All Free Biology MCQs with Answers

3891. Photosystem II occurs in:

• A. Stroma
• B. Grana
• C. On Surface of mitochondria
• D. Inner membrane of mitochondria

Explanation: Grana are structures within chloroplasts that consist of stacks of thylakoid discs. Thylakoids are flattened membrane sacs inside chloroplasts where the light-dependent reactions of photosynthesis take place.The primary function of grana is to enhance the efficiency of the light-dependent reactions of photosynthesis. The thylakoid membranes contain pigments like chlorophyll that capture light energy. This captured light energy is then used to convert water and carbon dioxide into oxygen and glucose during the process of photosynthesis.

3892. In the electron transport chain, which undergoes terminal oxidation?

• A. Cytochrome b
• B. Cytochrome a
• C. Cytochrome a3
• D. Cytochrome c

Explanation: Complex IV (Cytochrome c Oxidase) is the final enzyme complex in the electron transport chain. Within this complex, the final transfer of electrons to molecular oxygen occurs at a binuclear centre containing cytochrome a3 and a copper ion (CuB). This process is known as terminal oxidation because it is the last step where oxygen acts as the terminal electron acceptor, being reduced to water.

3893. During the light reaction of photosynthesis, which of the following phenomena are observed during cyclic phosphorylation as well as noncyclic phosphorylation?

• A. Production of ATP
• B. Photolysis
• C. Formation of NADPH
• D. Excitation of electrons by light

Explanation: During the light reactions of photosynthesis, excitation of electrons by light and production of ATP occur in both cyclic and non-cyclic phosphorylation. Splitting of water and formation of NADPH occur only in non-cyclic phosphorylation. Cyclic phosphorylation recycles electrons within photosystem I and does not produce NADPH or oxygen.

3894. During cyclic photophosphorylation, energy molecules are obtained in the form of:

• A. Oxygen
• B. ATP
• C. Glucose
• D. NADPH

Explanation: During cyclic photophosphorylation, energy is obtained in the form of ATP. NADPH and oxygen are not produced because electrons are recycled within photosystem I, and water is not split. Glucose is also not formed directly, as it requires both ATP and NADPH for its synthesis.

3895. The Calvin cycle occurs in:

• A. Chloroplast
• B. Mitochondria
• C. Glyoxysomes
• D. Cytoplasm

Explanation: In the stroma of the chloroplast, carbon dioxide is fixed into organic molecules using ATP and NADPH produced during the light reactions. Prokaryotes do not have chloroplasts, although some photosynthetic bacteria carry out similar processes in their cytoplasm. Mitochondria are the sites of cellular respiration, not photosynthesis, and are involved in energy release rather than carbon fixation. The cytoplasm is where other metabolic processes like glycolysis occur, but it does not carry out the Calvin cycle. Glyoxysomes are specialised organelles involved in fat metabolism in germinating seeds, not in photosynthesis. Therefore, the Calvin cycle occurs only in the chloroplasts.

3896. The dark reaction of photosynthesis occurs in:

• A. Stroma
• B. Thylakoid
• C. Grana
• D. Cytoplasm

Explanation: The dark reaction of photosynthesis, also known as the Calvin cycle, takes place in the stroma of the chloroplast. The stroma contains the necessary enzymes for carbon dioxide fixation, where carbon dioxide is converted into glucose using ATP and NADPH from the light reactions. The thylakoids and grana are involved in the light-dependent reactions, where light energy is captured to produce ATP and NADPH, but they are not sites of the dark reaction. The cytoplasm also does not take part in this process, as it lies outside the chloroplast. Therefore, the stroma is the correct location for the dark reaction of photosynthesis.

3897. During the dark reaction of photosynthesis:

• A. CO2 is reduced to organic compound
• B. Chlorophyll is activated
• C. 6C sugar is broken down into 3C sugar
• D. Photolysis

Explanation: The dark reactions occur in the stroma of the chloroplast and are also known as the Calvin cycle. In this stage, the enzyme RuBisCO helps combine carbon dioxide with ribulose-1,5-bisphosphate to form 3-phosphoglycerate, which is then converted into glucose and other carbohydrates using the ATP and NADPH produced during the light reactions. The other options are incorrect because chlorophyll activation and photolysis occur during the light reaction, and the breakdown of six-carbon sugars into three-carbon sugars is part of cellular respiration, not photosynthesis. Therefore, the correct answer is that carbon dioxide is reduced to organic compounds during the dark reaction.

3898. CO₂ joins the photosynthetic pathway during:

• A. Dark Reaction
• B. Photosystem I
• C. Photosystem II
• D. Photolysis

Explanation: The correct answer is the dark reaction, which is also referred to as the Calvin cycle. This is the stage of photosynthesis where carbon dioxide (CO₂) is incorporated into organic molecules, a process known as carbon fixation. This occurs in the stroma of the chloroplasts and does not require light to proceed. The Calvin cycle ultimately leads to the formation of glucose. The other options (Photosystem I, Photosystem II, and Photolysis) all pertain to the light-dependent reactions, which are primarily concerned with capturing light energy to produce ATP and NADPH, and do not directly involve CO₂ assimilation.

3899. The initial enzyme of the Calvin cycle is

• A. Ribulose 1,5 bisphosphate carboxylase
• B. Triose phosphate dehydrogenase
• C. Cytochrome oxidase
• D. Hexokinase

Explanation: RuBisCO is the correct enzyme because it catalyses the first step of the Calvin cycle by fixing carbon dioxide to ribulose-1,5-bisphosphate, forming two molecules of 3-phosphoglycerate. This marks the beginning of carbon fixation in photosynthesis. Triose phosphate dehydrogenase is involved later in the cycle, helping convert 3-phosphoglycerate into triose phosphates using ATP and NADPH, but does not initiate the process. Cytochrome oxidase functions in cellular respiration, not photosynthesis, as it transfers electrons to oxygen in mitochondria. Hexokinase is an enzyme of glycolysis that phosphorylates glucose to glucose-6-phosphate and has no role in the Calvin cycle. Therefore, only RuBisCO is responsible for starting the Calvin cycle.

3900. Which one of the following would NOT limit photosynthesis?

• A. Oxygen
• B. Carbon dioxide
• C. Light
• D. Chlorophyll

Explanation: The correct answer is oxygen. While oxygen is involved in the process of photorespiration, it is not a primary factor that limits photosynthesis. In photosynthesis, the availability of carbon dioxide and light is crucial, as they are required for the light-dependent and light-independent reactions. Chlorophyll is also essential for capturing light energy. Therefore, a lack of any of these three components can directly limit the rate of photosynthesis, unlike oxygen, which is not a direct limiting reactant.