Respiration

Question 1

Cellular respiration

Answer 1

• Process by which chemical energy in organic molecules is released by oxidation
• Aerobic (presence of O₂) → cytosol and mitochondria
• Anaerobic (absence of O₂) → cytosol
• Produce ATP

Question 2

Mitochondria

Answer 2

• Cristae → inholdings of inner membrane
• Circular DNA, 70S ribosomes
• Inner & outer membrane
• Intermembrane space vs mitochondrial matrix

Question 3

Aerobic respiration (4)

Answer 3

1. Glycolysis (In cytosol, rest in mitochondria)
2. Link reaction
3. Krebs cycle
4. Oxidative phosphorylation

Question 4

Key molecules besides ATP

Answer 4

• NADH (Reduced nicotinamide adenine dinucleotide)
• FADH₂ (Reduced flavin adenine dinucleotide)
• Serve as mobile electron carriers → transport high-energy electrons and protons

Question 5

Glycolysis (4)

Answer 5

1. Phosphorylation of glucose
2. Lysis
3. Oxidation by dehydrogenation
4. Substrate-level phosphorylation

Question 6

Phosphorylation of glucose

Answer 6

• Initial investment of 2 ATP molecules
• Glucose phosphorylated → fructose 1,6-bisphosphate
• Activate sugar, make it more reactive, commit it to glycolytic pathway
• Catalysed by phosphofructokinase (addition of 2nd phosphate group)

Question 7

Phosphofructokinase (PFK)

Answer 7

• Allosteric enzyme
• Inhibited by excess ATP/citrate
• End product inhibition (allosteric inhibition)
• Stimulated by AMP and ADP (allosteric activators)

Question 8

Lysis

Answer 8

• Fructose 1,6-bisphosphate split → glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP)
• G3P and DHAP are isomers → can be converted by isomerase

Question 9

Other names for G3P (2)

Answer 9

1. Triose phosphate (TP)

2. Phosphoglyceraldehyde (PGAL)

Question 10

Oxidation can be defined in 3 ways

Answer 10

1. Addition of oxygen
2. Removal of hydrogen (dehydrogenation)
3. Removal of electrons

Question 11

Oxidation by dehydrogenation

Answer 11

• G3P oxidised by dehydrogenation
• Coenzyme NAD⁺ reduced to NADH
• Highly exergonic, energy released adds 2nd phosphate group to G3P → 1,3-bisphosphoglycerate

Question 12

Substrate-level phosphorylation

Answer 12

• 1,3-bisphosphoglycerate dephosphorylated → glycerate phosphate (GP) → pyruvate
• 2 ATP formed via substrate-level phosphorylation

Question 13

Glycolysis overview

Answer 13

• Net gain of 2 ATP and 2 NADH per glucose
• Glucose + 2ADP + 2Pi + 2NAD⁺ → 2 pyruvate + 2ATP + 2NADH
• Energy transferred from glucose to pyruvate, ATP and NADH

Question 14

Link reaction/oxidative decarboxylation

Answer 14

• Transport protein in mitochondrial membrane translocates pyruvate from cytosol to mitochondria via active transport
• Pyruvate decarboxylated → loss of CO₂
• Oxidation by dehydrogenation → NADH + 2C compound
• 2C compound combines with coenzyme A → acetyl CoA
• 2 acetyl CoA, NADH and CO₂ per glucose/pyruvate

Question 15

Krebs cycle (3)

Answer 15

1. Acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C)
2. Citrate decarboxylated & dehydrogenated → α-ketoglutarate (5C) + NADH
3. Oxaloacetate (4C) regenerated → 1 decarboxylation step (1CO₂), 3 dehydrogenation (2NADH, 1FADH₂), 1 substrate-level phosphorylation (1ATP)

Question 16

Krebs cycle overview

Answer 16

• 2 rounds to completely oxidise 1 molecule of glucose

- 2 CO₂, 3NADH, 1 FADH₂, 1ATP per acetyl CoA

Question 17

Inner mitochondria membrane (cristae)

Answer 17

• Highly folded to increase surface area to accommodate:
  1. Electron transport chains (ETC) → series of electron carriers with increasing electronegativity
  2. ATP synthase → synthesise ATP

Question 18

Function of NAD⁺ and FAD

Answer 18

• Serve as mobile electron carriers
• Transport high energy e⁻ from organic molecules to ETC
• Pass electrons to ETC → oxidised → regenerated → pick up more electrons and protons

Question 19

Oxidative phosphorylation

Answer 19

1. NADH and FADH₂ transfer high energy electrons to electron carriers of ETC
2. As electrons travel down ETC, energy released coupled to pumping of H⁺ from mitochondrial matrix into intermembrane space → builds up proton gradient across cristae
3. As H⁺ flows through ATP synthase back into matrix down the gradient, ATP is produced from ADP and inorganic phosphate via chemiosmosis
4. O₂ acts as final electron acceptor, accepting electrons and combines with H⁺ to produce H₂O

Question 20

Chemiosmosis

Answer 20

Mechanism by which energy stored in proton gradient drives ATP synthesis

Question 21

Importance of O₂ (4)

Answer 21

1. Final electron acceptor that accepts electrons from the ned of the electron transport chain where it will combine with electrons and protons to form water
2. Allows electron carriers NADH and FADH₂ to continue donating their electrons to the chain by re-oxidising ETC, thereby generating ATP via chemiosmosis, allowing oxidative phosphorylation to continue
3. Allows regeneration of NAD⁺ and FAD → pick up more electrons and protons from glycolysis, link reaction and Krebs cycle
4. Reduction of O₂ to water removes H⁺ from matrix → contributes to generation of proton gradient

Question 22

ETC Function (2)

Answer 22

1. Generate proton motive force to produce ATP

2. Regenerate coenzymes NAD⁺ and FAD

Question 23

ATP produced per glucose molecule (Theoretical)

Answer 23

• NADH → 3ATP, FADH₂ → 2ATP (enters chain at lower energy potential)
• Glycolysis → 2ATP, 2NADH → 8 ATP
• Link reaction → 2NADH → 6 ATP
• Krebs cycle → 2ATP, 6NADH, 2FADH₂ → 24 ATP
• Total: 38 ATP

Question 24

ATP produced per glucose molecule (Reality)

Answer 24

• 36-38 ATP
• Mitochondrial membrane impermeable to NADH → cannot enter from glycolysis
• Electrons, H⁺ carried across via shuttle system → either to NADH or FADH₂

Question 25

What happens when there is no O₂

Answer 25

- No final e⁻ acceptor → e⁻ carriers remain reduce → NADH and FADH₂ can no longer donate e⁻ to ETC - NAD⁺ and FAD not regenerated to pick up more e⁻ from glycolysis, link reaction and Krebs cycle → 3 processes come to a halt

Question 26

Anaerobic respiration

Answer 26

- Takes place in cytosol - Glycolysis followed by fermentation - Produce small yield of energy - Regenerate NAD⁺ from NADH

Question 27

Fermentation

Answer 27

- Pathways to regenerate NAD⁺ from NADH → ensure steady supply of NAD⁺ for glycolysis to continue - Alcohol - Lactic acid - Pyruvate/ethanal become final e⁻ acceptor

Question 28

Alcohol fermentation

Answer 28

1. Pyruvate decarboxylase coverts pyruvate (3C) → ethanal/acetaldehyde (2C) 2. Alcohol dehydrogenase reduces ethanal to ethanol, removing H⁺ from NADH → NAD⁺

Question 29

Lactic acid fermentation

Answer 29

- When muscles contract, exceptionally high demand for ATP → dramatic increase in rate of glycolysis → rapidly depletes limited supply of NAD+ → oxidative phosphorylation may not be able to replenish NAD+ quickly enough - Lactate dehydrogenase reduces pyruvate → lactic acid/lactate - No CO₂ produced - Lactate carried to liver → converted back to pyruvate