All Free Biology MCQs with Answers

3831. In the Krebs cycle, for the formation of α-ketoglutarate, the following changes occur, EXCEPT:

• A. NAD-mediated oxidation
• B. Formation of NADH
• C. Decarboxylation
• D. Hydration

Explanation: In the formation of α-ketoglutarate during the Krebs cycle, several key changes occur that contribute to the cycle's energy yield. Isocitrate undergoes NAD-mediated oxidation, where electrons are transferred to NAD⁺, forming NADH. This step is crucial because it captures high-energy electrons that will later be used in ATP production. Simultaneously, the reaction involves decarboxylation, releasing a molecule of carbon dioxide and reducing the carbon count from six in isocitrate to five in α-ketoglutarate. Both the NAD-mediated oxidation and decarboxylation are essential for the proper transformation of isocitrate into α-ketoglutarate. The formation of NADH also occurs as a direct result of this oxidation, providing the reducing power necessary for the electron transport chain. Hydration, however, does not take place in this step. Hydration reactions occur in other parts of the Krebs cycle, such as the conversion of fumarate to malate, but the formation of α-ketoglutarate specifically involves only oxidation and decarboxylation, making hydration incorrect in this context.

3832. FAD is reduced to get FADH₂ in a step of the Krebs cycle, which involves the conversion of:

• A. Succinate to fumarate
• B. Malate to oxaloacetate
• C. Fumarate to malate
• D. α-ketoglutarate to succinate

Explanation: In the Krebs cycle, FAD is reduced to FADH₂ during the conversion of succinate to fumarate, a reaction catalyzed by the enzyme succinate dehydrogenase. This is the only step in the Krebs cycle where FAD is used as an electron acceptor. The other options involve different reactions: the conversion of malate to oxaloacetate involves NAD+ reduction, fumarate to malate involves hydration, and α-ketoglutarate to succinate involves NAD+ reduction.

3833. Succinate is converted into fumarate by the removal of:

• A. A single hydrogen atom
• B. CO2 molecule
• C. Two hydrogen atoms
• D. Water molecule

Explanation: In the Krebs cycle, succinate is converted to fumarate by the enzyme succinate dehydrogenase. This reaction involves the removal of two hydrogen atoms from succinate, which are then transferred to FAD to form FADH₂. Thus, option C is correct. Options A, B, and D are incorrect because they do not accurately describe the chemical changes occurring during this specific step of the Krebs cycle.

3834. Rearrangement followed by a second ATP phosphorylation involves step no. _ of glycolysis.

• A. 3
• B. 6
• C. 7
• D. 10

Explanation: Step number 7 of glycolysis is the correct option because it represents the second substrate-level phosphorylation in the pathway. In this step, 1,3-bisphosphoglycerate donates a high-energy phosphate group to ADP to form ATP and 3-phosphoglycerate, catalyzed by the enzyme phosphoglycerate kinase. This step is critical because it directly generates ATP, which is one of the main goals of glycolysis, and it follows the rearrangement and oxidation steps that prepare the molecule for efficient energy transfer. The reaction not only produces energy but also helps maintain the flow of intermediates through the pathway, ensuring that glycolysis proceeds efficiently to ultimately produce pyruvate.Step 3 is incorrect because it consumes ATP rather than producing it, as fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate. Step 6 is also incorrect because, although it produces a high-energy intermediate, no ATP is directly formed; it only sets the stage for the next ATP-generating step. Step 10 is not correct in this context because it is the third and final ATP-generating step, not the second, involving the conversion of phosphoenolpyruvate to pyruvate.

3835. In glycolysis, the six-carbon molecule is split into G3P and DAP, then DAP is also converted into G3P in step no. _.

• A. 2 & 3
• B. 3 & 4
• C. 4 & 5
• D. 5 & 6

Explanation: In glycolysis, during step 4, the six-carbon molecule fructose 1,6-bisphosphate is split into two three-carbon molecules: glyceraldehyde-3-phosphate (G-3-P) and dihydroxyacetone phosphate (DAP). Step 5 involves the conversion of DAP into another G-3-P molecule, ensuring both products of the split can continue through glycolysis. Options 2 & 3 and 3 & 4 are incorrect because they describe steps before or during the splitting process but do not include the conversion of DAP. Options 5 & 6 are incorrect because step 6 is where G-3-P is oxidized, unrelated to the splitting and conversion process.

3836. In glycolysis, oxidation followed by phosphorylation produces two NADH molecules and two molecules of BPG in step no:

• A. 4
• B. 5
• C. 6
• D. 7

Explanation: Step number 6 is the correct option because glyceraldehyde-3-phosphate is oxidized and phosphorylated to form 1,3-bisphosphoglycerate. In this reaction, NAD⁺ is reduced to NADH, and an inorganic phosphate is added, creating a high-energy intermediate. Since each glucose produces two glyceraldehyde-3-phosphate molecules, this step generates two NADH molecules and two molecules of 1,3-bisphosphoglycerate.The other steps are incorrect because they do not produce both NADH and 1,3-bisphosphoglycerate. Step 4 only splits fructose-1,6-bisphosphate, step 5 is just an isomerization, and step 7 generates ATP by using 1,3-bisphosphoglycerate rather than forming it. Only step 6 combines oxidation and phosphorylation to produce these molecules.

3837. The step of glycolysis that involves the removal of high-energy phosphate by two ADP molecules to get two ATP molecules and two 3PGA molecules is:

• A. 6
• B. 7
• C. 9
• D. 10

Explanation: Step number 7 is the correct step because it is the first ATP-generating reaction in glycolysis. In this step, 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP to form ATP and 3-phosphoglycerate, and since there are two molecules of 1,3-bisphosphoglycerate per glucose, this produces two ATP and two 3-phosphoglycerate molecules. Step 6 is incorrect because it forms 1,3-bisphosphoglycerate from glyceraldehyde-3-phosphate but does not produce ATP. Step 9 is wrong because it only dehydrates 2-phosphoglycerate to phosphoenolpyruvate, preparing the molecule for the next ATP-generating step without producing ATP itself. Step 10 produces ATP as well, but it is the final step of glycolysis and involves phosphoenolpyruvate, not 1,3-bisphosphoglycerate, so it is not the step described in the question.

3838. Removal of high-energy phosphate by two ADP molecules produces two ATP molecules and two pyruvate molecules in step no. _ of glycolysis.

• A. 7
• B. 8
• C. 9
• D. 10

Explanation: Step 10 of glycolysis is crucial, as it involves the conversion of phosphoenolpyruvate (PEP) into pyruvate, catalyzed by the enzyme pyruvate kinase. This reaction is coupled with the production of ATP from ADP, marking the final step of glycolysis, where energy is extracted in the form of ATP. Options 7, 8, and 9 are incorrect, as they correspond to earlier steps in glycolysis that do not result in the production of pyruvate and ATP simultaneously.

3839. The oxidation-reduction substances which take part in the respiratory chain are the following EXCEPT:

• A. Coenzyme Q
• B. Molecular oxygen
• C. Cytochromes b, c, a and a3
• D. Cytochrome f

Explanation: Cytochrome f is the correct option because it is not a component of the mitochondrial respiratory chain. Instead, it is found in the photosynthetic electron transport system of chloroplasts, where it forms part of the cytochrome b6f complex involved in the light-dependent reactions of photosynthesis. Its role there is to mediate the transfer of electrons between plastoquinone and plastocyanin, which is entirely different from the function of cytochromes in cellular respiration. Therefore, cytochrome f does not participate in the oxidative phosphorylation process of mitochondria and is excluded from the list of redox components in the respiratory chain.Coenzyme Q is an incorrect option because it acts as a mobile electron carrier within the mitochondrial inner membrane, transferring electrons between Complex I (or II) and Complex III.Molecular oxygen is an incorrect option because it serves as the final electron acceptor in the respiratory chain, combining with electrons and protons to form water.Cytochromes b, c, a, and a3 are incorrect options because they are essential components of the electron transport chain, facilitating the stepwise transfer of electrons that drives ATP synthesis.

3840. In the respiratory electron transport chain, the first ATP is formed from ADP and inorganic phosphate, utilizing the free energy obtained by oxidation of :

• A. NADH
• B. FADH
• C. Coenzyme Q
• D. Cytochrome C

Explanation: In the respiratory electron transport chain, the first ATP molecule is formed when electrons donated by NADH are transferred through the initial complexes of the chain. NADH is oxidized at Complex I, also called NADH dehydrogenase, releasing electrons that move through the chain and promote proton pumping across the inner mitochondrial membrane. The resulting proton gradient provides the free energy necessary for ATP synthesis by ATP synthase.FADH is incorrect because electrons from FADH₂ enter the chain at Complex II, bypassing Complex I, and therefore lead to the formation of fewer ATP molecules.Coenzyme Q is incorrect because it acts as an electron carrier, transferring electrons between Complexes I or II and III, but does not directly produce ATP through oxidation.Cytochrome c is incorrect because it transfers electrons between Complex III and Complex IV, yet the first ATP formation occurs before this step in the chain.