All Free Biology MCQs with Answers

3711. Energy transformation in biological systems is called

• A. Metabolism
• B. Photosynthesis
• C. Photorespiration
• D. Bioenergetics

Explanation: The correct answer is bioenergetics. Bioenergetics is dedicated to understanding how organisms acquire, transform, and utilize energy, which is essential for life processes. Metabolism refers to the broader set of chemical reactions and isn't specifically about energy transformation. Photosynthesis is a specific process of converting light energy to chemical energy, rather than a general term for energy transformation. Photorespiration is a plant-specific process that results in energy loss, not transformation.

3712. The difference between non-cyclic and cyclic phosphorylation:

• A. Types of photosystems involved
• B. Product form
• C. Time duration
• D. All of these

Explanation: In cyclic and non-cyclic photophosphorylation, the photosystems involved are different. Non-cyclic photophosphorylation uses both photosystem I and photosystem II, while cyclic photophosphorylation involves only photosystem I, as the electrons cycle back instead of moving linearly. Their end products also differ because non-cyclic photophosphorylation produces ATP, NADPH, and oxygen, whereas cyclic photophosphorylation produces only ATP since no water is split and no NADPH or oxygen is formed. The time of occurrence also varies, as cyclic photophosphorylation usually takes place when the cell needs extra ATP, while non-cyclic photophosphorylation occurs during normal photosynthesis. In summary, these processes differ in the photosystems they use, the products they form, and when they occur, allowing the plant to balance its energy requirements efficiently.

3713. Which one is not correct about chlorophyll b?

• A. It acts as an accessory pigment
• B. It reflects green light
• C. It absorbs light mainly in the blue and red regions
• D. It is the primary pigment in photosynthesis

Explanation: Chlorophyll b acts as an accessory pigment, helping chlorophyll a by capturing additional light energy and transferring it for use in photosynthesis. It reflects green light, which gives plants their green color, as it mainly absorbs light from other parts of the spectrum. This pigment is most effective in absorbing light from the blue and red regions, allowing plants to utilize a wider range of light for photosynthesis. However, chlorophyll b is not the primary pigment in this process. The main pigment responsible for capturing light energy and driving the reactions of photosynthesis is chlorophyll a, making the statement that chlorophyll b is the primary pigment incorrect.

3714. Ribulose bisphosphate is a:

• A. Protein
• B. Nucleic acid
• C. Lipid
• D. Carbohydrate

Explanation: Ribulose 1,5-bisphosphate (RuBP) is indeed a carbohydrate. It is a five-carbon sugar (ketopentose) with two phosphate groups and serves as the primary CO₂ acceptor in the Calvin cycle of photosynthesis. This makes it essential in the process of carbon fixation. The other options are incorrect because RuBP is not a protein, nucleic acid, or lipid. Proteins are made of amino acids, nucleic acids are composed of nucleotides, and lipids consist of fatty acids and glycerol.

3715. The first stable compound during light-independent reactions contains:

• A. 2 carbon atoms
• B. 3 carbon atoms
• C. 4 carbon atoms
• D. 5 carbon atoms

Explanation: The first stable compound formed during the light-independent reactions of photosynthesis contains three carbon atoms. This compound, 3-phosphoglyceric acid, marks the first stable product of carbon fixation in the Calvin cycle, highlighting the importance of the three-carbon pathway in C3 plants.

3716. Enzymes required for the respiratory chain are present on/in:

• A. Cristae
• B. Mitochondrial matrix
• C. Inter-membrane space
• D. Cytoplasm

Explanation: The correct answer is cristae. Cristae are the folds within the inner mitochondrial membrane that house the enzymes for the electron transport chain, crucial for ATP synthesis. The other options are incorrect because they describe locations within the cell that do not contain these specific enzymes: the mitochondrial matrix is the site of the Krebs cycle, the intermembrane space is involved in the proton gradient, and the cytoplasm is where glycolysis takes place, not the electron transport chain.

3717. During the Krebs cycle, the carbon number is reduced by decarboxylation. It is repeated _ time/s during one cycle.

• A. One
• B. Two
• C. Four
• D. Three

Explanation: The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide. During this cycle, decarboxylation occurs twice, leading to the production of two carbon dioxide molecules. This is why the carbon number is reduced twice per cycle. Options A, C, and D are incorrect, as they do not accurately reflect the number of decarboxylation events that occur in one full cycle of the Krebs cycle.

3718. The Calvin cycle is also named:

• A. C3 pathway
• B. Sugar synthesis phase
• C. Dark reactions
• D. All of these

Explanation: The Calvin cycle is known by several names that highlight its different characteristics and roles in photosynthesis: It is called the C3 pathway because the first stable product of carbon fixation is a three-carbon molecule, 3-phosphoglycerate (3-PGA).The cycle is referred to as the sugar synthesis phase because it is the stage where glucose and other carbohydrates are synthesized using ATP and NADPH from the light-dependent reactions.These reactions occur in the absence of light, utilizing the products of the light-dependent reactions to convert carbon dioxide into sugar.Therefore, option D, 'All of these,' is the correct answer, as each term provides a valid description of the Calvin cycle.

3719. It is the most abundant and most important photosynthetic pigment:

• A. Chlorophyll b
• B. Chlorophyll c
• C. Chlorophyll d
• D. Chlorophyll a

Explanation: Chlorophyll a is the correct answer because it is the most abundant and primary photosynthetic pigment in plants, algae, and cyanobacteria. It plays a vital role in absorbing light and converting it into chemical energy during photosynthesis. Other chlorophyll pigments like b, c, and d serve as accessory pigments, helping capture additional light wavelengths and transferring energy to chlorophyll a, but they are not the primary pigments.

3720. In which stage of aerobic respiration are 2-carbon molecules oxidized completely to carbon dioxide?

• A. Glycolysis
• B. Krebs cycle
• C. ETC
• D. Calvin cycle

Explanation: The correct answer is the Krebs cycle. This cycle occurs in the mitochondria and is responsible for the complete oxidation of acetyl-CoA to carbon dioxide. It is a crucial part of aerobic respiration, contributing to ATP production by generating NADH and FADH₂, which carry electrons to the electron transport chain.Glycolysis is incorrect because it occurs in the cytoplasm and only partially oxidizes glucose to pyruvate, without producing carbon dioxide. The electron transport chain, while essential for ATP synthesis, does not oxidize carbon molecules to carbon dioxide. The Calvin cycle is part of photosynthesis, not respiration, and involves the fixation of carbon dioxide into organic molecules, rather than its oxidation.