All Free Biology MCQs with Answers

3651. The following is the correct sequence of energy transfer between photosynthetic pigments:

• A. Chl.a→Chl.b→Carotenoids
• B. Carotenoids→Chl.b→Chl.a
• C. Chl.b→Carotenoids→Chl.a
• D. In any direction

Explanation: The correct sequence of energy transfer between photosynthetic pigments is carotenoids→Chl.b→Chl.a. Carotenoids and chlorophyll b absorb light across different parts of the spectrum and transfer that energy to chlorophyll a, which is the primary pigment responsible for driving the light reactions of photosynthesis. This structured pathway ensures maximum efficiency in capturing and utilizing light energy. Options A, C, and D are incorrect because they do not represent the true order of energy transfer that occurs in the photosynthetic process.

3652. All of the following are correct differences between Chl. 'a' and 'b' except:

• A. Chlorophyll 'a' has a methyl (-CH₃) group at the C-3 position, while chlorophyll 'b' has a formyl (-CHO) group at the same position
• B. Chlorophyll 'a' absorbs light primarily at 430 nm and 662 nm, whereas chlorophyll 'b' absorbs at 453 nm and 642 nm
• C. Chlorophyll 'a' is found only in plants, while chlorophyll 'b' is found only in algae
• D. Chlorophyll 'a' participates directly in the light reactions of photosynthesis, whereas Chlorophyll 'b' acts as an accessory pigment

Explanation: Chlorophyll 'a' and 'b' differ in their chemical structure, absorption spectra, and roles in photosynthesis. Structurally, chlorophyll 'a' has a methyl group (-CH₃) at the C-3 position of the porphyrin ring, whereas chlorophyll 'b' has a formyl group (-CHO), which slightly shifts its light absorption properties. In terms of absorption, chlorophyll 'a' absorbs light mainly at 430 nm (blue-violet) and 662 nm (red), while chlorophyll 'b' absorbs at 453 nm (blue) and 642 nm (red), complementing chlorophyll 'a' in capturing light energy. Functionally, chlorophyll 'a' is directly involved in the photochemical reactions of the light-dependent reactions of photosynthesis, whereas chlorophyll 'b' acts as an accessory pigment, transferring the absorbed energy to chlorophyll 'a'. However, it is incorrect to say that chlorophyll 'a' is found only in plants and chlorophyll 'b' only in algae, because both types are present in plants and green algae, making that statement the exception among the differences.

3653. The location of the phytol tail of chlorophyll b is

• A. Stroma
• B. Thylakoid membrane
• C. Thylakoid surface
• D. Thylakoid lumen

Explanation: Chlorophyll molecules, including chlorophyll b, have a hydrophobic phytol tail that allows them to embed firmly into the lipid bilayer of the thylakoid membrane. This hydrophobic tail ensures that the porphyrin ring, which absorbs light, is properly oriented to interact with other pigments and protein complexes in the photosystems. By anchoring chlorophyll in the thylakoid membrane, the phytol tail plays a crucial structural role, maintaining the correct organization of photosynthetic pigments and enabling efficient energy transfer during the light-dependent reactions of photosynthesis.

3654. Which one of the following is not a carotenoid?

• A. Convert light energy into chemical energy
• B. Protect chlorophyll
• C. Transfer energy
• D. Protect human eye

Explanation: Carotenoids are accessory pigments that have several important roles, including protecting chlorophyll from damage, transferring energy to chlorophyll, and protecting human eyes by absorbing blue light. However, they do not directly convert light energy into chemical energy; this function is primarily performed by chlorophyll and the photosystems within chloroplasts. Thus, the correct answer is the option that states carotenoids do not convert light energy into chemical energy.

3655. PS-I is named as it:

• A. Absorbs wavelength of 680 nm
• B. Discovered earlier than PS-II
• C. Absorbs the wavelength of 700 nm
• D. Slightly located upward than PS-II

Explanation: The correct answer is that PS-I is named so because it was discovered before PS-II. The names of photosystems are assigned based on the sequence of their discovery, not based on the wavelengths they absorb or their position within the thylakoid membrane. Option A is incorrect because PS-I absorbs approximately 700 nm, not 680 nm. Option C, while correct about the absorption wavelength, does not explain the naming basis. Option D incorrectly associates naming with spatial location, which is not a factor in naming photosystems.

3656. Photosystem II has a reaction centre of

• A. P680
• B. P700
• C. P730
• D. P660

Explanation: Photosystem II (PS II) and Photosystem I (PS I) are two distinct complexes in the light-dependent reactions of photosynthesis. PS II has a reaction center known as P680 because it absorbs light most efficiently at a wavelength of 680 nm. PS I, on the other hand, have a reaction center called P700, absorbing best at 700 nm. Thus, the correct answer for the reaction center of Photosystem II is P680. Other options, such as P700, P730, and P660, are incorrect because they either correspond to Photosystem I or are not relevant to the known reaction centers in the photosynthetic process.

3657. During cyclic photophosphorylation, electrons pass from all except:

• A. Photosystem I
• B. Ferredoxin
• C. Photosystem II
• D. Cytochrome complex

Explanation: In cyclic photophosphorylation, electrons are cycled back to Photosystem I after passing through components such as ferredoxin and the cytochrome complex. Photosystem II is excluded from this pathway because it is involved in non-cyclic photophosphorylation, where it initiates the process by splitting water molecules. Therefore, the correct answer is photosystem II, as it does not participate in the cyclic electron flow.

3658. Some photosynthetic organisms contain chloroplasts that lack photosystem II yet can survive. The best way to detect the "lack of photosystem II" in these organisms would be:

• A. To determine if they have thylakoids in the chloroplasts
• B. To test for liberation of O₂ in the light
• C. To test for CO2 fixation in the dark
• D. To do experiments to generate an action spectrum

Explanation: Photosystem II plays a vital role in the photolysis of water, which results in the release of oxygen. In organisms lacking Photosystem II, oxygen is not produced when exposed to light. Therefore, testing for oxygen liberation under light conditions is a direct and effective method to detect the absence of Photosystem II. Option A is incorrect because the presence of thylakoids does not confirm the presence of Photosystem II. Option C is incorrect, as CO₂ fixation in the dark is unrelated to Photosystem II. Option D is incorrect because an action spectrum does not specifically address the presence of Photosystem II.

3659. Where do the light reactions take place?

• A. Stroma
• B. Thylakoid membrane
• C. Chloroplast
• D. Leucoplast

Explanation: The light reactions of photosynthesis occur in the thylakoid membranes of the chloroplasts. During these reactions, light energy is captured by chlorophyll and other pigments, exciting electrons that drive the synthesis of ATP and NADPH. These molecules then provide the energy needed for the Calvin cycle in the stroma. The stroma is not the site of the light reactions but rather where the Calvin cycle, or light-independent reactions, occur. Chloroplasts as a whole are responsible for photosynthesis, but the distinction of regions is crucial for understanding the process. Leucoplasts, on the other hand, do not participate in photosynthesis at all, as they lack photosynthetic pigments.

3660. Each photon of light excites how many electrons?

• A. 1
• B. 4
• C. 2
• D. 8

Explanation: Each photon of light excites one electron in a pigment molecule, such as chlorophyll, during the light-dependent reactions of photosynthesis. When a photon of appropriate energy is absorbed by the chlorophyll's porphyrin ring, it elevates one electron from its ground state to a higher energy level, called an excited state. This excited electron can then be transferred to an electron acceptor, initiating a chain of electron transport reactions that ultimately lead to the synthesis of ATP and NADPH. The process is highly efficient because each photon contributes to the excitation of a single electron, ensuring precise energy transfer through the photosystems.