All Free Biology MCQs with Answers

3421. Before entering the Krebs cycle, the pyruvate is first decarboxylated and oxidized into:

• A. Acetyl-CoA
• B. Oxaloacetate
• C. Lactic acid
• D. Citric acid

Explanation: Before entering the Krebs cycle, pyruvate is decarboxylated and oxidized into acetyl-CoA by the pyruvate dehydrogenase complex. This reaction releases one molecule of CO₂ and generates NADH. The acetyl-CoA formed then enters the Krebs cycle to combine with oxaloacetate, forming citric acid. Other options like oxaloacetate, lactic acid, and citric acid are related to cellular respiration but occur at different stages or under different conditions. Thus, the correct answer is Acetyl-CoA.

3422. Some electron from the second primary acceptor may pass back to chlorophyll molecules by electron carrier system, yielding ATP. This process is called:

• A. Phosphorylation
• B. Photophosphorylation
• C. Non-Cyclic Phosphorylation
• D. Cyclic Phosphorylation

Explanation: In cyclic photophosphorylation, electrons released from the reaction center of Photosystem I (PSI) are cycled back to the chlorophyll molecules through an electron carrier system, ultimately leading to the generation of ATP. Cyclic photophosphorylation is particularly important in certain types of photosynthetic bacteria and in the light reactions of photosynthesis in the chloroplasts of some plants. It allows the cell to generate ATP without the simultaneous production of NADPH and oxygen.

3423. Z-scheme is used for:

• A. Non-Cyclic Photophosphorylation
• B. Cyclic Photophosphorylation
• C. Both Cyclic and Non-Cyclic Photophosphorylation
• D. Oxidative Phosphorylation

Explanation: The Z-scheme is a concept in photosynthesis that describes the electron flow through the photosystems during the light-dependent reactions. However, it is associated with cyclic and non-cyclic photophosphorylation.Non-cyclic photophosphorylation is the process in which light energy is used to generate both ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) by linear electron flow. The Z-scheme illustrates the flow of electrons between two photosystems, Photosystem II (PSII) and Photosystem I (PSI), and other electron carriers in a zigzag or Z-shaped pattern.

3424. Which one of the following is the site of oxidative phosphorylation in mitochondria?

• A. Cristae
• B. Matrix
• C. Outer membrane
• D. Ribosomes

Explanation: The site of oxidative phosphorylation in mitochondria is the inner mitochondrial membrane. While the inner mitochondrial membrane is the primary site of oxidative phosphorylation, it is essential to mention that the specific structures within the inner membrane called cristae play a crucial role in this process. Cristae are the infoldings or protrusions of the inner mitochondrial membrane, and they significantly increase the surface area available for the various proteins involved in oxidative phosphorylation. Within the inner mitochondrial membrane, there is a series of protein complexes collectively known as the electron transport chain (ETC). This is where the transfer of electrons takes place, and the energy released during this process is used to pump protons across the inner mitochondrial membrane, creating a proton gradient. The enzyme ATP synthase, also located in the inner mitochondrial membrane, utilizes the energy from the proton gradient to synthesize ATP from ADP and inorganic phosphate. This entire process of electron transport and ATP synthesis is referred to as oxidative phosphorylation.

3425. The most common respiratory substrate as an energy source 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).

3426. Oxidative phase of glycolysis starts with dehydrogenation of:

• A. Glycolysis
• B. Ribulose Bisphosphate
• C. Glyceraldehyde 3-phosphate
• D. NADH

Explanation: In the oxidative phase, each molecule of glyceraldehyde 3-phosphate (produced earlier in the preparatory phase) undergoes dehydrogenation. This involves the removal of a hydrogen atom from glyceraldehyde 3-phosphate, which is transferred to a molecule of NAD+ (nicotinamide adenine dinucleotide) to form NADH. The resulting molecule is 1,3-bisphosphoglycerate.

3427. In one turn, the Krebs's cycle produces one molecule of ATP, one molecule of FADH2 and _ molecules of NADH.

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: In one turn of the Krebs cycle, one molecule of ATP, one molecule of FADH2, and three molecules of NADH are produced. The ATP is generated through substrate-level phosphorylation, while FADH2 and NADH carry high-energy electrons that will be used in the electron transport chain to produce additional ATP during oxidative

3428. Which one of the following is the stage of cellular respiration for which oxygen is not essential:

• A. Glycolysis
• B. Pyruvate oxidation
• C. Krebs's cycle
• D. Electron Transport Chain

Explanation: The stage of cellular respiration for which oxygen is not essential is glycolysis. Glycolysis is the first step in cellular respiration and takes place in the cytoplasm of the cell. It does not require oxygen and can occur under both aerobic (with oxygen) and anaerobic (without oxygen) conditions. In the absence of oxygen, glycolysis is followed by either fermentation (in anaerobic conditions) or the continuation of the aerobic respiration process (if oxygen is available).

3429. Pyruvate, the end product of glycolysis moves from cytosol to mitochondrial matrix where it is oxidized into _ producing CO2 as a by-product.

• A. Acetic acid (active)
• B. Citrate
• C. NAD
• D. FAD

Explanation: Pyruvate, the end product of glycolysis, moves from the cytosol to the mitochondrial matrix, where it undergoes oxidative decarboxylation. During this process, pyruvate is converted into Active Acetic acid or acetyl-CoA (acetyl coenzyme A). The reaction involves the removal of a carbon dioxide (CO2) molecule as a by-product. This step is known as the pyruvate decarboxylation reaction. The acetyl-CoA then enters the citric acid cycle (also known as the Krebs cycle) within the mitochondrial matrix.

3430. Every molecule of NADH, fed into ETC produces:

• A. 2 ATP
• B. 3 ATP
• C. 4 ATP
• D. 6 ATP

Explanation: Every molecule of NADH (nicotinamide adenine dinucleotide) that enters the Electron Transport Chain (ETC) during cellular respiration produces approximately 3 molecules of ATP. This occurs through a process known as oxidative phosphorylation. The high-energy electrons carried by NADH are transferred along the electron transport chain, consisting of a series of protein complexes in the inner mitochondrial membrane. As electrons move through this chain, protons (H+) are pumped across the membrane, creating a proton gradient. This proton gradient is then used by the enzyme ATP synthase to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi). The exact number of ATP molecules generated per NADH can vary slightly, but, on average, it is considered to be around 3 molecules of ATP. The process is more complex for FADH2 (another electron carrier), which typically yields about 2 molecules of ATP. These ATP molecules contribute to the overall energy production in cellular respiration.