All Free Biology MCQs with Answers

3171. The final fate of glucose in yeast when it is deprived of oxygen:

• A. Lactate
• B. Pyruvate
• C. Acetaldehyde
• D. Alcohol

Explanation: In the absence of oxygen, yeast carries out alcoholic fermentation. Glucose is first converted to pyruvate, then to acetaldehyde, and finally reduced to ethanol (alcohol) as the end product.

3172. Water is released during the conversion of:

• A. 2-phosphoglycerate → PEP
• B. 3-phosphoglycerate → 2-phosphoglycerate
• C. 1,3-bisphosphoglycerate → 3-phosphoglycerate
• D. G3P → 1,3-bisphosphoglycerate

Explanation: During glycolysis, the conversion of 2-phosphoglycerate (2- PG) to phosphoenolpyruvate (PEP) is catalyzed by the enzyme enolase, where a molecule of water is removed (dehydration reaction). The reaction can be represented as follows: 2-phosphoglycerate (2-PG) ⇌ phosphoenolpyruvate (PEP) + H2O.

3173. Gross production of ATP molecules during glycolysis is

• A. 2
• B. 4
• C. 6
• D. 8

Explanation: In glycolysis, a total of 4 ATP molecules are produced in the energy payoff phase. However, since 2 ATP are consumed in the energy investment phase, the net gain is only 2 ATP.

3174. The oxidation of succinate produces:

• A. NADH
• B. FADH2
• C. Malate
• D. ATP

Explanation: During the Krebs cycle, succinate is oxidized to fumarate, producing FADH2 as an electron carrier in the process.

3175. The precursor of fumarate during Krebs cycle requires _ for fumarate synthesis:

• A. Amino acid aspartate
• B. Fumarate dehydrogenase
• C. Succinate dehydrogenase
• D. FAD+ oxidase

Explanation: Succinate dehydrogenase is the enzyme that catalyzes the conversion of succinate to fumarate in the Krebs cycle, making it the correct choice for fumarate synthesis.

3176. Starting from end products of glycolysis, how many molecules of CO2 are produced up to the formation of succinate in a single Krebs cycle?

• A. 2
• B. 12
• C. 6
• D. 3

Explanation: Starting from pyruvate (end product of glycolysis), three decarboxylation reactions occur: one during pyruvate oxidation and two during the Krebs cycle, producing a total of three CO₂ molecules per pyruvate.

3177. During the respiratory chain of cellular respiration, cytochrome c oxidizes:

• A. Oxygen
• B. Cytochrome a₃
• C. NADH
• D. FADH₂

Explanation: During the respiratory chain of cellular respiration, cytochrome c oxidizes cytochrome a₃. Cytochrome c acts as an electron carrier, transferring electrons from complex III to complex IV, where cytochrome a₃ is located. In this process, cytochrome c itself is oxidized as it donates electrons, and cytochrome a₃ is reduced. This step is crucial because it allows electrons to continue down the chain and ultimately reduces oxygen to water at the end of the respiratory chain. Cytochrome c does not oxidize oxygen directly, nor does it act on NADH or FADH₂, which serve as earlier electron donors in the chain.

3178. The electron transport chain in mitochondria is used to:

• A. Synthesize NADP+
• B. Synthesize ADP
• C. Create electron gradient
• D. Create proton gradient

Explanation: The electron transport chain in mitochondria creates a proton gradient across the inner mitochondrial membrane. This gradient is crucial for ATP synthesis via ATP synthase in a process called oxidative phosphorylation.

3179. Which of the following types of mammalian cell does not carry out oxidative phosporylation?

• A. Erythrocytes
• B. Neuron
• C. Oxyntic cell
• D. Cardiac muscle cell

Explanation: Oxidative phosphorylation takes place in mitochondria. Erythrocytes (red blood cells) lack mitochondria, so they cannot perform oxidative phosphorylation and rely only on glycolysis for ATP production. Other cells like neurons, oxyntic cells, and cardiac muscle cells contain mitochondria and carry out oxidative phosphorylation.

3180. The stage of cellular respiration producing maximum ATP:

• A. Glycolysis
• B. Oxidation of pyruvate
• C. Krebs cycle
• D. Chemiosmosis

Explanation: Chemiosmosis uses the proton gradient created by the electron transport chain to produce the majority of ATP in cellular respiration. Glycolysis and the Krebs cycle produce only small amounts of ATP directly, while chemiosmosis generates the majority.