All Free Biology MCQs with Answers

3851. The second stage of photosynthesis, in which carbon dioxide is reduced to carbohydrate and which occurs whether light is present or not, is called:

• A. Light reaction
• B. Light dependent reaction
• C. Light independent reaction
• D. Synthesis of ATP and NADPH2

Explanation: The correct answer is the light-independent reaction, also known as the Calvin cycle. This phase occurs in the stroma of chloroplasts and uses ATP and NADPH produced during the light-dependent reactions to fix carbon dioxide and produce glucose. It is termed 'light-independent' because it does not require light directly, although it relies on the products of the light-dependent reactions.The other options refer to processes that are part of the light-dependent reactions, which occur in the thylakoid membranes and require light. The 'light-dependent reaction' is a term used to describe the same phase where light energy is converted into chemical energy. 'Synthesis of ATP and NADPH2' is a process that happens during these light-dependent reactions, not the Calvin cycle.

3852. The removal of electrons from an atom or compound is called:

• A. Reduction
• B. Oxidative phosphorylation
• C. Oxidation
• D. Oxidation-reduction reaction

Explanation: The correct answer is oxidation. Oxidation involves the removal of electrons from an atom or compound, which results in an increase in its oxidation state. This process is often paired with reduction, where another species gains the electrons that were lost. The term "reduction" refers to the opposite process, where a molecule gains electrons. Oxidative phosphorylation, on the other hand, is a mechanism for ATP production in mitochondria, not directly related to electron removal. Finally, an oxidation-reduction reaction describes a full redox process where both oxidation and reduction occur, encompassing both electron loss and gain.

3853. The condition in which reduced metabolic products comprising the "debt" accumulate due to the inability of oxidative metabolism to function rapidly enough. The "debt" is paid off when the metabolism that produces reduced products slows. This is called:

• A. Electron debt
• B. Oxygen debt
• C. Hydrogen debt
• D. Carbon debt

Explanation: The correct answer is oxygen debt. During intense exercise, the body often performs anaerobic metabolism, leading to the buildup of metabolic byproducts like lactic acid due to the temporary insufficiency of oxygen. This state is termed "oxygen debt" because the body needs extra oxygen post-exercise to metabolize these byproducts and restore normal conditions. The other options are either not relevant to the context of exercise and metabolism or refer to unrelated scientific concepts.

3854. The two basic molecular systems for converting light to chemical energy during photosynthesis are called

• A. Photosystem I and II
• B. Light systems
• C. Pigment systems
• D. PS 660 and PS 730

Explanation: The correct answer is Photosystem I and II. These are the primary molecular systems in plants, algae, and cyanobacteria that convert light energy into chemical energy during photosynthesis. Photosystem II absorbs light, leading to the splitting of water molecules and the release of oxygen, while Photosystem I facilitates the formation of NADPH. Other options, such as 'Light systems' and 'PS 660 and PS 730,' do not exist in the scientific context of photosynthesis. 'Pigment systems' involve the absorption of light energy but do not directly convert it into chemical energy; this conversion is accomplished by the photosystems.

3855. The hydrogen ions move down their gradient from the thylakoid space to the outside through special complexes found in the thylakoid membrane called

• A. Ferredoxin
• B. ATP synthase
• C. Cytochrome complex
• D. Plastoquinone

Explanation: The correct answer is ATP synthase. During the light-dependent reactions of photosynthesis, ATP synthase uses the proton gradient created across the thylakoid membrane to produce ATP from ADP and inorganic phosphate. This movement of protons (hydrogen ions) down their gradient through ATP synthase is crucial for ATP synthesis. Other options, such as ferredoxin, cytochrome complex, and plastoquinone, play roles in electron transport but do not facilitate the movement of hydrogen ions across the membrane.

3856. Cyclic photophosphorylation involves:

• A. PS I
• B. PS I and PS II
• C. PS II
• D. P680

Explanation: Cyclic photophosphorylation involves only Photosystem I (PS I). In this process, electrons are transferred cyclically back to the PS I reaction center, allowing for the production of ATP without the generation of NADPH. This contrasts with non-cyclic photophosphorylation, which involves both PS I and PS II and leads to the production of both ATP and NADPH. Options B, C, and D are incorrect because they involve elements not part of the cyclic photophosphorylation pathway: PS II is not involved (hence options B and C are wrong), and P680 is a component of PS II, not PS I (hence option D is wrong).

3857. Photosystem I and photosystem II are found in:

• A. Stroma of chloroplast
• B. Grana of chloroplast
• C. Matrix of mitochondria
• D. Inner membrane of mitochondria

Explanation: Photosystem I and photosystem II are integral components of the light-dependent reactions in photosynthesis. They are located in the thylakoid membranes of the grana within chloroplasts. These photosystems capture light energy and facilitate the transfer of electrons, essential for the synthesis of ATP and NADPH. The stroma, on the other hand, is where the Calvin cycle (light-independent reactions) occurs, while the mitochondria are involved in cellular respiration, not photosynthesis.

3858. Photophosphorylation means:

• A. Formation of ATP from ADP in presence of light
• B. Formation of NADP
• C. Formation of ADP from ATP
• D. Formation of PGA

Explanation: Photophosphorylation is the process by which ATP is synthesized from ADP and inorganic phosphate using the energy of sunlight, which is captured during the light-dependent reactions of photosynthesis. This occurs in the thylakoid membranes of chloroplasts and is a crucial step in converting light energy into chemical energy.Option A is correct, as it accurately describes the formation of ATP in the presence of light. Option B is incorrect because NADP is a coenzyme that gets reduced to NADPH, not formed in this step. Option C is incorrect, as it describes the dephosphorylation of ATP, which is not what photophosphorylation entails. Option D is incorrect because the PGA formation is part of the Calvin cycle, not the light-dependent reactions.

3859. Photosynthesis is an oxidation-reduction process in which:

• A. Carbon dioxide is oxidized and water is reduced
• B. Water is oxidized and carbon dioxide is reduced
• C. Both CO₂ and water are oxidized
• D. Both CO₂ and water are reduced

Explanation: Photosynthesis is a redox process where light energy is used to convert water and carbon dioxide into glucose and oxygen. In this process, water (H₂O) is oxidized, meaning it loses electrons, and oxygen is released as a byproduct. Carbon dioxide (CO2), on the other hand, is reduced by gaining electrons to form glucose (C6H12O6). This transformation is crucial for converting solar energy into chemical energy. Option B is correct because it accurately describes the electron flow in this process. Options A, C, and D are incorrect, as they either reverse the roles of CO₂ and water or incorrectly claim both undergo the same redox reaction.

3860. The formation of acetyl-CoA from pyruvic acid is the result of its:

• A. Reduction
• B. Oxidation
• C. Hydrolysis
• D. Oxidative decarboxylation

Explanation: The conversion of pyruvic acid into acetyl-CoA takes place through the process of oxidative decarboxylation. In this reaction, pyruvic acid is oxidized, losing one carbon atom as carbon dioxide, and the remaining two-carbon acetyl group combines with coenzyme A to form acetyl-CoA. This process is catalyzed by the pyruvate dehydrogenase complex and links glycolysis to the Krebs cycle. Unlike reduction or hydrolysis, which involve different types of chemical changes, oxidative decarboxylation specifically involves both oxidation and the removal of carbon dioxide, making it the key step in preparing pyruvate for energy production in cellular respiration.