All Free Biology MCQs with Answers

3381. Tropical plants such as sugarcane show high efficiency of CO2 fixation because of:

• A. Calvin cycle
• B. Hatch-slack
• C. TCA-cycle
• D. All of these

Explanation: The high efficiency of carbon dioxide (CO2) fixation in C4 plants, including tropical plants like sugarcane, is indeed associated with the Hatch-Slack pathway. The Hatch-Slack pathway, also known as C4 photosynthesis, is a mechanism that allows these plants to efficiently fix carbon dioxide even under high temperatures and intense sunlight.

3382. Where does the molecular mechanism of oxidative phosphorylation take place?

• A. Cytosol
• B. Mitochondria
• C. Nucleus
• D. All of these

Explanation: Mitochondria is the only place where ATP can be synthesized as the compartmentalization within the mitochondria provides an efficient and regulated environment for ATP synthesis and efficient energy production.

3383. The light reaction takes place in?

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

Explanation: The light reactions of photosynthesis indeed take place in the thylakoid membranes within the chloroplasts of plant cells. Thylakoid membranes are flattened, disk-like structures within the chloroplasts. These membranes house the photosystems and other proteins involved in the light-dependent reactions. Grana are stacks of thylakoid membranes. The arrangement of thylakoids in grana provides a large surface area for the organization of pigments, including chlorophyll, necessary for capturing light energy. The overall process in the thylakoid membranes is crucial for capturing and converting light energy into chemical energy, which is then used in the synthesis of ATP and NADPH. These energy-rich molecules, produced during the light reactions, play a key role in powering the subsequent dark reactions (Calvin Cycle) where carbon dioxide is fixed and organic molecules are synthesized.

3384. The part of the chlorophyll molecule is embedded in the core of the thylakoid membrane, which is?

• A. Hydrophilic
• B. Hydrophobic
• C. Both A and B
• D. None of these

Explanation: The part of the chlorophyll molecule that is embedded in the core of the thylakoid membrane is the hydrophobic phytol tail. The phytol tail is a long, hydrophobic, and nonpolar structure that anchors the hydrophilic head of the chlorophyll molecule within the thylakoid membrane. This arrangement allows the chlorophyll molecules to be positioned in such a way that their hydrophilic heads are exposed to the aqueous environment of the thylakoid lumen, while their hydrophobic tails are embedded within the hydrophobic interior of the membrane. This organization is essential for the proper functioning of chlorophyll in capturing light energy during the light-dependent reactions of photosynthesis.

3385. Which cells absorb carbon dioxide in the leaf?

• A. Neutrophil cells
• B. Basophil cells
• C. Mesophyll cells
• D. All of these

Explanation: Option C is correct. Mesophyll cells are found in the spongy and palisade layers of the leaf, where they are responsible for photosynthesis and gas exchange. They absorb carbon dioxide from the intercellular spaces, facilitating its diffusion into the cells for use in photosynthesis.Option A is incorrect because neutrophil cells are part of the human immune system, not plants. Option B is incorrect because basophil cells are also part of the immune system. Option D is incorrect because it incorrectly suggests that all listed cells are involved in carbon dioxide absorption, which is not the case; only mesophyll cells fulfill this function in leaves.

3386. The dark reaction in photosynthesis, where sugars are synthesized, is called?

• A. Krebs cycle
• B. Calvin Cycle
• C. Galvin cycle
• D. Glycolysis

Explanation: The dark reaction in photosynthesis, where sugars are synthesized, is known as the Calvin Cycle or the light-independent reactions. Despite the name "dark reaction," the Calvin Cycle does not require direct light; however, it relies on the products of the light-dependent reactions. During the Calvin Cycle, carbon dioxide is fixed and reduced to produce carbohydrates, particularly glucose. The key steps of the Calvin Cycle include carbon fixation, reduction, regeneration of the starting molecule, and the eventual synthesis of sugars. This process takes place in the stroma of chloroplasts, and it utilizes the ATP and NADPH produced during the light-dependent reactions to drive the chemical reactions leading to the formation of glucose and other organic molecules.

3387. What does ATP provide during photosynthesis?

• A. Mechanical energy
• B. Physical energy
• C. Chemical energy
• D. All of these

Explanation: The correct answer is Option C: Chemical energy. ATP (adenosine triphosphate) is a critical molecule in energy transfer within cells. It stores energy in its high-energy phosphate bonds and releases this energy to power various cellular activities, including the biochemical reactions in photosynthesis. Options A, B, and D are incorrect because they do not accurately describe the primary function of ATP, which is to provide chemical energy.

3388. NADH is oxidised by which coenzyme in the ETC?

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

Explanation: In the electron transport chain (ETC), NADH acts as an electron donor, not an oxidizer. It transfers its electrons to Coenzyme Q (also called ubiquinone) through a series of carriers. Coenzyme Q accepts these electrons and becomes reduced to ubiquinol (CoQH₂), while NADH is oxidized back to NAD⁺. This transfer helps drive the flow of electrons that ultimately produces ATP."

3389. What is the most common electron transport chain?

• A. Non-cyclic electron flow
• B. Cyclic electron flow
• C. Circular electron flow
• D. Both b and c

Explanation: The most common electron transport chain in photosynthesis is the non-cyclic electron flow. In this process, electrons move linearly through Photosystem II and Photosystem I. Water molecules are split in Photosystem II, releasing oxygen as a by-product. In contrast, the electrons finally reduce NADP⁺ to form NADPH. This pathway also generates ATP, providing the energy and reducing power needed for the Calvin cycle."

3390. In which of the following components of the body does lactic acid fermentation take place?

• A. Brain
• B. Heart
• C. Liver
• D. Muscles

Explanation: Lactic acid fermentation primarily occurs in skeletal muscles under anaerobic conditions, such as during intense exercise when oxygen supply is limited. Pyruvate, a product of glycolysis, is converted to lactic acid, leading to its accumulation and subsequent muscle fatigue. The brain, heart, and liver do not typically undergo lactic acid fermentation. The brain relies heavily on aerobic respiration for energy due to its high oxygen requirement. The heart muscles are designed to resist fatigue and maintain continuous aerobic respiration. The liver's role is to convert lactic acid back to glucose, not to accumulate it.