1. What is Glycolysis? Describe its role in cellular metabolism.
Answer: Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, generating small amounts of ATP and NADH in the process. It occurs in the cytoplasm of both prokaryotic and eukaryotic cells and is an anaerobic pathway, meaning it does not require oxygen. Glycolysis is critical as it is the first step in glucose metabolism and provides energy in the form of ATP and NADH for cells, especially under low-oxygen conditions. It serves as the precursor for further metabolic pathways like the citric acid cycle (aerobic respiration) and fermentation (anaerobic respiration).
2. Explain the two phases of glycolysis.
Answer: Glycolysis consists of two phases: the energy investment phase and the energy payoff phase.
- Energy Investment Phase: In this phase, two ATP molecules are consumed to activate glucose. The process starts with the phosphorylation of glucose to form glucose-6-phosphate. This is followed by a series of steps that involve the conversion of glucose-6-phosphate to fructose-1,6-bisphosphate.
- Energy Payoff Phase: In this phase, four ATP molecules and two NADH molecules are generated. The fructose-1,6-bisphosphate is split into two three-carbon molecules, which undergo oxidation and phosphorylation to form pyruvate, along with the production of ATP and NADH.
The overall reaction of glycolysis is the conversion of one molecule of glucose into two molecules of pyruvate, with a net gain of 2 ATP and 2 NADH.
3. What is the role of hexokinase in glycolysis?
Answer: Hexokinase is the enzyme responsible for the first step in glycolysis, where it catalyzes the phosphorylation of glucose to form glucose-6-phosphate. This step is crucial because it traps glucose inside the cell and prevents it from diffusing out. The addition of a phosphate group also prepares glucose for subsequent metabolic reactions. The reaction requires ATP as a phosphate donor.
4. Describe the function of phosphofructokinase (PFK) in glycolysis.
Answer: Phosphofructokinase (PFK) is one of the key regulatory enzymes in glycolysis. It catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, a critical step in glycolysis. This step is irreversible and is considered the rate-limiting step of glycolysis. PFK activity is tightly regulated by the levels of ATP, ADP, and other molecules like citrate. High ATP concentrations inhibit PFK, slowing down glycolysis, while high ADP levels activate PFK, accelerating glycolysis. This regulation helps balance energy production and consumption in cells.
5. Explain the role of NAD+ and NADH in glycolysis.
Answer: NAD+ is an essential coenzyme involved in the oxidation-reduction reactions of glycolysis. During the conversion of glyceraldehyde-3-phosphate (G3P) to 1,3-bisphosphoglycerate, NAD+ is reduced to NADH. This reduction is coupled with the oxidation of G3P, ensuring that the energy produced in glycolysis is stored in the form of NADH. The NADH molecules produced in glycolysis can later be used in oxidative phosphorylation (if oxygen is available) to generate more ATP. NAD+ must be regenerated for glycolysis to continue, and this is achieved through either aerobic or anaerobic pathways.
6. What happens during the cleavage of fructose-1,6-bisphosphate in glycolysis?
Answer: Fructose-1,6-bisphosphate is cleaved into two three-carbon molecules by the enzyme aldolase. This reaction splits the six-carbon sugar into two three-carbon compounds: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). DHAP can be converted into G3P by the enzyme triose phosphate isomerase. Thus, for every molecule of fructose-1,6-bisphosphate, two molecules of G3P are produced, which can then proceed through the rest of the glycolytic pathway.
7. Describe the importance of substrate-level phosphorylation in glycolysis.
Answer: Substrate-level phosphorylation is the process by which ATP is synthesized directly from a phosphorylated substrate, without the involvement of an electron transport chain or oxygen. In glycolysis, substrate-level phosphorylation occurs twice: once when 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate, and once when phosphoenolpyruvate is converted to pyruvate. These steps produce a net gain of 2 ATP molecules per glucose molecule, which is essential for energy supply in cells, particularly under anaerobic conditions.
8. What is the significance of pyruvate in glycolysis?
Answer: Pyruvate is the end product of glycolysis and plays a crucial role in cellular metabolism. Depending on the availability of oxygen, pyruvate can enter the mitochondria for aerobic respiration (citric acid cycle and oxidative phosphorylation) or can be converted to lactate or ethanol in anaerobic conditions. Pyruvate is a key link between glycolysis and other metabolic pathways. Its fate determines whether cells will produce more ATP via oxidative phosphorylation or rely on fermentation for energy production.
9. What are the net products of glycolysis?
Answer: The net products of glycolysis, starting with one molecule of glucose, are:
- 2 molecules of pyruvate
- 2 molecules of NADH
- 2 molecules of ATP (4 ATP are produced, but 2 ATP are consumed in the energy investment phase)
These products can then be used for energy production or metabolic intermediates for other biosynthetic pathways.
10. How is glycolysis regulated in response to the cell’s energy status?
Answer: Glycolysis is regulated through key enzymes that respond to the cell’s energy status:
- Hexokinase: Inhibited by its product, glucose-6-phosphate, to prevent excessive glucose phosphorylation when energy is abundant.
- Phosphofructokinase (PFK): The rate-limiting enzyme of glycolysis, which is inhibited by high ATP levels and activated by AMP, ADP, or fructose-2,6-bisphosphate. These signals indicate the cell’s energy needs.
- Pyruvate kinase: Inhibited by ATP and acetyl-CoA and activated by fructose-1,6-bisphosphate.
This regulation ensures that glycolysis occurs when the cell needs ATP and slows down when energy is abundant.
11. Describe the fate of pyruvate in aerobic conditions after glycolysis.
Answer: Under aerobic conditions, pyruvate produced in glycolysis is transported into the mitochondria, where it undergoes decarboxylation to form acetyl-CoA. Acetyl-CoA enters the citric acid cycle (Krebs cycle) and is further oxidized to produce NADH, FADH2, and ATP. These high-energy molecules then enter the electron transport chain to generate a large amount of ATP through oxidative phosphorylation. Oxygen acts as the final electron acceptor in the electron transport chain, allowing the entire process to be efficient and energy-producing.
12. How does glycolysis contribute to anaerobic respiration in muscles?
Answer: In the absence of oxygen, such as during intense exercise, glycolysis provides ATP through anaerobic respiration. The pyruvate produced in glycolysis is converted to lactate by the enzyme lactate dehydrogenase, regenerating NAD+ from NADH. This allows glycolysis to continue and produce ATP, even in the absence of oxygen. However, the accumulation of lactate can lead to muscle fatigue, and this process is less efficient in terms of ATP production compared to aerobic respiration.
13. Explain the role of the enzyme lactate dehydrogenase in anaerobic glycolysis.
Answer: Lactate dehydrogenase (LDH) plays a crucial role in anaerobic glycolysis by converting pyruvate to lactate. This reaction also involves the conversion of NADH to NAD+, which is essential for maintaining the continuation of glycolysis. By regenerating NAD+, LDH ensures that glycolysis can continue producing ATP, even when oxygen is not available. The buildup of lactate, however, can lower the pH of the muscle cells, contributing to fatigue and soreness.
14. How does glycolysis differ in prokaryotes and eukaryotes?
Answer: The process of glycolysis itself is very similar in both prokaryotes and eukaryotes, involving the same enzymes and steps. However, the key difference is the location: glycolysis occurs in the cytoplasm of both prokaryotic and eukaryotic cells. In eukaryotes, after glycolysis, pyruvate is transported into the mitochondria for further processing (aerobic respiration), while in prokaryotes, the entire process takes place in the cytoplasm due to the absence of mitochondria.
15. What is the role of phosphoenolpyruvate in glycolysis?
Answer: Phosphoenolpyruvate (PEP) is one of the intermediates in glycolysis and is involved in the final step, where it is converted to pyruvate by the enzyme pyruvate kinase. This step also involves the production of ATP through substrate-level phosphorylation. PEP is one of the highest-energy intermediates in glycolysis, and its conversion to pyruvate is essential for the generation of ATP.
16. What is the significance of the enzyme triose phosphate isomerase in glycolysis?
Answer: Triose phosphate isomerase (TPI) is an enzyme that catalyzes the reversible conversion of dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (G3P) during glycolysis. This is an important step because both DHAP and G3P can be processed further in the pathway, but only G3P continues through glycolysis. TPI ensures that the two molecules produced from the cleavage of fructose-1,6-bisphosphate are in equilibrium, allowing the pathway to proceed efficiently.
17. How is ATP produced in glycolysis without the involvement of oxygen?
Answer: ATP is produced in glycolysis through substrate-level phosphorylation, a process that does not require oxygen. In this process, high-energy phosphorylated intermediates (such as 1,3-bisphosphoglycerate and phosphoenolpyruvate) transfer a phosphate group to ADP, forming ATP. This allows cells to generate energy in anaerobic conditions when oxygen is not available, although this method of ATP production is less efficient than aerobic respiration.
18. What is the effect of low oxygen on glycolysis?
Answer: Under low oxygen conditions, glycolysis becomes the primary source of ATP production through anaerobic respiration. Since the electron transport chain cannot operate efficiently without oxygen, cells rely on glycolysis to produce ATP. The pyruvate produced in glycolysis is converted into lactate or ethanol, depending on the organism, to regenerate NAD+ and allow glycolysis to continue. However, this shift results in less ATP production compared to aerobic respiration.
19. Explain the connection between glycolysis and the citric acid cycle.
Answer: The connection between glycolysis and the citric acid cycle lies in the conversion of pyruvate (the end product of glycolysis) to acetyl-CoA. In aerobic conditions, pyruvate is transported into the mitochondria, where it is decarboxylated to form acetyl-CoA. Acetyl-CoA then enters the citric acid cycle, where it undergoes further oxidation to generate ATP, NADH, and FADH2. These high-energy molecules are used in the electron transport chain to produce a larger amount of ATP.
20. What are the key enzymes involved in the regulation of glycolysis?
Answer: The key enzymes involved in the regulation of glycolysis are:
- Hexokinase: Regulated by glucose-6-phosphate levels.
- Phosphofructokinase (PFK): The rate-limiting enzyme, regulated by ATP, ADP, and fructose-2,6-bisphosphate.
- Pyruvate kinase: Regulated by ATP and fructose-1,6-bisphosphate. These enzymes ensure that glycolysis is activated or inhibited based on the cell’s energy needs.
