All Free Biology MCQs with Answers

3571. Cyclic phosphorylation is stimulated by:

• A. Low level of NADPH
• B. High level of NADPH
• C. Low light intensity
• D. High light intensity

Explanation: Cyclic phosphorylation is a process that occurs in the light-dependent reactions of photosynthesis, where electrons are recycled within the photosystem to generate ATP without producing NADPH. It is stimulated by a high level of NADPH because this indicates that the cell has sufficient reducing agents and needs more ATP to maintain balance in the energy supply. Therefore, when NADPH levels are high, the demand for ATP increases, leading to the activation of cyclic phosphorylation. The other options are incorrect because:Low level of NADPH would not stimulate cyclic phosphorylation as it indicates a shortage of reducing power needed for other processes.Low light intensity affects the overall efficiency of photosynthesis but does not specifically trigger cyclic phosphorylation.High light intensity could lead to increased overall activity but does not directly correlate with the stimulation of cyclic phosphorylation as it depends more on the ATP/NADPH ratio than light intensity alone.

3572. The first stable product of the light-independent reaction having 3-carbons is:

• A. 3-phosphoglyceraldehyde
• B. 3-phosphoglyceric acid
• C. 1,3-biphosphate glycerate
• D. Dihydroxyacetone phosphate

Explanation: 3-phosphoglyceric acid - Correct. This is the first stable 3-carbon compound formed in the Calvin cycle.

3573. In cyclic photophosphorylation, the flow of electrons will be:

• A. PEA > Cytochrome complex > Plastocyanin
• B. PEA > Ferredoxin > Plastocyanin
• C. PEA > Cytochrome complex > Ferredoxin
• D. PEA > Ferredoxin > Cytochrome complex > Plastocyanin

Explanation: PEA > Ferredoxin > Cytochrome complex > Plastocyanin - Correct. In cyclic photophosphorylation, electrons flow from the primary electron acceptor (PEA) to ferredoxin

3574. The NADPH and ATP from the light-dependent reactions are used to:

• A. Regenerate RuBP
• B. Fix CO₂
• C. Convert PGAL to RuP
• D. Convert PGA to PGAL

Explanation: Convert PGA to PGAL - Correct. ATP and NADPH are used in the Calvin cycle to reduce 3-phosphoglycerate (PGA) to glyceraldehyde-3-phosphate (PGAL), which is a key step in sugar production.

3575. Carbon dioxide fixation can be enhanced by enhancing the efficiency of:

• A. Auxins
• B. Ribulose bisphosphate
• C. Lactoferrin
• D. Agrobacterium

Explanation: Carbon dioxide fixation, particularly in the context of photosynthesis, can be enhanced by enhancing the efficiency of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). RuBisCO is a key enzyme involved in the first step of the Calvin Cycle, where carbon dioxide is fixed into organic molecules.

3576. The end product of glycolysis in anaerobic respiration is:

• A. Ethanol and Carbon dioxide
• B. Lactate
• C. Pyruvate
• D. Both A and B

Explanation: The end product of glycolysis is always pyruvate

3577. The element in basic structure of proteins which differentiate them from carbohydrates is?

• A. Carbon
• B. Hydrogen
• C. Nitrogen
• D. None of these options is correct.

Explanation: Proteins, carbohydrates, and fats are made from three basic molecules: "carbon", "hydrogen", and "oxygen". However, all proteins contain an element not found in "carbohydrates" and "fats"; nitrogen, while some proteins also contain sulfur.

3578. Pathway of energy used by muscles converted from food is:

• A. Food-ATP-creatine phosphate- protein
• B. Food-glycogen- ATP-creatine phosphate
• C. Food-glycogen-creatine phosphate- ATP
• D. Food-protein-creatine phosphate- ATP

Explanation: Muscles convert food to energy by first breaking down glycogen to glucose. Glucose is then used to form creatine phosphate which is then metabolized to form ATP.

3579. The following flowchart depicts the steps of the Calvin Cycle. Which option, according to you, fits in as the correct answer of the missing step?

• A. Pyruvate
• B. Hydrogenase
• C. Ribulose bisphosphate
• D. Oxaloacetate

Explanation: The first step involves the fixing of carbon (derived from CO2) into molecules of Ribulose Bisphosphate (RuBP) via the action of the enzyme 'rubisco', resulting in the formation of 3-phosphoglycerate. This is further processed into 1,3-BPG and eventually, G-3-P, before the cycle starts again. The chart above only has RuBP missing.

3580. In which of the following does not net gaseous exchange take place between leaves and the atmosphere?

• A. Day
• B. Night
• C. Dawn and Dusk
• D. Midnight

Explanation: At dawn and dusk, the intensity of light is low. At this moment the amount of oxygen released from photosynthesis is equal to the amount of oxygen used in respiration. Similarly, the amount of carbon dioxide released from respiration is equal to the amount of carbon dioxide in photosynthesis. It is called "Compensation Point." Hence, option C is correct.