Questions & explanations
1. Substrate-level phosphorylation in glycolysis directly produces ATP by transferring a phosphate group from which intermediate to ADP?
- 1,3-bisphosphoglycerate
- Glucose-6-phosphate
- Fructose-1,6-bisphosphate
- Pyruvate
Answer: 1,3-bisphosphoglycerate
In glycolysis, substrate-level phosphorylation occurs when 1,3-bisphosphoglycerate donates a phosphate group to ADP via phosphoglycerate kinase, forming ATP. The other intermediates are involved in other steps but not directly in ATP generation via substrate-level phosphorylation.
2. Which of the following best describes respiration in living organisms?
- Oxidation of glucose to release energy
- Reduction of glucose to form lactic acid
- Synthesis of glucose from carbon dioxide
- Breakdown of proteins to release energy
Answer: Oxidation of glucose to release energy
Respiration is the process by which organisms oxidize glucose to release energy in the form of ATP. The other options describe processes like anaerobic fermentation, photosynthesis, or protein catabolism, which are not the primary definition of respiration.
3. Under anaerobic conditions in human muscle cells, pyruvate is converted to:
- Lactate
- Ethanol
- Acetyl-CoA
- Oxaloacetate
Answer: Lactate
In the absence of oxygen, human muscle cells convert pyruvate to lactate via lactate dehydrogenase, regenerating NAD+ for glycolysis. Ethanol is produced in yeast, while acetyl-CoA and oxaloacetate are involved in aerobic pathways.
4. Glycolysis occurs in which part of the cell?
- Cytoplasm
- Mitochondrial matrix
- Inner mitochondrial membrane
- Nucleus
Answer: Cytoplasm
Glycolysis takes place in the cytoplasm of the cell, regardless of whether oxygen is present. The other locations are involved in later stages of respiration (Krebs cycle, electron transport chain) or in other cellular processes.
5. What is the end product of glycolysis?
- Two molecules of pyruvate
- One molecule of glucose
- Two molecules of acetyl-CoA
- One molecule of lactate
Answer: Two molecules of pyruvate
Glycolysis converts one molecule of glucose into two molecules of pyruvate. Acetyl-CoA is formed from pyruvate in the mitochondria, and lactate is produced under anaerobic conditions in some organisms.
6. Lactic acid fermentation occurs in which of the following?
- Yeast cells under aerobic conditions
- Muscle cells during intense exercise
- Plant cells during photosynthesis
- Bacterial cells during aerobic respiration
Answer: Muscle cells during intense exercise
During intense exercise, oxygen supply is limited, and muscle cells convert pyruvate to lactic acid via lactic acid fermentation. Yeast undergoes alcoholic fermentation, not lactic acid fermentation.
7. What is the net ATP yield from fermentation of one molecule of glucose?
- 2 ATP
- 4 ATP
- 36 ATP
- 38 ATP
Answer: 2 ATP
Fermentation only yields 2 ATP per glucose from glycolysis, as no additional ATP is produced in the absence of oxygen. Aerobic respiration yields a net of 36 ATP per glucose molecule as per NCERT.
8. Which of the following processes involves incomplete oxidation of glucose?
- Aerobic respiration
- Anaerobic respiration
- Krebs cycle
- Electron transport chain
Answer: Anaerobic respiration
Anaerobic respiration involves incomplete oxidation of glucose, yielding only 2 ATP per glucose, whereas aerobic respiration fully oxidizes glucose to CO2 and water, producing much more ATP.
9. During glycolysis, ATP is consumed in the early steps and produced later. Which phase is associated with net ATP production?
- Energy payoff phase
- Energy investment phase
- Both phases equally
- Neither phase
Answer: Energy payoff phase
In glycolysis, the energy investment phase uses 2 ATP, while the energy payoff phase produces 4 ATP, resulting in a net gain of 2 ATP. Thus, the payoff phase is where net ATP is generated.
10. In alcoholic fermentation, pyruvate is converted to ethanol and CO2. Which enzymes catalyze this conversion?
- Pyruvate dehydrogenase and alcohol dehydrogenase
- Pyruvic acid decarboxylase and alcohol dehydrogenase
- Pyruvate kinase and alcohol dehydrogenase
- Pyruvic acid decarboxylase and lactate dehydrogenase
Answer: Pyruvic acid decarboxylase and alcohol dehydrogenase
In yeast, pyruvate is first decarboxylated to acetaldehyde by pyruvic acid decarboxylase, then reduced to ethanol by alcohol dehydrogenase. The other options involve incorrect enzymes.
11. Which of the following respiratory quotient (RQ) values is correct for carbohydrates?
- 1
- Less than 1
- Greater than 1
- 0
Answer: 1
For carbohydrates, RQ = 1 because the amount of CO2 evolved equals the amount of O2 consumed. Fats and proteins have RQ < 1, organic acids have RQ > 1.
12. In which cellular compartment does the oxidative decarboxylation of pyruvate to acetyl-CoA occur?
- A. Cytoplasm
- B. Mitochondrial matrix
- C. Inner mitochondrial membrane
- D. Nucleus
Answer: B. Mitochondrial matrix
The link reaction, which converts pyruvate to acetyl-CoA, occurs in the mitochondrial matrix where the pyruvate dehydrogenase complex is located.