All Free Biology MCQs with Answers

3701. Dark reactions of carbon assimilation occur in:

• A. Cytoplasmic matrix
• B. Leucoplasts
• C. Mitochondria
• D. Chloroplasts

Explanation: The correct answer is chloroplast. Chloroplasts are the sites of photosynthesis in plant cells, encompassing both the light-dependent and light-independent reactions. The light-independent reactions, also known as the Calvin cycle or dark reactions, occur in the stroma of chloroplasts. During these reactions, carbon dioxide is assimilated into organic molecules like glucose. The other options are incorrect because they do not involve the photosynthetic process: the cytoplasmic matrix is involved in general cellular activities, leucoplasts are for storage, and mitochondria are involved in energy production through cellular respiration, not photosynthesis.

3702. ATP molecules required for the synthesis of a glucose molecule in the Benson-Calvin cycle are:

• A. 36
• B. 12
• C. 38
• D. 18

Explanation: The Benson-Calvin cycle, which occurs in the stroma of chloroplasts, is the light-independent phase of photosynthesis. During this cycle, carbon dioxide is fixed into organic molecules, eventually forming glucose. To produce one molecule of glucose, the cycle must turn six times, utilizing 18 ATP and 12 NADPH molecules. Each turn of the cycle fixes one carbon atom, requiring 3 ATP molecules. Therefore, 18 ATP molecules are necessary for the entire process. The other options (36, 12, 38) are incorrect, as they either represent energy needs in different processes or do not meet the specific requirements of the Benson-Calvin cycle.

3703. Photophosphorylation is a synthesis of:

• A. Glucose from carbon dioxide and water
• B. ATP from ADP and inorganic phosphate
• C. Oxygen from water molecules
• D. NADPH from NADP⁺ and hydrogen ions

Explanation: During photophosphorylation, light energy is captured by chlorophyll and other pigment molecules in the thylakoid membranes of chloroplasts. This energy is then used to generate ATP through the phosphorylation of ADP (adenosine diphosphate). In other words, photophosphorylation involves the synthesis of ATP from ADP and inorganic phosphate (Pi) using the energy obtained from light. This process occurs via two main mechanisms: cyclic and non-cyclic photophosphorylation. In cyclic photophosphorylation, only ATP is produced by utilizing light energy and an electron transport chain within the thylakoid membrane. Non-cyclic photophosphorylation involves both ATP and NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) synthesis and requires the participation of both photosystem I and photosystem II. Therefore, photophosphorylation is the synthesis of ATP from ADP.

3704. The following is/are obtained during cyclic photophosphorylation:

• A. ATP
• B. NADPH2
• C. O2
• D. All of these

Explanation: In cyclic photophosphorylation, electrons released by chlorophyll return to the same chlorophyll molecule, and ATP is produced. This process occurs in the thylakoid membranes and is part of the light-dependent reactions of photosynthesis. Unlike non-cyclic photophosphorylation, cyclic photophosphorylation does not produce NADPH₂ or oxygen (O₂), as there is no photolysis of water involved. Therefore, the correct answer is Option A: ATP.

3705. ATP formation occurs during all of the following steps of aerobic respiration except:

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

Explanation: Pyruvic acid oxidation does not directly produce ATP. Instead, it involves the conversion of pyruvate into acetyl-CoA, yielding NADH but no ATP. In contrast, glycolysis produces a net gain of 2 ATP per glucose molecule, the Krebs cycle generates ATP through substrate-level phosphorylation, and the electron transport chain produces ATP through oxidative phosphorylation. Therefore, pyruvic acid oxidation is the correct answer, as it is the step in aerobic respiration where ATP formation does not occur directly.

3706. The action spectrum of photosynthesis was described in 1883 by:

• A. Robert Hill
• B. C. Calvin
• C. T.W. Engelmann
• D. Hatch and Slack

Explanation: The correct answer is T.W. Engelmann. In 1883, Engelmann conducted experiments using a prism to disperse light across a filament of algae and used oxygen-sensitive bacteria to determine where photosynthesis was most active. This led to the first description of the action spectrum of photosynthesis, highlighting the effectiveness of different wavelengths of light. The other options involve significant figures in photosynthesis research but are associated with different discoveries: Robert Hill with the Hill reaction, C. Calvin with the Calvin cycle, and Hatch and Slack with the C4 pathway.

3707. Conversion of NAD+ into NADH requires:

• A. 2 electrons, 1 proton
• B. 2 Electrons 2 protons
• C. 1 Electron 2 protons
• D. 1 Electron 1 proton

Explanation: The conversion of NAD+ into NADH is a reduction process that requires the addition of 2 electrons and 1 proton. This results in the formation of NADH from NAD+. The correct option (A) reflects this requirement. Options B, C, and D are incorrect because they do not represent the accurate stoichiometry needed for the reduction of NAD+ to NADH.

3708. Which of the following molecules is reduced by accepting hydrogen in the Calvin cycle?

• A. 1,3-Bisphosphoglycerate
• B. Ribulose-1,5-bisphosphate
• C. 3-phosphoglycerate
• D. Glyceraldehyde-3-phosphate

Explanation: The molecule that is reduced by accepting hydrogen in the Calvin cycle is 1,3-bisphosphoglycerate. This reduction step, driven by NADPH, converts 1,3-bisphosphoglycerate into glyceraldehyde-3-phosphate, representing the key phase where energy and reducing power from light reactions are used to form carbohydrate precursors.

3709. The immediate source of energy for cellular metabolism is

• A. Lipids
• B. Carbohydrates
• C. ATP
• D. Proteins

Explanation: ATP (adenosine triphosphate) is the immediate source of energy for cellular metabolism because it directly powers cellular activities by releasing energy upon the hydrolysis of its phosphate bonds. Other options, such as lipids and carbohydrates, are involved in energy storage and production but do not directly provide immediate energy to cells. Lipids are used for long-term energy storage, carbohydrates are metabolized to produce ATP, and proteins primarily serve structural and functional roles, not as direct energy sources.

3710. Chlorophyll a is present in all except:

• A. Kelps
• B. Spirogyra
• C. Wheat
• D. Purple sulphur bacteria

Explanation: The correct answer is the D: purple sulfur bacteria these bacteria do not contain chlorophyll a; they utilise bacteriochlorophylls for photosynthesis in environments lacking oxygen. In contrast, kelps, Spirogyra, and wheat are all photosynthetic organisms that contain chlorophyll a, which is a crucial component for capturing light energy in the photosynthetic process.