All Free Biology MCQs with Answers

3461. Which substance enters and leaves a mitochondrion during aerobic respiration?

• A. Pyruvate enters and ATP leaves
• B. Glucose enters and carbon dioxide leaves
• C. Oxygen enters and water leaves
• D. NADH enters and FAD leaves

Explanation: During aerobic respiration, pyruvate produced from glycolysis enters the mitochondria, where it is converted into acetyl-CoA, which enters the citric acid cycle. ATP, the energy currency of the cell, is generated within the mitochondria during oxidative phosphorylation and exits the mitochondria to be utilized by the cell. Other substances, like oxygen, also enter the mitochondria, while carbon dioxide and water are produced as byproducts inside the mitochondria.The correct answer is that pyruvate enters and ATP leaves, as this highlights the critical inputs and outputs of the mitochondria in the context of aerobic respiration.

3462. Which of the following equation most adequately sums up the photosynthetic reaction?

• A. CO2 + H2O + Light → CH2O + O2
• B. C6H12O6 + 6H2O → 6CO2 + 6H2O
• C. 6CO2 + 6H2O → C6H12O6 + 6O2
• D. 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O

Explanation: Option D is correct because it describes the complete photosynthetic process where carbon dioxide (6CO2) and water (12H2O), in the presence of light energy, are converted into glucose (C6H12O6) and oxygen (6O2). This equation accurately balances the water molecules, with 12 being consumed and 6 being produced. Option A oversimplifies the reaction by not representing the full glucose molecule. Option B incorrectly describes cellular respiration, a process opposite to photosynthesis. Option C omits the essential role of light energy and does not account for the correct water balance.

3463. If a photosynthesizing plant releases oxygen containing more than the normal amount of 18O, it is concluded that the plant has been supplied with:

• A. C6HI2O6 containing18O
• B. H2O containing 18O
• C. CO6 containing 18O
• D. Oxygen in the form of ozone

Explanation: If a photosynthesizing plant releases oxygen containing more than the normal amount of 18O, it is concluded that the plant has been supplied with H2O containing 18O.

3464. Where do the energy capturing reactions of photosynthesis occur?

• A. Plasma membrane
• B. Thylakoids
• C. Cytoplasm
• D. Stroma

Explanation: The energy-capturing reactions of photosynthesis, commonly referred to as the light-dependent reactions, occur in the thylakoids of chloroplasts. During these reactions, sunlight is absorbed by chlorophyll and other pigments in the thylakoid membranes, leading to the production of ATP and NADPH, which are crucial for the subsequent Calvin cycle in the stroma. The plasma membrane and cytoplasm are not involved in these specific reactions, and although the stroma is part of the chloroplast, it is the site of the Calvin cycle, not the light-dependent reactions.

3465. The final acceptor of electrons during the non-cyclic electron pathway is:

• A. Photosystem I
• B. Photosystem II
• C. ATP
• D. NADP+

Explanation: NADP+ acts as the final electron acceptor during non-cyclic electron flow, while electrons are moved back from ferredoxin to the cytochrome complex to generate assimilating power via cyclic electron flow.

3466. NADP is converted into NADPH₂ in:

• A. Dark reaction
• B. Non-cyclic photophosphorylation
• C. Photorespiration
• D. Cyclic photophosphorylation

Explanation: NADP is converted into NADPH₂ during non-cyclic photophosphorylation, which occurs in the light reactions of photosynthesis. In this process, electrons from water are excited by light, passed through photosystems II and I, and finally transferred to NADP⁺, forming NADPH₂. Other processes like cyclic photophosphorylation, dark reactions, and photorespiration do not produce NADPH₂.

3467. The primary acceptor during CO2 fixation in C3 plants is

• A. Ribulose bisphosphate
• B. Glycolate
• C. Phosphoenolpyruvate
• D. Triosephosphate

Explanation: In C3 plants, the Calvin cycle is the series of reactions that occur in the light-independent phase of photosynthesis. The primary acceptor of CO2 during this cycle is Ribulose bisphosphate (RuBP). RuBP combines with CO2 to form a 6-carbon intermediate, which quickly splits into two molecules of 3-phosphoglycerate. This is a key step in the process of carbon fixation. Glycolate, on the other hand, is related to photorespiration, not carbon fixation. Phosphoenolpyruvate (PEP) serves as the primary CO2 acceptor in C4 plants, not C3. Triosephosphate is a molecule produced later in the Calvin cycle, not during the initial fixation of CO2.

3468. Which of following does not occur during light reaction?

• A. Electron transfer
• B. Release of O2
• C. Photolysis of H2O
• D. Release of H2

Explanation: The light-dependent reactions of photosynthesis, also known as the light reactions, occur in the thylakoid membranes of chloroplasts. During these reactions, sunlight is absorbed by chlorophyll, exciting electrons which are then transferred through the electron transport chain. This process results in the production of ATP and NADPH, which are essential for the Calvin cycle. Additionally, the photolysis of water occurs, splitting water into oxygen, protons, and electrons. Oxygen is released as a byproduct, while the protons contribute to the proton gradient used to produce ATP. The release of hydrogen gas (H2) does not occur during these reactions. Therefore, the correct answer is Option D: Release of H2.

3469. The source of hydrogen atom for the synthesis of glucose is

• A. NADP
• B. FADH2
• C. H2O
• D. ATP

Explanation: In the light-dependent reactions of photosynthesis, water (H2O) is split in a process known as photolysis. This process releases electrons, protons (hydrogen ions), and oxygen. The protons are used in the Calvin cycle to help reduce carbon dioxide into glucose. Therefore, water is the source of hydrogen atoms for glucose synthesis. NADP acts as an electron carrier, becoming NADPH after accepting electrons and protons but is not the initial source of hydrogen. FADH2 is not involved in photosynthesis, and ATP provides energy rather than hydrogen.

3470. Cu is present in:

• A. Plastocyannin
• B. Cytochrome b6f
• C. Ferredoxin
• D. Chlorophyll

Explanation: Copper is present in plastocyanin, a protein that transfers electrons from the cytochrome b6f complex to Photosystem I during the light reactions of photosynthesis. Other options like ferredoxin, cytochrome b6f, and chlorophyll do not contain copper; they use iron or magnesium instead and have different roles in photosynthesis.