Cellular Respiration 🌝

Explain the roles of NAD+ and FAD in aerobic respiration

1. NAD+ and FAD are coenzymes which function as electron acceptors and electron donors.

2. In their oxidised states, NAD+ and FAD act as electron acceptors.

3. In their reduced states, NADH and FADH2 act as electron donors and carry electrons from glycolysis, link reaction and Krebs cycle to electron carriers of Electron Transport Chain.

4. During oxidative phosphorylation, NAD+ and FAD are regenerated.

State the location, raw materials and products of glycolysis

Location: Cytosol.

Raw materials: Glucose, 4 ADP, 2 NAD+.

Products: 2 pyruvate, 4 ATP (net 2 ATP), 2 NADH.

Outline the process of glycolysis

1. 1 glucose is phosphorylated by 1 ATP to form glucose-6-phosphate which converts to fructose-6-phosphate.

2. Fructose-6-phosphate is phosphorylated by 1 ATP to form fructose-1,6,-biphosphate.

3. Fructose-1,6-biphosphate is cleaved to form 2 triose phosphates.

4. 2 triose phosphates are converted to 2 pyruvate and in the process, oxidation occurs to synthesise 2 NADH and substrate-level phosphorylation occurs to synthesise 4 ATP (net 2 ATP),

Explain the significance of phosphorylation of hexose/ activation of glucose

1. Maintain a steep concentration gradient so more glucose diffuses into the cell.

2. Traps glucose inside the cell due to charged phosphate group of glucose-6-phosphate. Glucose-6-phosphate is charged and unable to directly diffuse across the hydrophobic core of the phospholipid bilayer.

3. Raises glucose levels for subsequent ATP synthesis via substrate-level phosphorylation.

Explain how the rate of glycolysis is regulated

1. The rate of glycolysis is regulated via allosteric regulation of enzymes.

2. Glucose is phosphorylated by 1 ATP to form glucose-6-phosphate, catalysed by enzyme hexokinase.

3. Hexokinase is allosterically inhibited by its product glucose-6-phosphate.

4. Fructose-6-phosphate is phosphorylated by 1 ATP form form fructose-1,6-biphosphate, catalysed by enzyme phosphofructokinase (PFK1)

5. PFK1 is allosterically inhibited by ATP.

State the location, raw materials and products of link reaction

Location: Mitochondrial matrix.

Raw materials: 2 pyruvate, 2 coenzyme A, 2 NAD+.

Products: 2 acetyl-coA, 2 CO2, 2 NADH.

Outline the process of link reaction

1. Pyruvate undergoes oxidative decarboxylation to form acetyl-coA, catalysed by enzyme pyruvate dehydrogenase.

2. Pyruvate combines with coenzyme A.

3. Pyruvate is decarboxylated to release 1 CO2.

4. NAD+ is reduced to NADH.

State the location, raw materials and products of Krebs cycle

Location: Mitochondrial matrix.

Raw materials: 2 acetyl-coA, 2 oxaloacetate, 6 NAD+, 2 FAD.

Product: 2 oxaloacetate regenerated, 4CO2, 6 NADH, 2 FADH2, 2 ATP via substrate-level phosphorylation.

Outline the process of Krebs cycle

1. Acetyl-coA combines with oxaloacetate to form citrate.

2. Citrate undergoes oxidative decarboxylation to form alpha-ketoglutarate. 1 CO2 is released and 1 NAD+ is reduced to NADH.

3. Alpha-ketoglutarate undergoes oxidative decarboxylation to form succinyl coA. 1 CO2 is released and 1 NAD+ is reduced to NADH.

4. Succinyl coA is converted to succinate. 1 ATP is synthesised via substrate-level phosphorylation.

5. Succinate is oxidised to malate. 1 FAD is reduced to FADH2.

6. Malate is oxidised to regenerate oxaloacetate. 1 NAD+ is reduced to NADH.

Explain why Krebs cycle is called a cycle

1. For 1 turn of the cycle, 2 carbons enter and 2 carbons exit.

2. At the start of the cycle, oxaloacetate combines with acetyl-coA.

3. At the end of the cycle, oxaloacetate is regenerated.

State the location, raw materials and products of oxidative phosphorylation

Location: Inner mitochondrial membrane.

Raw materials: 10 NADH, 2 FADH2.

Products: 34 ATP, NAD+, FAD.

Describe the features of electron transport chain

1. Electron transport chain (ETC) is located in inner mitochondrial membrane.

2. It consists of electron carriers of decreasing energy levels that undergo a series of redox reactions.

3. As electrons flow down ETC, energy released drives chemiosmosis for ATP synthesis.

Outline the process of oxidative phosphorylation

1. NADH and FADH2 donate electrons to electron carriers of Electron Transport Chain (ETC).

2. Electron is passed down electron carriers of decreasing energy levels.

3. Electron is then passed to final electron acceptor O2 to form water.

4. 4H+ + 4e + O2–> 2H2O.

5. Energy released from flow of electrons is used to pump H+ from matrix into intermembrane space.

6. This creates a steep proton gradient.

7. Diffusion of H+ from intermembrane space into matrix through hydrophilic channel of ATP synthase down concentration gradient releases energy which is coupled to ATP synthesis, catalysed by enzyme ATP synthase via chemiosmosis.

8. Oxidation of 1 NADH yields 3 ATP.

9. Oxidation of 1 FADH2 yield 2 ATP.

10. Oxidative phosphorylation yields 34 ATP.

Explain why only glycolysis can occur under anaerobic conditions

1. In the absence of oxygen, there is no oxygen as final electron acceptor.

2. Electron Transport chain cannot function.

3. Oxidative phosphorylation cannot occur.

4. NAD+ and FAD cannot be regenerated.

5. Krebs cycle and link reaction cannot occur.

6. Only glycolysis can occur to produce net 2 ATP per glucose via substrate-level phosphorylation as fermentation regenerates only a small amount of NAD+.

Outline the process of lactate fermentation

1. Pyruvate is reduced to lactate by NADH, catalysed by enzyme lactate dehydrogenase.

2. NAD+ is regenerated for glycolysis.

Outline the process of ethanol fermentation

1. Pyruvate is decarboxylated to ethanal, catalysed by enzyme pyruvate decarboxylase.

2. Ethanal is reduced to ethanol by NADH, catalysed by enzyme alcohol dehydrogenase.

3. NAD+ is regenerated for glycolysis.

State the efficiency of aerobic respiration and anaerobic respiration

Aerobic respiration: 40.5%.

Anaerobic respiration: 2.1%.

To produce the same number of 38 ATP from 1 glucose, aerobic respiration is 19 times more efficient than anaerobic respiration.

State three differences between substrate-level phosphorylation and oxidative phosphorylation

1. Substrate-level phosphorylation occurs in cytosol and mitochondrial matrix while oxidative phosphorylation occurs in inner mitochondrial membrane.

2. Substrate-level phosphorylation does not involve Electron Transport Chain while oxidative phosphorylation involves Electron Transport Chain.

3. Substrate-level phosphorylation does not involve chemiosmosis while oxidative phosphorylation involves chemiosmosis.

State three differences between oxidative decarboxylation and oxidative phosphorylation

1. Oxidative decarboxylation occurs in mitochondrial matrix while oxidative phosphorylation occurs in inner mitochondrial membrane.

2. Oxidative decarboxylation does not involve Electron Transport Chain while oxidative phosphorylation involves Electron Transport Chain.

3. Oxidative decarboxylation does not involve chemiosmosis while oxidative phosphorylation involves chemiosis.

State the location of aerobic respiration in prokaryotes

Location: Cell surface membrane.