All Free Biology MCQs with Answers

3191. What is the coenzyme that facilitates the oxidation of fumarate?

• A. FAD
• B. FADH2
• C. NAD
• D. NADPH

Explanation: FAD (Flavin Adenine Dinucleotide) acts as a coenzyme in the oxidation of succinate to fumarate during the Krebs cycle. It is reduced to FADH in this process, which occurs in the mitochondrial matrix.

3192. Function of succinate dehydrogenase is aided by:

• A. NAD
• B. FAD
• C. Metal ion
• D. Vitamin

Explanation: Succinate dehydrogenase is an integral enzyme in the citric acid cycle and the electron transport chain. It catalyzes the oxidation of succinate to fumarate, using FAD as a cofactor. FAD accepts electrons, forming FADH , which then donates electrons to the electron transport chain, contributing to ATP synthesis through oxidative phosphorylation. NAD, metal ions, and vitamins do not serve as direct cofactors for this enzyme's specific action within these pathways.

3193. Which is most closely associated with the calvin cycle ?

• A. ATP production.
• B. Sugar production.
• C. Oxygen production.
• D. Carbon dioxide production.

Explanation: The Calvin cycle uses CO₂, ATP, and NADPH to synthesize glucose (sugars) during photosynthesis.

3194. Water vapors exit and carbon dioxide enters a leaf through:

• A. Stomata
• B. Grana
• C. Porphyrin ring
• D. Photons

Explanation: Stomata are small pores found on the surface of plant leaves and stems. They regulate gas exchange, allowing water vapor to exit the leaf during transpiration and enabling the entry of carbon dioxide for photosynthesis.

3195. How are photosynthetic pigments organized?

• A. Clusters
• B. Photosystems
• C. Stacks
• D. Both A and B are correct

Explanation: Photosynthetic pigments are organized into clusters called 'photosystems' in the thylakoid membranes. These clusters are crucial for capturing light energy during the lightdependent reactions of photosynthesis. Option D is correct because it acknowledges both the clustering of pigments and their designation as photosystems. Option A is partially correct but does not specify the term 'photosystems'. Option B correctly identifies the term 'photosystems' but does not highlight the clustering aspect. Option C is incorrect because 'stacks' refer to the thylakoid membrane arrangement, not the organization of pigments.

3196. Excretory products of autotrophic plants:

• A. CO2
• B. O2
• C. H2O
• D. All of these

Explanation: During photosynthesis, plants use carbon dioxide and water to produce glucose and oxygen. The oxygen is released as an excretory product because it is not needed by the plant for photosynthesis. Therefore, the correct answer is oxygen (O ). Carbon dioxide (CO ) and water (H O) are essential reactants in this process and thus are not excretory products.

3197. In all plants, the major sites of photosynthesis are:

• A. Leaf.
• B. Stems
• C. Roots
• D. Branches

Explanation: The correct answer is 'Leaf'. In most plants, leaves are the primary sites for photosynthesis. They possess mesophyll cells filled with chloroplasts, which contain chlorophyll to capture sunlight and convert it into chemical energy. This process mainly occurs in the leaves because they are structured to maximize light absorption. Stems, roots, and branches, while essential for other functions such as support, transport, and nutrient absorption, do not have the necessary chloroplasts to perform photosynthesis effectively.

3198. Carotenoids perform protective function in which of the following organisms?

• A. Animal
• B. Plants
• C. Both A and B
• D. None of these

Explanation: This option is correct as carotenoids perform protective functions in both animals and plants.

3199. Identify the missing step in the image below:

• A. Glyceraldehyde-3-phosphate
• B. Dihydroxyacetone phosphate
• C. Fructose-1-phosphate
• D. Phosphofructokinase

Explanation: The missing step involves phosphorylation of fructose-6- phosphate by phosphofructokinase-1 (PFK-1), forming fructose-1,6-bisphosphate, a key rate-limiting step in glycolysis.

3200. Identify the steps where ATP is consumed in glycolysis:

• A. G6P → F6P
• B. F6P → F1,6BP
• C. G3P → 1,3-BPG
• D. 1,3-BPG → 3-PG

Explanation: ATP is consumed twice - once during Glucose → G6P (hexokinase) and once during F6P → F1,6BP (PFK-1). The question targets the second ATP-consuming step.