2 Most FAQ’s of Respiration in Plants Chapter in Inter 2nd Year Botany (TS/AP)

8 Marks

LAQ-1 : Give an account of glycolysis. Where does it occur? What are the end products? Trace the fate of these products in both acrobic and anaerobic respiration. (OR) Describe the process of various biochemical reactions that occur during Glycolysis.

Introduction

Glycolysis is a fundamental metabolic pathway in all living organisms. It represents the initial step in the breakdown of glucose to extract energy for cellular metabolism. This process occurs in the cytoplasm of cells, resulting in the production of pyruvic acid, ATP (adenosine triphosphate), and NADH (nicotinamide adenine dinucleotide plus hydrogen).

Glycolysis Process

1. Phosphorylation: Glucose combines with ATP to form Glucose-6-phosphate and ADP (adenosine diphosphate), catalyzed by Hexokinase.
2. Isomerization: Glucose-6-phosphate is transformed into Fructose-6-phosphate by Hexophosphate isomerase.
3. Second Phosphorylation: Fructose-6-phosphate reacts with another ATP molecule to form Fructose1,6-biphosphate and another ADP, facilitated by Phosphofructokinase.
4. Cleavage: Fructose1,6-biphosphate is split into Glyceraldehyde-3-phosphate and DHAP (dihydroxyacetone phosphate) by Aldolase.
5. Second Isomerization: DHAP is converted into Glyceraldehyde-3-phosphate by Triose phosphate isomerase.
6. Oxidation: Glyceraldehyde-3-phosphate is oxidized, forming 1,3-biphosphoglycerate and NADH, via G-3P dehydrogenase.
7. First Dephosphorylation: 1,3-biphosphoglycerate donates a phosphate to ADP, producing ATP and 3-phosphoglycerate, catalyzed by Phosphoglycerokinase.
8. Intramolecular Shift: 3-phosphoglycerate is rearranged to 2-phosphoglycerate by Phosphoglyceromutase.
9. Dehydration: 2-phosphoglycerate undergoes dehydration, forming Phosphoenolpyruvic acid (PEP), facilitated by Enolase.
10. Second Dephosphorylation: PEP donates a phosphate to ADP, forming Pyruvic acid and another ATP, catalyzed by Pyruvate Kinase.

Fate of Pyruvic Acid

1. Aerobic Respiration: Under aerobic conditions, pyruvic acid is fully oxidized into carbon dioxide (CO₂) and water (H₂O) via the Krebs cycle.
2. Anaerobic Respiration: In the absence of oxygen, pyruvic acid is converted into either ethyl alcohol or lactic acid through fermentation.

Summary

Glycolysis involves a series of ten biochemical reactions converting glucose into pyruvic acid, ATP, and NADH in the cell’s cytoplasm. The fate of pyruvic acid depends on oxygen availability: fully oxidized into CO₂ and H₂O under aerobic conditions or converted into ethyl alcohol or lactic acid anaerobically.

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LAQ-2 : Explain the reactions of Krebs cycle.

Introduction

The Krebs Cycle, also known as the citric acid cycle or tricarboxylic acid cycle (TCA), is a sequence of chemical reactions utilized by all aerobic organisms. It generates energy by oxidizing Acetyl-CoA into ATP (Adenosine Triphosphate) and carbon dioxide. This cycle takes place in the mitochondrial matrix.

Step-by-Step Breakdown of the Krebs Cycle

1. Condensation: Acetyl CoA combines with Oxaloacetic acid and water to form Citric acid and Coenzyme A, catalyzed by Citrate Synthase.
2. Dehydration: Citric acid is transformed into Cis-aconitic acid and water by the Aconitase enzyme.
3. Hydration: Cis-aconitic acid is converted to Isocitric acid through Aconitase.
4. Oxidation I: Isocitric acid reacts with NAD+ to form Oxalosuccinic acid, NADH, and a hydrogen ion, catalyzed by Isocitrate Dehydrogenase.
5. Decarboxylation: Oxalosuccinic acid is converted to α-Ketoglutaric acid and CO₂ via α-Ketoglutarate Dehydrogenase.
6. Oxidation II: α-Ketoglutaric acid reacts with NAD+ and CoA to form Succinyl CoA, NADH, a hydrogen ion, and CO₂, catalyzed by α-Ketoglutarate Dehydrogenase.
7. Cleavage: Succinyl CoA reacts with ADP and Pi (inorganic phosphate) to form Succinic acid, ATP, and CoA, facilitated by Succinyl-CoA Synthetase.
8. Oxidation III: Succinic acid reacts with FAD (Flavin Adenine Dinucleotide) to form Fumaric acid and FADH2, catalyzed by Succinate Dehydrogenase.
9. Hydration: Fumaric acid reacts with water to form Malic acid, catalyzed by Fumarase.
10. Oxidation IV: Malic acid reacts with NAD+ to form Oxaloacetic acid, NADH, and a hydrogen ion, catalyzed by Malate Dehydrogenase.

Summary

At the cycle’s conclusion, Oxaloacetate is regenerated, allowing the cycle to continue. Each turn of the cycle produces three NADH molecules, one FADH2 molecule, and one ATP (or GTP) molecule. NADH and FADH2 contribute to oxidative phosphorylation to generate ATP, while carbon dioxide is expelled as a waste product.