All Free Biology MCQs with Answers

3471. Glyceraldehyde phosphate is oxidized during glycolysis. What happens to hydrogen atom and the electron that are removed during oxidation?

• A. They reduce NADP
• B. They are transferred to pyruvic acid
• C. They are eliminated in the form of methane
• D. They reduce NAD+

Explanation: During glycolysis, specifically in the step where glyceraldehyde 3-phosphate is converted to 1,3-bisphosphoglycerate, NAD+ serves as an electron carrier. It accepts electrons and hydrogen atoms that are removed from glyceraldehyde phosphate, becoming reduced to NADH. This is a crucial step in glycolysis as it contributes to the cell's energy production by providing electrons for the electron transport chain. Option A is incorrect because NADP is not involved in glycolysis. Option B is incorrect because pyruvic acid is formed later in the glycolytic pathway and does not directly accept electrons in this step. Option C is incorrect because methane production is unrelated to glycolysis and is instead a part of anaerobic digestion processes in some microorganisms.

3472. Succinate is oxidized and form:

• A. FAD
• B. FADH
• C. FADH2
• D. NADH2

Explanation: In the citric acid cycle, succinate undergoes oxidation to form fumarate. During this reaction, FAD (flavin adenine dinucleotide) acts as the oxidizing agent and is reduced to FADH2. This process is crucial for transferring electrons to the electron transport chain, which ultimately leads to ATP production.Let's break down why the other options are incorrect:Option A: FAD - This is incorrect because FAD is the molecule that gets reduced, not the product of the reaction.Option B: FADH - This is incorrect as FAD is reduced to FADH2, not FADH.Option D: NADH2 - This is incorrect because NADH2 is not a product of this reaction; it involves the reduction of FAD to FADH2.

3473. Fatty acid releases a considerable amount of energy in oxidation during:

• A. Calvin cycle
• B. Krebs cycle
• C. Dark reaction
• D. Light reaction

Explanation: Fatty acids are broken down into acetyl-CoA molecules through a process called beta-oxidation. Acetyl-CoA then enters the Krebs cycle, where it is further oxidized to produce energy. The Krebs cycle is a series of chemical reactions that occur in the mitochondria of cells. These reactions produce carbon dioxide, ATP (adenosine triphosphate), and other energy-carrying molecules.

3474. End product of citric acid cycle:

• A. Pyruvate
• B. CO2 and H2O
• C. CO2
• D. Lactic acid

Explanation: The correct answer is CO2 and H2O. During the citric acid cycle, acetyl-CoA is oxidized, resulting in the release of carbon dioxide (CO2). The high-energy electrons captured by NADH and FADH2 are eventually used in the electron transport chain to produce water (H2O) as the electrons combine with oxygen. Therefore, the combined end products of the citric acid cycle and subsequent processes in aerobic respiration are CO2 and H2O.Pyruvate is incorrect because it is not an end product of the citric acid cycle; it is a precursor that is converted to acetyl-CoA. CO2 alone is incomplete as an answer because it leaves out the contribution of water formed in the electron transport chain. Lactic acid is not involved in the citric acid cycle but is produced under anaerobic conditions when oxygen is not present.

3475. Molecular oxygen is released during:

• A. Calvin cycle
• B. Light reaction in photosynthesis
• C. Krebs cycle
• D. Glycolysis

Explanation: The light-dependent reactions of photosynthesis, also known as the light reactions, occur in the thylakoid membranes of chloroplasts and are responsible for the release of molecular oxygen. During these reactions, light energy is absorbed by chlorophyll, which drives the photolysis of water. This process splits water molecules into hydrogen ions, electrons, and oxygen atoms. The oxygen atoms then combine to form molecular oxygen (O2), which is released as a byproduct. This is the source of atmospheric oxygen replenishment.In contrast, the Calvin cycle (Option A) focuses on the fixation of carbon dioxide and does not produce oxygen. The Krebs cycle (Option C) and glycolysis (Option D) are part of cellular respiration, not photosynthesis, and do not release molecular oxygen.

3476. The molecular formula of chlorophyll b is:

• A. C₅₅H₇₂O₅N₄Mg
• B. C₅₅H₇₀O₆N₄Mg
• C. C₅₅H₇₄O₅N₄Mg
• D. C₅₅H₇₀O₄N₄Mg

Explanation: Among the given options, option B (C₅₅H₇₀O₆N₄Mg) correctly represents the molecular formula of chlorophyll b, the pigment responsible for capturing light energy and complementing chlorophyll a in photosynthesis. Chlorophyll b differs from chlorophyll a by having a formyl (-CHO) group instead of a methyl (-CH₃) group, resulting in two fewer hydrogen atoms and one more oxygen atom. The other options are incorrect: Option A describes chlorophyll a, option C has an unrealistically high hydrogen count, and option D lacks the proper number of oxygen atoms required for the formyl substitution in chlorophyll b.

3477. Location of dark reactions in chloroplast:

• A. Inner membrane
• B. Grana
• C. Stroma
• D. Thylakoid

Explanation: The correct answer is the stroma of the chloroplast. This is where the Calvin Cycle, or dark reactions, occur. The stroma contains all the enzymes necessary for carbon fixation and sugar synthesis. The inner membrane primarily serves to enclose the stroma and is not involved in the Calvin Cycle. The grana are stacks of thylakoids where light-dependent reactions take place, not the Calvin Cycle. Finally, the thylakoid membranes are involved in the light-dependent reactions, not the dark reactions.

3478. How many number of carbon atoms are present in a molecule of Rubisco?

• A. 4
• B. 6
• C. 5
• D. 7

Explanation: Rubisco takes carbon dioxide and attaches it to ribulose bisphosphate, a short sugar chain with five carbon atoms.

3479. The process in which energy-poor, inorganic, and oxidized compounds of carbon and hydrogen are reduced to energy-rich carbohydrates (glucose) using light energy that is converted into chemical energy by chlorophyll and other pigments is called:

• A. Bioenergetics
• B. Photosynthesis
• C. Aerobic respiration
• D. Anerobic respiration

Explanation: Photosynthesis is the process in which light energy is absorbed by chlorophyll and other pigments to convert inorganic compounds such as carbon dioxide and water into energy-rich carbohydrates like glucose. This process is an essential part of bioenergetics, which studies the conversion and flow of energy in biological systems. In contrast, aerobic respiration is the complete oxidation of glucose in the presence of oxygen, producing carbon dioxide, water, and a large amount of energy. Anaerobic respiration, on the other hand, occurs without oxygen and yields less energy, resulting in byproducts such as lactic acid.

3480. Carbon dioxide released by respiration equals the quantity required by photosynthesizing cells. At this moment there is no net gas exchange between the leaves and the atmosphere. This is termed as what?

• A. Bacteriochlorophylls
• B. Redox process
• C. Photosystems
• D. Compensation point

Explanation: The condition in which the carbon dioxide released during respiration equals the amount of carbon dioxide utilized in photosynthesis is known as the compensation point. At this stage, the rate of photosynthesis exactly balances the rate of respiration, resulting in no net exchange of gases between the leaf and the surrounding atmosphere. In other words, the carbon dioxide produced by respiration is completely used up by photosynthesis, and the oxygen generated during photosynthesis is entirely consumed by respiration. This point represents a state of equilibrium in gas exchange and is often referred to specifically as the CO₂ compensation point.