All Free Biology MCQs with Answers

3281. The spent energy in the form of ADP is regenerated by mitochondria into which of the following form?

• A. AMP
• B. ATP
• C. ADP
• D. All of these options are correct

Explanation: Mitochondria are membrane-bound organelles, and those membranes function for the purpose of mitochondria, which is essential to produce energy. That energy is produced by having chemicals within the cell go through pathways. And the process of that conversion produces energy in the form of ATP because phosphate is a high-energy bond and provides energy for other reactions within the cell. So the mitochondria's purpose is to produce that energy. Hence, option C is incorrect.

3282. How is pyruvate produced in anaerobic conditions?

• A. Alcoholic fermentation
• B. Lactic acid fermentation
• C. Glycolysis
• D. None of these

Explanation: Pyruvate is produced through the process of glycolysis, which is the initial step in cellular respiration that occurs in the cytoplasm of the cell. Glycolysis does not require oxygen, allowing it to function under anaerobic conditions. During glycolysis, one molecule of glucose is broken down into two molecules of pyruvate, along with the production of a small amount of ATP. While both alcoholic and lactic acid fermentation utilize pyruvate, they do not produce it; instead, they are processes that occur after glycolysis to regenerate NAD+ for continued glycolysis under anaerobic conditions. Therefore, the correct answer is glycolysis.

3283. The water splitting step of photosynthesis is called:

• A. Hydrolysis
• B. Chemiosmosis
• C. Photolysis
• D. Photosynthesis

Explanation: The correct answer is Photolysis. Photolysis refers to the light-dependent splitting of water molecules into hydrogen ions, oxygen, and electrons. This process occurs during photosynthesis and is crucial for providing the electrons needed for the synthesis of ATP and NADPH. The other options are incorrect because:Hydrolysis refers to the general process of breaking bonds using water, not specifically related to light-dependent reactions.Chemiosmosis is the process of generating ATP through the movement of hydrogen ions across a membrane, not the splitting of water.Photosynthesis is the general term for the process of converting light energy to chemical energy, which includes photolysis as one of its steps, but it is not specifically the water-splitting step.

3284. Which statement correctly outlines some of the main events in photosynthesis?

• A. A 5-Carbon carbohydrate accepts carbon dioxide and is then reduced by NADPH derived from photophosphorylation
• B. A 3-Carbon carbohydrate is regenerated and reduced by hydrogen molecules derived from photophosphorylation
• C. Photolysis uses light to produce reduced NADP and oxygen which are used to reduce a 3-Carbon carbohydrate
• D. Photolysis produces NADPH and ATP which are used to reduce a 5-Carbon carbohydrate

Explanation: During the light-independent reactions, a 5-carbon carbohydrate called RuBP accepts carbon dioxide to form 3-carbon phosphoglycerate, which is reduced by NADPH formed in the light reaction.

3285. In photosynthesis, the dark reaction is called so because:

• A. It takes place only in the dark
• B. It does not require light directly
• C. It occurs during the night only
• D. It involves only dark-colored pigments

Explanation: The dark reaction is called so because it does not need light directly and uses ATP and NADPH from the light reaction to make glucose from carbon dioxide, occurring in both light and dark conditions. a) It takes place only in the dark-Wrong, because it can occur in light as well. c) It occurs during nighttime only-Wrong, because it is not limited to nighttime. d) It involves only dark-colored pigments-Wrong, because no pigments are involved in the dark reaction.

3286. What do the two peaks in the action spectrum represent?

• A. Absorption of light
• B. Carbon dioxide consumption
• C. Emission of light
• D. Both Options A and B are correct

Explanation: The action spectrum represents the efficiency with which different wavelengths of light are absorbed by a photosynthetic pigment to drive the process of photosynthesis. The two peaks in the action spectrum indicate the wavelengths of light that are most efficiently absorbed by the photosynthetic pigment(s) in question. These absorbed photons provide the energy needed to initiate the light-dependent reactions of photosynthesis.

3287. Cytochrome c reduces which coenzyme in the ETC?

• A. Cytochrome a
• B. Coenzyme Q
• C. Coenzyme b
• D. Coenzyme a3

Explanation: In the ETC, electrons are passed down a chain of carriers from higher to lower energy states. Cytochrome c is a mobile electron carrier that shuttles electrons from Complex III to Complex IV. Within Complex IV, cytochrome a is the first molecule that accepts electrons from cytochrome c. Therefore, cytochrome a is the correct answer, while the other options are either upstream components, not directly involved, or do not exist in this context.

3288. In the plants, there are 50 or more thylakoids piled upon each other to form?

• A. Granum
• B. Centrosome
• C. Stroma
• D. Multinucleate

Explanation: The correct answer is Granum. In plants, thylakoids are organized into stacks called grana, which are essential for the light-dependent reactions of photosynthesis. These reactions occur in the thylakoid membranes, where light energy is converted into chemical energy. The other options are incorrect: a Centrosome is found in animal cells, not plant cells; the Stroma is the fluid surrounding the thylakoids in chloroplasts, not a stack of them; and Multinucleate refers to cells with multiple nuclei, which is unrelated to thylakoid organization.

3289. From one pyruvate passing through the Krebs cycle, how many NADH are formed?

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: From one molecule of pyruvate passing through the Krebs cycle, three molecules of NADH are formed. During the cycle, pyruvate is first converted into acetyl-CoA, which then enters the Krebs cycle. As the acetyl-CoA is oxidized, hydrogen atoms are transferred to NAD⁺, forming NADH at three different steps. These NADH molecules later carry high-energy electrons to the electron transport chain, where they are used to produce ATP. Thus, each pyruvate generates three NADH molecules in the Krebs cycle.

3290. When is sugar formed in photosynthesis?

• A. Light independent reactions
• B. Light dependent reactions
• C. Both A and B
• D. None of these

Explanation: The light-independent reactions, also known as the Calvin Cycle, are where sugar formation occurs in photosynthesis. These reactions take place in the stroma of chloroplasts and involve the fixation of carbon dioxide into sugar molecules like glucose, using ATP and NADPH from the light-dependent reactions. The light-dependent reactions do not form sugar directly but provide the energy and reducing power needed for the Calvin Cycle. Therefore, the correct answer is the light-independent reactions, while the other options are incorrect as they either describe processes that do not directly synthesize sugar or incorrectly suggest that sugar is not formed in photosynthesis.