All Free Biology MCQs with Answers

3901. The specific part of a chloroplast that releases O₂ is the:

• A. Stroma
• B. Cristae
• C. Thylakoids
• D. Chlorophyll

Explanation: The correct part of the chloroplast that releases oxygen is the thylakoid because this is where the light-dependent reactions of photosynthesis take place. Within the thylakoid membranes, water molecules are split in Photosystem II through a process called photolysis, releasing oxygen as a by-product. The stroma, on the other hand, is where the Calvin cycle occurs, which involves the fixation of carbon dioxide, not the release of oxygen. The term "cristae" refers to the folds inside mitochondria, not chloroplasts, and is related to cellular respiration. Chlorophyll is a pigment found in the thylakoid membranes that captures light energy but does not release oxygen itself. Therefore, oxygen is specifically released from the thylakoid region of the chloroplast.

3902. Which of the following does not occur during the light reaction?

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

Explanation: During the light reaction of photosynthesis, light energy is absorbed by chlorophyll, leading to the excitation of electrons that move through the electron transport chain. This process includes the transfer of electrons, the splitting of water molecules (photolysis), and the release of oxygen as a by-product. These steps are essential for producing ATP and NADPH, which are later used in the Calvin cycle. The release of hydrogen, however, does not occur as a separate event. Instead, hydrogen from water is released in the form of protons and electrons, which are used to form NADPH. Therefore, the release of free hydrogen gas does not happen during the light reaction, making it the correct answer as the process that does not occur.

3903. The source of O₂ liberated in photosynthesis is:

• A. Photosynthetic enzyme
• B. Carbohydrates
• C. Water
• D. Carbon dioxide

Explanation: The correct source of oxygen released during photosynthesis is water because it is split in the light-dependent reactions through a process called photolysis. When light energy strikes Photosystem II, water molecules are broken down into hydrogen ions, electrons, and oxygen. The oxygen is then released as a byproduct into the atmosphere. Photosynthetic enzymes only catalyse reactions but do not provide oxygen themselves. Carbohydrates are products of photosynthesis, not sources of oxygen, and carbon dioxide contributes carbon and oxygen atoms to glucose formation, but not to the oxygen gas that is released. Hence, only water serves as the true source of the oxygen liberated during photosynthesis.

3904. Which enzyme is most abundantly found on Earth?

• A. Rubisco
• B. Nitrogenase
• C. Invertase
• D. Catalase

Explanation: Rubisco is the most abundant enzyme on Earth, primarily due to its critical role in the Calvin cycle of photosynthesis, where it facilitates the fixation of carbon dioxide into organic compounds. This process is fundamental for producing the organic materials that form the basis of most life on Earth. Other enzymes like nitrogenase, invertase, and catalase have important but more specialized roles and are not as universally abundant as Rubisco.

3905. The source of hydrogen atoms for the synthesis of glucose is

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

Explanation: Water is the correct source of hydrogen atoms for glucose synthesis because, during photosynthesis, it is split in the light-dependent reactions to release hydrogen ions, electrons, and oxygen. The hydrogen ions and electrons are then used to reduce carbon dioxide into glucose in the Calvin cycle. NADH, FADH₂, and ATP are not sources of hydrogen in photosynthesis because they are mainly involved in cellular respiration, not in providing hydrogen for sugar formation. While ATP provides energy and NADPH supplies reducing power during photosynthesis, the actual hydrogen atoms come from water molecules.

3906. The electron transport chain of the photosynthetic process is

• A. Non-cyclic photophosphorylation
• B. Cyclic photophosphorylation
• C. Non-photochemical quenching chain
• D. Photorespiratory electron transport

Explanation: The correct electron transport chain in photosynthesis is non-cyclic photophosphorylation. In this process, electrons move from water through Photosystem II and Photosystem I to NADP⁺, forming NADPH, releasing oxygen, and producing ATP. Cyclic photophosphorylation involves only Photosystem I, where electrons circulate to generate ATP but not NADPH or oxygen. The non-photochemical quenching chain is not part of electron transport; it helps plants release excess light energy as heat to prevent damage. Photorespiratory electron transport occurs when oxygen is fixed instead of carbon dioxide, leading to energy loss and redox balance adjustments, but it is not part of the main photosynthetic chain. Only non-cyclic photophosphorylation is the true electron transport chain of photosynthesis.

3907. The photosystem I conducts:

• A. Cyclic photophosphorylation
• B. Oxidative phosphorylation
• C. Photolysis of water
• D. Non-cyclic photophosphorylation

Explanation: In photosynthesis, Photosystem I is primarily involved in cyclic photophosphorylation, which is the correct option. In this process, light energy is absorbed by Photosystem I to generate ATP without producing NADPH. The electrons excited by light energy circulate back to the same photosystem, allowing the continuous production of ATP to meet the energy needs of the cell. This process is particularly important when the Calvin cycle requires more ATP than NADPH. The other options are incorrect for different reasons. Oxidative phosphorylation occurs in mitochondria during cellular respiration and does not involve light energy or Photosystem I, so it is not part of photosynthesis. Photolysis of water takes place in Photosystem II, where light energy splits water molecules into oxygen, protons, and electrons; this process supports the light reactions but is not associated with Photosystem I. Non-cyclic photophosphorylation involves both Photosystem II and Photosystem I to produce ATP and NADPH, so it is not exclusive to Photosystem I. These processes either occur in other parts of the cell or involve multiple photosystems, which is why they are not the correct answer for the role of Photosystem I.

3908. How many turns of the Calvin cycle yield one molecule of glucose?

• A. Four
• B. Six
• C. Eight
• D. Two

Explanation: The Calvin cycle is integral to photosynthesis, as it incorporates carbon dioxide into organic molecules. Each turn of the cycle fixes one carbon dioxide molecule, contributing one carbon atom. Glucose, a hexose sugar, has a six-carbon structure. Therefore, it takes six turns of the Calvin cycle to fix enough carbon to form one glucose molecule. Option B is correct because it accounts for the necessary carbon fixation. Option A is incorrect, as four turns only fix four carbon atoms. Option C is incorrect, as eight turns result in excess carbon fixation. Option D is incorrect because two turns only produce G3P, not a complete glucose molecule.

3909. Which of the following absorbs light energy for photosynthesis?

• A. Chlorophyll
• B. Water molecules
• C. O2
• D. RuBP

Explanation: The correct answer is chlorophyll. Chlorophyll is the green pigment in chloroplasts responsible for absorbing light energy, which is vital for photosynthesis. This energy is used to split water molecules, releasing oxygen, and to convert carbon dioxide into glucose. Water molecules are involved in the light-dependent reactions but do not absorb light. Oxygen (O₂) is simply a byproduct of these reactions, with no role in light absorption. RuBP is part of the Calvin cycle, contributing to carbon fixation but not light absorption.

3910. In photosynthesis energy from the light reaction to the dark reaction is transferred in the form of:

• A. ADP
• B. AMP
• C. NADPH
• D. Chlorophyll

Explanation: In photosynthesis, the energy from the light reactions is transferred to the dark reactions in the form of NADPH. NADPH is a molecule that carries high-energy electrons produced during the light reactions, and these electrons are used in the dark reactions, also known as the Calvin cycle, to reduce carbon dioxide into glucose and other carbohydrates. ADP is incorrect because it is the breakdown product of ATP and is used to store energy, but it does not carry electrons, so it cannot transfer energy from the light to the dark reactions. AMP is also incorrect because it is a simpler molecule than ATP, inefficient at storing energy, and similarly, it cannot carry electrons needed for the dark reactions. Chlorophyll is incorrect as well because its role is to absorb light energy and drive the light reactions; it does not participate in transferring energy to the dark reactions. Therefore, NADPH is the correct molecule responsible for carrying energy from the light reactions to the dark reactions in photosynthesis.