All Free Biology MCQs with Answers

3951. What energy-rich organic compound is produced as a result of the Calvin cycle?

• A. NADPH
• B. CO2
• C. ATP
• D. Glucose

Explanation: As a result of the Calvin cycle, the energy-rich organic compound produced is glucose. The Calvin cycle is the second stage of photosynthesis and takes place in the stroma of chloroplasts. It converts carbon dioxide (CO2) and energy-rich molecules like ATP and NADPH (produced during the light-dependent reactions) into glucose. This process is also known as carbon fixation, as it fixes carbon from the atmosphere into organic molecules. Glucose serves as a primary energy source for plants and is used in various metabolic processes or stored as starch to support growth and development.

3952. Does Oxaloacetate combine with which molecule to enter the Krebs cycle again?

• A. ATP
• B. NADPH
• C. FAD
• D. Acetyl CoA

Explanation: Oxaloacetate combines with acetyl-CoA to enter the Krebs cycle again. The Krebs cycle, also known as the citric acid cycle or TCA cycle (tricarboxylic acid cycle), is a metabolic pathway that takes place in the mitochondrial matrix. It plays a crucial role in the aerobic respiration of carbohydrates, fats, and proteins, generating energy in the form of ATP and producing electron carriers (NADH and FADH2) that feed into the electron transport chain. The first step of the Krebs cycle involves the combination of a two-carbon acetyl group (from acetyl-CoA) with a four-carbon compound, oxaloacetate, to form a six-carbon molecule called citrate. This reaction is catalyzed by an enzyme called citrate synthase. The overall reaction can be represented as follows: Acetyl-CoA + Oxaloacetate → Citrate + CoA.

3953. Hexokinase plays a role in:

• A. Krebs cycle
• B. Electron transport chain
• C. Glycolysis
• D. Pyruvate oxidation

Explanation: Glycolysis is a metabolic pathway that occurs in the cytoplasm of cells and is responsible for the breakdown of glucose into two molecules of pyruvate. It is the first step in cellular respiration and is a central pathway for glucose metabolism. Hexokinase plays a crucial role in the early steps of glycolysis. The reaction catalyzed by hexokinase in glycolysis is as follows: Glucose + ATP → Glucose-6-phosphate + ADP. In this step, hexokinase phosphorylates glucose using ATP as a phosphate donor, producing glucose-6-phosphate and ADP. This phosphorylation of glucose is essential for trapping glucose inside the cell and initiating its metabolism.

3954. Hexokinase is the enzyme found in:

• A. Glycolysis and pentose pathway
• B. Pentose pathway only
• C. Glycolysis only
• D. Krebs cycle

Explanation: Glycolysis is a metabolic pathway that occurs in the cytoplasm of cells and is responsible for the breakdown of glucose into two molecules of pyruvate. It is the first step in cellular respiration and is a central pathway for glucose metabolism. Hexokinase plays a crucial role in the early steps of glycolysis. The reaction catalyzed by hexokinase in glycolysis is as follows: Glucose + ATP → Glucose-6-phosphate + ADP. In this step, hexokinase phosphorylates glucose using ATP as a phosphate donor, producing glucose-6-phosphate and ADP. This phosphorylation of glucose is essential for trapping glucose inside the cell and initiating its metabolism. Pentose Phosphate Pathway (also known as the pentose phosphate shunt or the hexose monophosphate pathway): The pentose phosphate pathway is an alternative pathway for glucose metabolism. It occurs in the cytoplasm of cells and serves various purposes, including the production of NADPH (a reducing agent) and ribose-5-phosphate (used in nucleotide synthesis). In the pentose phosphate pathway, hexokinase is also involved in the first step: Glucose + ATP → Glucose-6-phosphate + ADP. Similarly to glycolysis, hexokinase phosphorylates glucose to produce glucose-6-phosphate, which can then be further metabolized through the pentose phosphate pathway. In summary, hexokinase is found in glycolysis and the pentose phosphate pathway, where it plays a key role in phosphorylating glucose to initiate its metabolism in these pathways. It is not present in the Krebs cycle.

3955. ATP synthetase is located in the _ of the mitochondrion.

• A. Intermembrane space
• B. Outer membrane
• C. Matrix
• D. Inner membrane

Explanation: The inner mitochondrial membrane is the site where ATP synthase is located. This membrane is impermeable to most ions and molecules and plays a crucial role in establishing the proton gradient necessary for ATP synthesis. The electron transport chain, which is part of oxidative phosphorylation, takes place on the inner membrane. As electrons pass through the electron transport chain, protons are pumped from the matrix to the intermembrane space, creating a proton gradient. ATP synthase is embedded in the inner membrane and utilizes this proton gradient to convert ADP (adenosine diphosphate) and inorganic phosphate (Pi) into ATP.

3956. What are the products of respiration in plants?

• A. CO2 and H2O
• B. CO2, H2O and ATP
• C. C6H12O6 and H2O
• D. None

Explanation: Respiration in plants involves the oxidation of glucose to produce energy, with carbon dioxide (CO2) and water (H2O) as byproducts. The primary purpose is to generate ATP, the energy currency of the cell, which is used for various cellular processes. Option B is correct because it includes ATP, which is crucial for energy transfer within cells. Option A is incorrect because it omits ATP, a key product of respiration. Option C incorrectly lists glucose as a product instead of a reactant. Option D is incorrect as it states that there are no products, whereas respiration clearly produces CO2, H2O, and ATP.

3957. The pay-off phase of glycolysis conserves:

• A. Molecules of glucose
• B. ATP
• C. Molecules of fructose
• D. Water

Explanation: Oxidative phase is pay-off phase in which not only ATPs are produced through substrate level phosphorylation but it also produces NADH which upon further oxidation in respiratory electron transport chain yields more ATPs.

3958. End product of citric acid cycle is:

• A. Pyruvate
• B. CO2 and H2O
• C. CO2
• D. Lactic acid

Explanation: The correct answer is CO2. The citric acid cycle, also known as the Krebs cycle, plays a crucial role in cellular respiration. During the cycle, acetyl-CoA undergoes oxidation, leading to the release of carbon dioxide (CO2) as a waste product. While CO2 is the direct product of this cycle, water (H2O) is primarily formed during the later stages of cellular respiration, specifically in the electron transport chain. Pyruvate is a precursor to the cycle, being converted into acetyl-CoA, while lactic acid is a product of anaerobic processes, not the citric acid cycle.

3959. Oaxaloacetate contains how many carbon atoms?

• A. 4
• B. 5
• C. 6
• D. 2

Explanation: Acetyl CoA (2-carbon compound) and a water molecule combine with oxaloacetate (4-carbon compound) to form a 6-carbon rnmpound called citrate (citric acid). It is the first product of Krebs cycle. CoA is liberated.

3960. ATP molecules are consumed during which process?

• A. Glycolysis
• B. Light dependent phase
• C. Krebs cycle
• D. None

Explanation: Glycolysis is the correct answer because it begins with the investment of 2 ATP molecules to phosphorylate glucose, which is necessary to drive the subsequent reactions leading to pyruvate production. This initial consumption of ATP is crucial for the glycolytic pathway to proceed.The light-dependent phase of photosynthesis, on the other hand, is involved in ATP production, not consumption, as it captures light energy to generate ATP and NADPH.The Krebs cycle is mainly involved in the extraction of energy from acetyl-CoA, resulting in the production of ATP, NADH, and FADH2 without directly consuming ATP in its reactions. Therefore, glycolysis is the only process among the options where ATP consumption is an integral part of its mechanism.