All Free Biology MCQs with Answers

3601. Where does the dark reaction of photosynthesis take place?

• A. Thylakoid
• B. Grana
• C. Stroma
• D. All of these options are correct

Explanation: The dark reactions of photosynthesis occur in the stroma of the chloroplast where they utilize the products of the light reaction.

3602. How many ATP molecules are made from one molecule of glucose during anaerobic cellular respiration?

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: Without oxygen, organisms must use anaerobic respiration to produce ATP, and this process produces only two molecules of ATP per molecule of glucose.

3603. C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. The reaction mentioned above is:

• A. Aerobic respiration
• B. Photosynthesis
• C. None of these
• D. Anaerobic respiration

Explanation: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy is the equation of aerobic respiration. Aerobic respiration is the process by which cells generate energy (in the form of ATP) in the presence of oxygen. It is the most efficient way for cells to extract energy from organic molecules.

3604. Total NADH formed by one glucose molecule during Krebs's Cycle are:

• A. 6
• B. 3
• C. 8
• D. 18

Explanation: During the Krebs cycle, which is also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, one molecule of glucose results in the formation of a total of 6 molecules of NADH. The Krebs cycle involves a series of reactions in which acetyl-CoA, derived from the breakdown of glucose, is further oxidized. For each turn of the Krebs cycle, three molecules of NADH are produced. Since glycolysis produces two molecules of pyruvate (and therefore, two molecules of acetyl-CoA), and each molecule of glucose generates two molecules of pyruvate, a complete oxidation of one glucose molecule through the Krebs cycle results in the formation of 6 molecules of NADH.

3605. One molecule of FADH2 is produced in Krebs's cycle during conversion of:

• A. Fumarate to Malate
• B. Succinate to Fumarate
• C. Malate to Oxaloacetate
• D. α-Ketoglutarate to Succinate

Explanation: One molecule of FADH2 is produced in the Krebs cycle during the conversion of succinate to fumarate. The specific reaction involves the enzyme succinate dehydrogenase, which catalyzes the oxidation of succinate. The chemical reaction can be represented as follows: Succinate + FAD (flavin adenine dinucleotide) → Fumarate + FADH2 In this reaction, succinate is oxidized to fumarate, and FAD is reduced to form FADH2. The electrons removed from succinate are transferred to FAD, and the resulting FADH2 carries these electrons to the electron transport chain, where they contribute to the generation of ATP during oxidative phosphorylation in later stages of cellular respiration.

3606. Enzyme succinate dehydrogenase converts succinate into:

• A. Malate
• B. Citrate
• C. Malonic acid
• D. Fumarate

Explanation: D is correct. It is an important step of Kreb's cycle as succinate dehydrogenase converts succinate into fumurate.

3607. Oxidative phosphorylation, synthesis of ATP in the presence of oxygen occurs in:

• A. All Types of Cells
• B. All Anaerobic Cells
• C. All Primitive Cells
• D. All Aerobic Cells

Explanation: Oxidative phosphorylation, which involves the synthesis of ATP in the presence of oxygen, occurs in aerobic cells. Aerobic cells are those that use oxygen as a final electron acceptor in the electron transport chain, leading to the production of water. This process takes place in the mitochondria of eukaryotic cells and in the inner membrane of the cell's mitochondria.

3608. Before entering Krebs's cycle, the pyruvate is first decarboxylated and oxidized into:

• A. Alpha Ketoglutaric Acid
• B. Citric Acid
• C. Glyceric Acid
• D. Acetic Acid

Explanation: The statement you've provided is partially correct. Before entering the Krebs (citric acid) cycle, each pyruvate molecule, which is a product of glycolysis, undergoes a process called pyruvate decarboxylation. This occurs in the mitochondria. However, the resulting product is not directly acetic acid, but rather acetyl-CoA (acetyl coenzyme A). The conversion of pyruvate to acetyl-CoA involves two main steps: Decarboxylation: One of the three carbon atoms in pyruvate is removed in the form of carbon dioxide (CO2), leading to a two-carbon molecule called acetyl group. Oxidation: The remaining two-carbon acetyl group is oxidized, and the electrons are transferred to NAD+ (nicotinamide adenine dinucleotide), forming NADH. The oxidized acetyl group is then combined with coenzyme A to form acetyl-CoA.

3609. The most common respiratory substrate as a source of energy is:

• A. Glucose
• B. Sucrose
• C. Fructose
• D. Insulin

Explanation: The most common respiratory substrate (a substance that undergoes oxidation in cellular respiration to produce energy) is glucose. Glucose is a simple sugar and is a primary source of energy for many organisms. In cellular respiration, glucose is oxidized to produce carbon dioxide, water, and energy in the form of adenosine triphosphate (ATP).

3610. Which part of chlorophyll molecule absorbs light?

• A. Phytol
• B. Porphyrin ring
• C. Pyrrole
• D. Thylakoid membrane

Explanation: The part of the chlorophyll molecule that absorbs light is the porphyrin ring, which is a complex cyclic structure containing a centrally coordinated magnesium ion (Mg2+). The porphyrin ring is responsible for the pigment's ability to absorb light energy during the process of photosynthesis. The porphyrin ring consists of alternating single and double bonds, creating a conjugated system. This conjugated system allows chlorophyll to absorb light in the visible spectrum.The absorption of light by chlorophyll is most efficient in the blue and red regions of the electromagnetic spectrum.