Respiration in Plants (LAQs)

Botany-2 | 5. Respiration In Plants – LAQs:
Welcome to “LAQs” in “Chapter 5: Respiration In Plants”. This page includes detailed explanations for each LAQ, provided in simple English, followed by a Telugu explanation, and presented in the exam format. This approach helps you tackle each question effectively and aim for great results in your final exams.

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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 crucial process that occurs in every living organism. It represents the first step in the breakdown of glucose to release energy, which cells need to function. This process happens in the cytoplasm of cells, where glucose is converted into simpler molecules, yielding energy in the form of ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide plus hydrogen). The main product of glycolysis is pyruvic acid.

The Glycolysis Process

Glycolysis involves a sequence of ten biochemical reactions, each of which is catalyzed by a specific enzyme. Let’s walk through these reactions to understand how glucose is transformed and how energy is produced.

1. Phosphorylation of Glucose: The process begins when glucose is phosphorylated by the enzyme hexokinase. In this step, glucose combines with ATP to form glucose-6-phosphate and ADP (adenosine diphosphate). This reaction is important because it traps glucose inside the cell, making it available for further breakdown.
2. Isomerization of Glucose-6-Phosphate: The glucose-6-phosphate is then rearranged by the enzyme hexose phosphate isomerase to form fructose-6-phosphate. This rearrangement changes the structure of the molecule, preparing it for further reactions.
3. Second Phosphorylation: The enzyme phosphofructokinase catalyzes the addition of another phosphate group to fructose-6-phosphate, using a second ATP molecule. This step produces fructose-1,6-bisphosphate. This is a crucial regulatory step in glycolysis, determining the pathway’s continuation.
4. Cleavage of Fructose-1,6-Bisphosphate: The fructose-1,6-bisphosphate is then split into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) by the enzyme aldolase. Both of these molecules can be further processed in the glycolysis pathway.
5. Conversion of DHAP to G3P: The enzyme triose phosphate isomerase quickly converts DHAP into G3P, so that both molecules entering the next steps of glycolysis are the same, simplifying the pathway.
6. Oxidation and Phosphate Addition: Each G3P molecule undergoes oxidation, where it loses electrons, resulting in the formation of 1,3-bisphosphoglycerate. This reaction also generates NADH from NAD+, a coenzyme that plays a key role in cellular respiration.
7. First ATP Generation: The next step involves the transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP, producing ATP and 3-phosphoglycerate. This reaction is catalyzed by the enzyme phosphoglycerate kinase and is the first step in glycolysis that directly generates ATP, the energy currency of the cell.
8. Intramolecular Rearrangement: The 3-phosphoglycerate is then rearranged by the enzyme phosphoglyceromutase to form 2-phosphoglycerate.
9. Dehydration: The enzyme enolase catalyzes the removal of a water molecule from 2-phosphoglycerate, resulting in the formation of phosphoenolpyruvate (PEP).
10. Second ATP Generation: In the final step, the enzyme pyruvate kinase transfers a phosphate group from PEP to ADP, producing another molecule of ATP and the final product of glycolysis, pyruvic acid.

Fate of Pyruvic Acid

The fate of the pyruvic acid produced in glycolysis depends on whether oxygen is available.

1. Aerobic Respiration: When oxygen is present, pyruvic acid enters the mitochondria and is fully oxidized into carbon dioxide (CO2) and water (H2O) through the Krebs cycle and electron transport chain. This process generates a large amount of ATP, providing energy for the cell’s activities.
2. Anaerobic Respiration: In the absence of oxygen, pyruvic acid undergoes fermentation. Depending on the type of organism or cell, this can result in the production of lactic acid (as seen in muscle cells during intense exercise) or ethyl alcohol (as seen in yeast). This process produces far less energy compared to aerobic respiration but allows cells to continue generating ATP under low-oxygen conditions.

Summary

In summary, glycolysis is a vital metabolic pathway that breaks down glucose into pyruvic acid, yielding ATP and NADH in the process. This occurs in the cytoplasm of cells and involves a series of ten enzyme-driven reactions. The fate of pyruvic acid varies depending on oxygen availability: it is either fully oxidized into CO2 and H2O in the presence of oxygen or converted into lactic acid or ethyl alcohol under anaerobic conditions. This entire process is essential for cells to derive energy and sustain life.

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LAQ-2 : Explain the reactions of Krebs cycle.

Introduction

The Krebs Cycle, also known as the citric acid cycle or the tricarboxylic acid cycle (TCA), is a critical sequence of chemical reactions that occurs in all aerobic organisms. This cycle is essential for generating energy by oxidizing a molecule called Acetyl-CoA. The energy produced is captured in the form of ATP (Adenosine Triphosphate), the energy currency of the cell, and carbon dioxide is released as a byproduct. The Krebs cycle takes place in the mitochondrial matrix, a specialized compartment within the cell.

Understanding the Krebs Cycle

The Krebs Cycle is a multi-step process, and each step is crucial for extracting energy from Acetyl-CoA. Let’s explore how this process works in a more detailed and relatable way.

When we eat food, our body breaks it down into smaller molecules, one of which is glucose. Glucose is further broken down through a process called glycolysis, which occurs in the cytoplasm of the cell. This results in the formation of a compound known as pyruvate. Under aerobic conditions, meaning when oxygen is available, pyruvate is transported into the mitochondria. Here, it is converted into Acetyl-CoA, which then enters the Krebs Cycle.

Step-by-Step Breakdown of the Krebs Cycle

1. Formation of Citric Acid: The cycle begins when Acetyl-CoA combines with a four-carbon molecule called Oxaloacetic acid and a molecule of water. This reaction produces a six-carbon molecule called Citric acid and releases Coenzyme A. This step is catalyzed by an enzyme called Citrate Synthase. You can think of this step as the starting point of a journey where smaller units (like Acetyl-CoA) are combined to create something larger (Citric acid).
2. Conversion of Citric Acid to Isocitric Acid: In the next stage, Citric acid undergoes a transformation to form Cis-aconitic acid, facilitated by the enzyme Aconitase. This is quickly followed by another reaction where Cis-aconitic acid is converted into Isocitric acid. These transformations are necessary to prepare the molecule for subsequent steps where energy will be extracted.
3. First Oxidation and Decarboxylation: Isocitric acid then reacts with NAD+ (a coenzyme) to form Oxalosuccinic acid. This reaction, catalyzed by Isocitrate Dehydrogenase, produces NADH and releases a hydrogen ion. The next step involves the decarboxylation of Oxalosuccinic acid, where it loses a carbon dioxide molecule to form α-Ketoglutaric acid. This step is significant because it is one of the points where energy is captured in the form of NADH and CO2 is released as a byproduct.
4. Second Oxidation and Decarboxylation: Next, α-Ketoglutaric acid undergoes another oxidation reaction, again reacting with NAD+ and Coenzyme A (CoA). This reaction, catalyzed by the enzyme α-Ketoglutarate Dehydrogenase, produces Succinyl CoA, NADH, another hydrogen ion, and releases carbon dioxide. This step is another critical point in the cycle where energy is extracted and carbon dioxide is expelled.
5. Generation of ATP: The high-energy molecule Succinyl CoA then reacts with ADP (Adenosine Diphosphate) and inorganic phosphate (Pi), resulting in the formation of Succinic acid, ATP, and Coenzyme A. This step, catalyzed by Succinyl-CoA Synthetase, is one of the few direct points in cellular respiration where ATP is generated. Think of this step as the reward for all the earlier work—the cell finally gets a direct shot of energy in the form of ATP.
6. Further Oxidation and Regeneration of Oxaloacetate: The cycle continues with the conversion of Succinic acid into Fumaric acid through a reaction with FAD (Flavin Adenine Dinucleotide), catalyzed by Succinate Dehydrogenase. This reaction produces FADH2, another molecule that will eventually contribute to ATP production.
   Next, Fumaric acid reacts with water to form Malic acid, catalyzed by the enzyme Fumarase. Finally, Malic acid undergoes oxidation, reacting with NAD+ to form Oxaloacetic acid, NADH, and a hydrogen ion. This last step is facilitated by Malate Dehydrogenase and regenerates Oxaloacetate, the molecule that started the cycle, allowing the process to continue.

Summary

To sum up, the Krebs Cycle is a complex but incredibly efficient process that takes place in the mitochondrial matrix. It breaks down Acetyl-CoA into carbon dioxide and generates energy-rich molecules like ATP, NADH, and FADH2. Each turn of the cycle produces three NADH molecules, one FADH2 molecule, and one ATP (or GTP) molecule. These energy carriers, NADH and FADH2, later enter the electron transport chain to generate more ATP, while carbon dioxide is expelled as a waste product. This cycle is fundamental to life because it provides the energy needed for countless cellular processes, ensuring that our bodies can function efficiently, much like how fuel keeps a car running smoothly.