn of the Krebs cycle.

Question 14

What is the function of Coenzyme A?

Answer 14

The reaction of oxaloacetate with acetyl CoA is catalysed by an enzyme, which requires a coenzyme (Coenzyme A) to function properly and ensure the Krebs cycle can take place.

Question 15

What is the fourth stage of aerobic respiration and where does it take place?

Answer 15

Oxidative phosphorylation - it takes place in the inner mitochondrial membrane, between the matrix and the intermembrane space.

Question 16

Describe the third stage of aerobic respiration (oxidative phosphorylation).

Answer 16

• ATP synthase catalyses the production of ATP from ADP and an inorganic phosphate (Pi).
• Energy for this is provided by the diffusion of protons down an electrochemical gradient through ATP synthase from the intermembrane space to the matrix (chemiosmosis).
• The electrochemical/proton gradient is maintained by protons being actively transported from the matrix to the intermembrane space.
• The energy for this active transport is supplied by the electron transport chain. At the start, NADH and FADH2 (from glycolysis, the link reaction and the Krebs cycle) each donate two electrons (to different membrane proteins), and these electrons are transferred along the membrane by a series of oxidation-reduction reactions that release energy, enabling the active transport of protons.
• Once electrons reach the final protein, they react with oxygen and protons to form water. Oxygen is the final electron acceptor and allows the ETC to continue.
• Oxidative phosphorylation produces about 34 ATP molecules.

Question 17

How are mitochondria adapted for respiration?

Answer 17

• They have an inner mitochondrial membrane that folds to form cristae, which provides a large surface area for oxidative phosphorylation.
• The more ATP a cell needs, the more cristae it will have.

Question 18

Where does anaerobic respiration occur?

Answer 18

Cytoplasm

Question 19

Describe the second stage of anaerobic respiration in animals.

Answer 19

Pyruvate (3C) is reduced (by NADH from the conversion of TP to pyruvate in glycolysis) to form lactate (3C). The NAD molecule then returns to glycolysis to ensure glycolysis can continue.

Question 20

Describe the second stage of anaerobic respiration in plants and microorganisms.

Answer 20

Pyruvate (3C) is reduced (by NADH from the conversion of TP to pyruvate in glycolysis) to form ethanol (2C) and a carbon dioxide molecule. The NAD molecule returns to glycolysis to ensure glycolysis can continue.

Question 21

Why do different respiratory substrates (lipids, proteins and carbohydrates) produce different amounts of energy when they are broken down?

Answer 21

• The amount of energy a substrate releases depends on the number of protons it releases. These protons react to form NADH and FADH2, which then donate electrons to the ETC, to produce ATP.
• Lipids release the most energy during respiration, and carbohydrates the least.

Question 22

How do you calculate respiratory quotient?

Answer 22

RQ = (CO2 produced)/(O2 consumed)

Question 23

What is the respiratory quotient of carbohydrates?

Answer 23

1

Question 24

What is the respiratory quotient of proteins?

Answer 24

0.9

Question 25

What is the respiratory quotient of lipids?

Answer 25

0.7

Question 26

Why are there differences in respiratory quotients?

Answer 26

• The greater the amount of C-H bonds in a molecule, the more oxygen is required to break down the molecules, as oxygen is used to break these C-H bonds, so the RQ is smaller.