All Free Biology MCQs with Answers

3391. How does pyruvate enter the 'Krebs cycle' after glycolysis?

• A. Pyruvic acid
• B. Acetyl CoA
• C. Carbon dioxide
• D. Both A and B

Explanation: Option B is correct since, for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes to become 'acetyl Coenzyme A' (acetyl CoA). Pyruvate enters the Krebs cycle after glycolysis in the form of acetyl-CoA. After glycolysis, pyruvate is transported from the cytoplasm of the cell into the mitochondrial matrix, where it undergoes Pyruvic Acid Oxidation. In the presence of oxygen, pyruvate is first converted into acetyl-CoA by a complex of enzymes known as the pyruvate dehydrogenase complex.

3392. What percentage of the leaf surface does the stomata cover?

• A. 10 %
• B. 50 %
• C. 1-2 %
• D. 0.3 to 0.4 %

Explanation: Option C is correct since stomata are the microscopic pores on leaf surfaces that facilitate gas exchange with the atmosphere, namely, CO2, O2, and H2O, and cover 1-2 % of the leaf surface.

3393. For every 3 molecules of carbon dioxide in Calvin cycle how much G3P is produced?

• A. 6
• B. 5
• C. 3
• D. 8

Explanation: In the Calvin cycle, 3 molecules of carbon dioxide are fixed by an enzyme called RuBisCO, resulting in 6 molecules of G3P. These are the initial products of carbon fixation. Out of these 6 G3P molecules, 5 are used to regenerate RuBP, the CO2 acceptor molecule, and 1 is available for synthesis of glucose and other carbohydrates. Therefore, while the Calvin cycle produces 6 G3P molecules for every 3 CO2 molecules, only one contributes directly to biosynthesis outside the cycle. The incorrect options either underestimate or overestimate the number of G3P molecules produced.

3394. Water vapours exit and CO2 enters a leaf through the _.

• A. Stomata
• B. Grana
• C. Porphyrin ring
• D. Photons

Explanation: Stomatal opening is the place where gaseous exchange occurs in the plant. The vapours from transpiration leave the plant and CO2 for photsynthesis enters the plant through it.

3395. The ETC in plants consists of an electron carrier called _.

• A. Pq
• B. Pm
• C. Pt
• D. Po

Explanation: Plastoquinone, abbreviated as Pq, is a crucial electron carrier in the light-dependent reactions of photosynthesis. It transfers electrons from Photosystem II to the cytochrome b6f complex, facilitating the continuation of the electron transport chain and contributing to the generation of a proton gradient used in ATP synthesis. Options B (Pm), C (Pt), and D (Po) do not correspond to any known electron carriers in the photosynthesis process, making them incorrect answers.

3396. What was the source of hydrogen for first photosynthetic organisms?

• A. Water
• B. Hydrogen present in soil
• C. Hydrogen sulphate
• D. Hydrogen sulphide

Explanation: Cyanobacteria are the first photosynthetic organisms. They used hydrogen sulfide as source of hydrogen.

3397. How is ATP molecule used by the cells?

• A. Synthesis of complex compounds
• B. Active transport
• C. Muscular contraction
• D. All of these

Explanation: The correct answer is d) All of these. ATP is the universal energy currency of the cell, and it is utilized in numerous cellular activities that require energy. It is involved in the synthesis of complex compounds by providing the energy for bond formation. It powers active transport mechanisms by enabling the movement of substances against their concentration gradients. Moreover, ATP is essential for muscular contractions, facilitating the interaction between actin and myosin filaments. Each of these processes relies on ATP to proceed efficiently, highlighting its critical role in cellular energetics.

3398. In this stage of photosynthesis, ATP and NADPH are converted to ADP+Pi and NADP+:

• A. Light dependent reaction
• B. Light independent reaction
• C. Both of these
• D. None of above

Explanation: The light-independent reaction, also known as the Calvin cycle or the dark reactions, is the second stage of photosynthesis. It takes place in the stroma of chloroplasts and does not directly rely on light energy. The primary purpose of the light-independent reaction is to convert carbon dioxide (CO2) into glucose and other organic molecules. In the Calvin cycle, ATP and NADPH (generated during the light-dependent reactions) are used as energy sources. Carbon dioxide molecules are combined with a five-carbon compound called ribulose bisphosphate (RuBP) to form a six-carbon molecule. This molecule is then converted into various organic compounds through a series of enzyme-catalyzed reactions. The cycle regenerates RuBP, allowing it to be reused in the process.

3399. The respiratory chain electron carrier that directly binds oxygen is:

• A. Cytochrome a
• B. Cytochrome a3
• C. Cytochrome c1
• D. Cytochrome b

Explanation: The respiratory chain electron carrier that directly binds oxygen is cytochrome a3. Cytochrome a3 is the final electron carrier in the electron transport chain, and it is located in complex IV. When electrons are transferred to cytochrome a3, they are oxidized and combine with oxygen to form water.

3400. Which one is a light gathering structure?

• A. Antenna complex
• B. Reaction center
• C. Photosystem
• D. None of these

Explanation: The correct answer is the antenna complex. It is specifically designed to gather light by capturing photons through its pigments, such as chlorophyll and carotenoids, and transferring the energy to the reaction center. The reaction center itself is not a light-gathering structure but a site where the energy is utilized to initiate electron transport. The photosystem consists of both the antenna complex and the reaction center, but the antenna complex is the component responsible for light gathering. Therefore, 'None of these' is incorrect because the antenna complex is indeed a light-gathering structure.