All Free Biology MCQs with Answers

3771. The absorptive peaks in the absorption spectrum of chlorophyll b are at the wavelength of:

• A. 453 nm and 642 nm
• B. 430 nm and 662 nm
• C. 480 nm and 520 nm
• D. 500 nm and 700 nm

Explanation: Among the given options, the most accurate ones are 453 nm and 642 nm. These wavelengths represent the two main absorption peaks of chlorophyll b, showing its ability to absorb light efficiently in both the blue and red regions of the visible spectrum. This absorption pattern allows chlorophyll b to assist chlorophyll a in capturing a broader range of light energy for photosynthesis. While the other options refer to wavelengths either absorbed by chlorophyll a or in regions where absorption is minimal, the combination of 453 nm and 642 nm accurately reflects the true absorption behaviour of chlorophyll b.

3772. Photosynthesis is a process in which:

• A. Oxidation of CO2 occurs
• B. Oxidation of H2O occurs
• C. Reduction of CO2 occurs
• D. Both B and C

Explanation: Option D is correct because photosynthesis is a redox process that encompasses both the oxidation of water and the reduction of carbon dioxide. During this process, water molecules are oxidized, meaning they lose electrons, which are then utilized in reducing carbon dioxide to form glucose. This is represented by the chemical equation:6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂Option A is incorrect since carbon dioxide undergoes reduction, not oxidation. Option B correctly identifies the oxidation of water, while Option C correctly identifies the reduction of carbon dioxide. However, only Option D captures the complete process of photosynthesis as involving both oxidation and reduction.

3773. The reactions of photosynthesis consist of:

• A. Two phases
• B. Four phases
• C. Three phases
• D. Many phases

Explanation: The correct answer is Option A. Photosynthesis comprises two main phases: the light-dependent reactions and the Calvin cycle. During the light-dependent reactions, which take place in the thylakoid membranes, light energy is used to produce ATP and NADPH while splitting water molecules into oxygen and hydrogen ions. The Calvin cycle occurs in the stroma of chloroplasts and utilizes ATP and NADPH to fix carbon dioxide into glucose. Options B, C, and D are incorrect, as they suggest incorrect numbers of phases, deviating from the established two-phase process of photosynthesis.

3774. In photosynthesis, reducing power and assimilatory power are synthesized during:

• A. Dark reaction
• B. Light reaction
• C. Calvin cycle
• D. Oxidation phosphorylation

Explanation: Option B is correct.The reducing power and assimilatory power are synthesized during the light-dependent reactions of photosynthesis. These reactions take place in the thylakoid membranes of chloroplasts. They use light energy to split water molecules into oxygen and hydrogen ions. The hydrogen ions are then used to produce ATP, a molecule that stores energy. NADPH is also produced during the light-dependent reactions. NADPH is a molecule that carries electrons, and it is used as a reducing agent in the Calvin cycle. Reducing power is a term used to describe the ability of a molecule to donate electrons. In photosynthesis, reducing power is provided by NADPH, which is produced in the light-dependent reactions. NADPH is used by the Calvin cycle to fix carbon dioxide into glucose.Assimilative power is a term used to describe the ability of a molecule to store energy. In photosynthesis, assimilatory power is provided by ATP, which is also produced in the light-dependent reactions. ATP is used by the Calvin cycle to drive the reactions that fix carbon dioxide into glucose.

3775. For the synthesis of sugar by reducing CO₂, NADPH₂ provides:

• A. Enzymes
• B. Co-enzymes
• C. Energized electrons
• D. Both B and C

Explanation: NADPH is vital in the Calvin cycle, where it acts both as a coenzyme and a provider of energized electrons. These electrons are used to reduce carbon dioxide into glucose. Option D, 'Both B and C,' captures the dual function of NADPH, making it the correct answer. Option A is incorrect because enzymes are not supplied by NADPH. Option B mentions the coenzyme role but misses the primary function of donating electrons. Option C correctly identifies the provision of energized electrons but does not acknowledge the coenzyme aspect, which is why Option D is the most comprehensive choice.

3776. The phase of photosynthesis in which sugar is synthesized by reducing CO₂ is also called the dark reaction because:

• A. It requires darkness
• B. It does not require light
• C. It requires night period
• D. It produces darkness in the chloroplast

Explanation: The dark reaction, also known as the Calvin cycle, is called so because it does not require light directly to carry out its chemical processes. During this phase, carbon dioxide is reduced to form sugars using the energy stored in ATP and the reducing power of NADPH, both of which are produced in the light-dependent reactions. Although it relies on the products of light reactions, the chemical steps of carbon fixation, reduction, and regeneration can proceed without light, which is why this phase is referred to as the dark reaction.

3777. Photosynthetic pigments are organized into clusters called:

• A. Antenna pigments
• B. Reaction centre
• C. Photosynthetic system
• D. Photosystems

Explanation: The correct answer is photosystems. Photosynthetic pigments are organized into clusters known as photosystems, which are crucial for the efficient absorption and conversion of light energy into chemical energy during photosynthesis. Photosystems are located in the thylakoid membranes and include Photosystem I (PSI) and Photosystem II (PSII), both of which play integral roles in the light-dependent reactions of photosynthesis.Other options like antenna pigments, reaction center, and photosynthetic system refer to components or broader aspects of the photosynthetic process but do not specifically denote the clusters of pigments.

3778. Each photosystem consists of a light-gathering:

• A. Antenna complex
• B. Reaction complex
• C. Both A and B
• D. Primary electron acceptor

Explanation: Each photosystem contains an antenna complex and a reaction center. The antenna complex is responsible for capturing light energy using pigments like chlorophyll and carotenoids and directing it toward the reaction center. The reaction center contains chlorophyll a and a primary electron acceptor, where the energy from the captured light excites electrons and initiates the electron transport chain. This division of roles ensures that light energy is efficiently harvested by the antenna complex and then converted into chemical energy in the reaction center, enabling the production of ATP and NADPH for the Calvin cycle.

3779. The reaction center of photosystem has:

• A. One molecule of chlorophyll a and a primary electron acceptor
• B. Many molecules of chlorophyll a and primary electron acceptor
• C. One or more molecules of chlorophyll a and the primary electron acceptor
• D. Chlorophyll a, chlorophyll b, and the primary electron acceptor

Explanation: The reaction center of a photosystem is crucial for the initial steps of photosynthesis. It consists of one or more molecules of chlorophyll a, which are responsible for capturing light energy, and a primary electron acceptor that facilitates the transfer of electrons. Option C correctly identifies these components. Option A is incorrect, as it suggests only one molecule of chlorophyll a, whereas the center can have multiple such molecules. Option B is misleading, as it suggests an unspecified large number. Option D incorrectly includes chlorophyll b, which is not part of the reaction center but is involved in light harvesting.

3780. There are two photosystems associated with photosynthesis, which have been named in order of their discovery as

• A. P680 and P700
• B. PS-I and PS-II
• C. PS-II and PS-I
• D. Photo system alpha and photo system beta

Explanation: The correct answer is PS-I and PS-II. Photosystem I (PS-I) was discovered before Photosystem II (PS-II), which is why they are named in this sequential order of their discovery. Even though Photosystem II functions first in the linear electron flow during the light-dependent reactions of photosynthesis, it was identified after Photosystem I.