Questions and Answers on “Enzymes in Photosynthesis and Respiration”

1. Describe the role of RuBisCO in photosynthesis.

Answer: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant enzyme in plants and plays a crucial role in the Calvin cycle of photosynthesis. It catalyzes the fixation of carbon dioxide by attaching it to ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar. This reaction forms an unstable six-carbon compound that quickly breaks down into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. This step is vital for the synthesis of sugars in plants, which are used as energy sources and building blocks for growth.

2. Explain the function of ATP synthase in cellular respiration.

Answer: ATP synthase is an enzyme found in the mitochondria that plays a key role in cellular respiration. It is responsible for the synthesis of ATP from ADP and inorganic phosphate. The enzyme operates in the inner mitochondrial membrane, utilizing the proton gradient generated by the electron transport chain (ETC). Protons (H⁺) are pumped across the membrane into the intermembrane space, creating a gradient. ATP synthase uses the energy from this gradient as protons flow back into the mitochondrial matrix to catalyze the production of ATP. This process is known as chemiosmosis.

3. How does phosphofructokinase regulate glycolysis?

Answer: Phosphofructokinase (PFK) is a crucial enzyme in the regulation of glycolysis, the metabolic pathway that breaks down glucose to produce energy. PFK catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate using ATP. This reaction is considered the committed step of glycolysis because it commits the glucose molecule to continue through the pathway. PFK is allosterically regulated by several metabolites, including ATP, citrate, and AMP. High levels of ATP and citrate inhibit PFK activity, signaling that the cell has sufficient energy, while high levels of AMP activate PFK, indicating that energy is needed.

4. Discuss the role of NADP+ reductase in photosynthesis.

Answer: NADP+ reductase is an enzyme involved in the light-dependent reactions of photosynthesis, specifically in the production of NADPH. It catalyzes the final step in the electron transport chain, where NADP⁺ (nicotinamide adenine dinucleotide phosphate) is reduced to NADPH by accepting electrons and a proton. NADPH is essential for the Calvin cycle, where it acts as a reducing agent to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate, a sugar used in the formation of glucose and other organic molecules. NADPH, along with ATP produced in the light-dependent reactions, provides the energy needed for the Calvin cycle.

5. Explain the importance of pyruvate dehydrogenase in cellular respiration.

Answer: Pyruvate dehydrogenase is a multi-enzyme complex that plays a critical role in cellular respiration. It catalyzes the conversion of pyruvate, which is produced during glycolysis, into acetyl-CoA, a key molecule that enters the citric acid cycle (Krebs cycle). This reaction involves the decarboxylation of pyruvate, releasing carbon dioxide, and the transfer of electrons to NAD+, forming NADH. Acetyl-CoA is essential for energy production, as it combines with oxaloacetate to form citrate, initiating the citric acid cycle, which leads to further ATP production through oxidative phosphorylation.

6. What is the role of succinate dehydrogenase in the citric acid cycle?

Answer: Succinate dehydrogenase is an enzyme involved in the citric acid cycle (Krebs cycle), specifically in the conversion of succinate to fumarate. During this reaction, succinate is oxidized, and electrons are transferred to FAD (flavin adenine dinucleotide), reducing it to FADH2. This is the only step in the citric acid cycle where FAD is used as the electron carrier. The electrons from FADH2 are subsequently passed to the electron transport chain to generate ATP. Succinate dehydrogenase is also part of the electron transport chain, located in the inner mitochondrial membrane, linking both the citric acid cycle and oxidative phosphorylation.

7. Describe the process of glycolysis and the enzymes involved.

Answer: Glycolysis is the initial stage of glucose metabolism, occurring in the cytoplasm, where glucose (a six-carbon molecule) is broken down into two molecules of pyruvate (a three-carbon compound). The process involves a series of enzyme-catalyzed reactions:

1. Hexokinase catalyzes the phosphorylation of glucose to glucose-6-phosphate.
2. Phosphoglucose isomerase converts glucose-6-phosphate to fructose-6-phosphate.
3. Phosphofructokinase (PFK) adds another phosphate group to form fructose-1,6-bisphosphate.
4. Aldolase splits fructose-1,6-bisphosphate into two three-carbon molecules.
5. Glyceraldehyde-3-phosphate dehydrogenase converts glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate.
6. Phosphoglycerate kinase produces ATP by converting 1,3-bisphosphoglycerate to 3-phosphoglycerate.
7. Pyruvate kinase finalizes the process by converting phosphoenolpyruvate to pyruvate, producing ATP.

Glycolysis produces a net gain of 2 ATP and 2 NADH molecules, which are used in further stages of cellular respiration.

8. How does lactate dehydrogenase function during anaerobic respiration?

Answer: Lactate dehydrogenase (LDH) is an enzyme that catalyzes the conversion of pyruvate to lactate in anaerobic respiration. This process occurs when oxygen is scarce, such as during intense exercise, when the electron transport chain cannot operate efficiently due to a lack of oxygen. The conversion of pyruvate to lactate allows NADH to be oxidized to NAD+, which is required to continue glycolysis and produce ATP in the absence of oxygen. This mechanism enables cells to produce energy under anaerobic conditions but results in the accumulation of lactate, which can cause muscle fatigue and acidosis.

9. What is the role of cytochrome c in cellular respiration?

Answer: Cytochrome c is a small heme protein located in the intermembrane space of mitochondria, and it plays a vital role in the electron transport chain (ETC). It acts as an electron carrier between two enzyme complexes—complex III (cytochrome b-c1 complex) and complex IV (cytochrome c oxidase). Electrons from NADH and FADH2 are passed through the complexes, and cytochrome c shuttles electrons to complex IV. In complex IV, cytochrome c transfers the electrons to oxygen, which is reduced to form water. This transfer of electrons facilitates the proton pumping across the inner mitochondrial membrane, ultimately contributing to the proton gradient that drives ATP synthesis by ATP synthase.

10. Explain the role of ATP synthase in the process of oxidative phosphorylation.

Answer: ATP synthase is a crucial enzyme that catalyzes the synthesis of ATP in oxidative phosphorylation, which occurs in the mitochondria. During oxidative phosphorylation, electrons from NADH and FADH2 are transferred through the electron transport chain, resulting in the pumping of protons (H⁺) across the inner mitochondrial membrane. This creates a proton gradient across the membrane. ATP synthase utilizes the energy from this gradient, as protons flow back into the mitochondrial matrix through the enzyme, to drive the conversion of ADP and inorganic phosphate into ATP. This process is known as chemiosmosis.

11. What is the significance of NAD+ and FAD in cellular respiration?

Answer: NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are essential electron carriers in cellular respiration. During glycolysis, the citric acid cycle, and the oxidation of glucose, these molecules accept electrons from glucose and other substrates, becoming NADH and FADH2, respectively. These reduced forms of NAD+ and FAD transport electrons to the electron transport chain (ETC) in the mitochondria. The electrons are passed through various protein complexes in the ETC, leading to the production of ATP via oxidative phosphorylation. NAD+ and FAD are recycled back to their oxidized forms during this process, enabling continuous ATP production.

12. Discuss the role of the enzyme NADH dehydrogenase in the electron transport chain.

Answer: NADH dehydrogenase is the first enzyme complex in the electron transport chain (ETC) located in the inner mitochondrial membrane. It is responsible for transferring electrons from NADH to the electron transport chain. NADH dehydrogenase accepts two electrons from NADH, which are then passed to ubiquinone (CoQ). In this process, NADH dehydrogenase also pumps protons (H⁺) from the mitochondrial matrix into the intermembrane space, contributing to the creation of a proton gradient that is later used to generate ATP by ATP synthase. The enzyme plays a crucial role in energy production by initiating the chain of reactions that leads to ATP synthesis.

13. What is the function of hexokinase in glycolysis?

Answer: Hexokinase is the first enzyme in the glycolytic pathway, and it catalyzes the phosphorylation of glucose to form glucose-6-phosphate. This reaction uses ATP as the phosphate donor. The phosphorylation of glucose serves to trap the glucose inside the cell, as glucose-6-phosphate cannot easily cross the plasma membrane. This step is important because it commits glucose to the glycolytic pathway, allowing it to be further metabolized to produce ATP. Hexokinase is tightly regulated to prevent unnecessary energy expenditure, especially in tissues where glucose is already abundant.

14. How does the enzyme RuBisCO contribute to carbon fixation?

Answer: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme responsible for catalyzing the first step of the Calvin cycle, known as carbon fixation. In this step, RuBisCO binds carbon dioxide from the atmosphere to ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar. This reaction results in the formation of an unstable six-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. This is the first step in converting atmospheric carbon dioxide into organic compounds that the plant can use for energy and growth.

15. What is the role of the enzyme ATP synthase in photosynthesis?

Answer: ATP synthase in photosynthesis is responsible for the production of ATP during the light-dependent reactions. It is located in the thylakoid membrane of the chloroplast. During the light-dependent reactions, light energy is used to excite electrons in chlorophyll, which are passed along an electron transport chain. This movement of electrons creates a proton gradient across the thylakoid membrane. As protons flow back into the stroma through ATP synthase, the enzyme uses the energy from this flow to synthesize ATP from ADP and inorganic phosphate. This ATP is then used in the Calvin cycle to power the production of sugars from carbon dioxide.

16. How does phosphoenolpyruvate carboxylase (PEP carboxylase) aid in CAM photosynthesis?

Answer: Phosphoenolpyruvate carboxylase (PEP carboxylase) plays a central role in CAM (Crassulacean Acid Metabolism) photosynthesis, a process adapted by plants in arid environments. Unlike the conventional C3 pathway, CAM plants fix carbon at night when stomata are open, allowing the plant to minimize water loss. PEP carboxylase catalyzes the conversion of phosphoenolpyruvate (PEP) to oxaloacetate, which is then converted to malate. The malate is stored in vacuoles overnight and is decarboxylated during the day to release CO2, which enters the Calvin cycle. This mechanism allows CAM plants to conserve water while still conducting photosynthesis.

17. What role do enzymes play in the regulation of the Calvin cycle?

Answer: Enzymes play a critical role in the regulation of the Calvin cycle by controlling the rate of key reactions and ensuring that the cycle operates efficiently. Key enzymes include RuBisCO, which catalyzes the fixation of CO2 into ribulose-1,5-bisphosphate, and glyceraldehyde-3-phosphate dehydrogenase, which is involved in the reduction of 3-phosphoglycerate. The activity of these enzymes is influenced by factors such as light, CO2 concentration, and the availability of ATP and NADPH. In particular, RuBisCO is regulated by magnesium ions and the presence of carbon dioxide, ensuring that the Calvin cycle proceeds only when conditions are favorable for carbon fixation.

18. What is the importance of NADH and FADH2 in cellular respiration?

Answer: NADH and FADH2 are essential electron carriers in cellular respiration. They are produced during glycolysis, the citric acid cycle, and fatty acid oxidation. Both NADH and FADH2 carry high-energy electrons to the electron transport chain (ETC) in the mitochondria. As electrons pass through the ETC, they release energy that is used to pump protons across the mitochondrial membrane, creating a proton gradient. This gradient is then used by ATP synthase to produce ATP. The electrons from NADH and FADH2 ultimately combine with oxygen to form water, which is essential for the final step of oxidative phosphorylation.

19. How does the enzyme citrate synthase function in the citric acid cycle?

Answer: Citrate synthase is an enzyme that catalyzes the first step of the citric acid cycle (Krebs cycle). It combines acetyl-CoA with oxaloacetate to form citrate, a six-carbon compound. This reaction is crucial because it initiates the cycle, leading to the production of high-energy electron carriers (NADH and FADH2) that will be used in the electron transport chain to generate ATP. Citrate synthase is regulated by the availability of acetyl-CoA and oxaloacetate, ensuring that the citric acid cycle proceeds when there is sufficient substrate for energy production.

20. What role do enzymes play in the fermentation process?

Answer: Enzymes play an essential role in the fermentation process, which occurs when oxygen is unavailable for cellular respiration. During fermentation, enzymes such as hexokinase, pyruvate kinase, and lactate dehydrogenase help in the breakdown of glucose to generate ATP. In lactic acid fermentation, pyruvate is converted into lactate by lactate dehydrogenase, which regenerates NAD+ so glycolysis can continue. In alcoholic fermentation, pyruvate is decarboxylated to form ethanol and CO2, catalyzed by enzymes like pyruvate decarboxylase and alcohol dehydrogenase. These enzyme-driven pathways allow cells to continue producing ATP under anaerobic conditions.