All Free Biology MCQs with Answers

3721. The type of respiration that involves the step-by-step breakdown of carbon chain molecules in the cell is called

• A. External respiration
• B. Pulmonary respiration
• C. Cellular respiration
• D. Cutaneous respiration

Explanation: The correct answer is cellular respiration. This process involves the breakdown of carbon chain molecules, such as glucose, within cells to produce ATP, the energy currency of the cell. The breakdown occurs in several stages, including glycolysis, the citric acid cycle, and oxidative phosphorylation. External respiration and pulmonary respiration refer to the exchange of gases at the respiratory system level, not at the cellular level. Cutaneous respiration involves gas exchange through the skin and is unrelated to the cellular metabolic processes of energy conversion.

3722. End products of yeast fermentation, bacterial fermentation, and anaerobic respiration are:

• A. Ethyl alcohol, lactic acid, and carbon dioxide
• B. Ethyl alcohol, lactic acid, carbon dioxide, and water
• C. Glucose, oxygen, and water
• D. Pyruvic acid, carbon dioxide, and oxygen

Explanation: The correct end products of yeast fermentation, bacterial fermentation, and anaerobic respiration are ethyl alcohol, lactic acid, and carbon dioxide. These products result from energy-yielding processes that occur in the absence of oxygen, allowing organisms to survive under anaerobic conditions.

3723. Identify the correct option concerning the absorption spectrum of photosynthesis:

• A. Chlorophyll absorbs light mainly in the blue and red regions of the spectrum
• B. Chlorophyll absorbs light mainly in the green region of the spectrum
• C. Carotenoids absorb light only in the red region of the spectrum
• D. The absorption spectrum of chlorophyll and the action spectrum of photosynthesis are completely different

Explanation: The correct statement is Option A: Chlorophyll absorbs light mainly in the blue and red regions of the spectrum. This matches the actual absorption characteristics of chlorophyll, which align closely with the regions where photosynthesis is most efficient. Other options are incorrect because chlorophyll does not absorb green light, carotenoids do not absorb only red light, and the absorption and action spectra are similar, not completely different.

3724. The electrons excited from photo II pass directly to:

• A. Ferredoxin
• B. Plastocyanin
• C. Plastoquinone
• D. Carotenoids

Explanation: The correct answer is plastoquinone. It directly accepts electrons from photosystem II and transfers them to the cytochrome b6f complex, which is a critical step in the electron transport chain of photosynthesis. This movement of electrons is essential for establishing a proton gradient used in ATP synthesis.Ferredoxin and plastocyanin are involved in later stages of electron transport, with ferredoxin functioning after photosystem I and plastocyanin transferring electrons between the cytochrome b6f complex and photosystem I. Carotenoids, on the other hand, are not directly involved in electron transport but are important for light absorption and protection against photooxidative damage.

3725. Formation of acetyl-CoA from pyruvic acid needs:

• A. A single enzyme
• B. Two enzymes
• C. Multi-enzyme complex
• D. No enzyme

Explanation: The conversion of pyruvic acid to acetyl CoA is a crucial step in cellular respiration and is catalyzed by the pyruvate dehydrogenase complex, a multi-enzyme system. This complex includes several enzymes, such as pyruvate dehydrogenase, dihydrolipoamide acetyltransferase, and dihydrolipoamide dehydrogenase, along with multiple cofactors. The complexity of this system allows for efficient transformation of pyruvic acid into acetyl-CoA, a key molecule for entry into the citric acid cycle. The incorrect options suggest fewer enzymes or no enzymes at all, which would be inadequate for this biochemical process.

3726. Which one of the following is Complex V of the ETS on the inner membrane of mitochondria?

• A. NADH dehydrogenase
• B. Cytochrome oxidase
• C. Ubiquinase
• D. ATP synthase

Explanation: ATP synthase, or Complex V, is the enzyme that synthesizes ATP by utilizing the proton gradient created by the other complexes in the electron transport chain. This process, known as oxidative phosphorylation, occurs on the inner mitochondrial membrane. In contrast, NADH dehydrogenase (Complex I) and cytochrome oxidase (Complex IV) are involved in electron transfer processes, and 'ubiquinase' does not exist as an enzyme in this context.

3727. Inside the stroma of the chloroplast, there is a suspended:

• A. Membrane system
• B. Set of enzymes
• C. Both A and B
• D. Chlorophyll

Explanation: The stroma is the fluid-filled matrix of the chloroplast that surrounds the thylakoid membranes. It serves as the site for many biochemical reactions, including the Calvin cycle, which synthesizes glucose from carbon dioxide and water. Within the stroma, a complex membrane system composed of thylakoids is suspended; these flattened sacs are arranged in stacks called grana and are responsible for the light-dependent reactions of photosynthesis. In addition, the stroma contains a set of enzymes that catalyze the light-independent reactions, as well as chloroplast DNA, ribosomes, and various organic molecules. Thus, the stroma provides both the structural framework and the enzymatic environment necessary for the complete process of photosynthesis.

3728. Chlorophyll molecules are found embedded in:

• A. Thylakoid membranes
• B. Outer chloroplast membrane
• C. Lamellar membranes
• D. Inner chloroplast membrane

Explanation: Chlorophyll molecules are crucial for the light-dependent reactions of photosynthesis and are located in the thylakoid membranes within chloroplasts. These membranes provide the structure needed to organize chlorophyll and other pigments to capture light energy efficiently. The outer and inner chloroplast membranes, while essential for chloroplast structure and function, do not contain chlorophyll. 'Lamellar membranes' is a more general term and does not specifically refer to the location of chlorophyll in the process of photosynthesis.

3729. Electron acceptors of the photosynthetic electron transport chain are parts of:

• A. Thylakoid membranes
• B. Outer chloroplast membrane
• C. Lamellar membranes
• D. Inner chloroplast membrane

Explanation: The correct answer is that thylakoid membranes are crucial structures within chloroplasts where the light-dependent reactions of photosynthesis take place. They contain the components of the photosynthetic electron transport chain, including electron acceptors and other molecules necessary for the conversion of light energy into chemical energy in the form of ATP and NADPH.Option B, the outer chloroplast membrane, and Option D, the inner chloroplast membrane, are both structural components of the chloroplast envelope but do not house the electron transport chain. Option C, lamellar membrane, is a general term for layered structures and does not specifically refer to the site of the photosynthetic electron transport chain.

3730. Chlorophyll and other pigments absorb light energy, which is converted into chemical energy:

• A. NADH and NADPH
• B. ATP and NADPH
• C. ATP and NADH
• D. FADH and NADPH

Explanation: During the light-dependent reactions of photosynthesis, chlorophyll and other pigments absorb light energy, which is then converted into chemical energy stored in the form of ATP and NADPH. ATP acts as the energy currency, driving various cellular processes, while NADPH provides the reducing power needed for the synthesis of glucose in the Calvin cycle. Other options are incorrect because they include NADH and FADH, which are not involved in photosynthesis but rather in cellular respiration.