All Free Biology MCQs with Answers

3201. At which two steps is ATP generated in glycolysis?

• A. G3P → 1,3-BPG and 1,3-BPG → 3-PG
• B. 1,3-BPG → 3-PG and PEP → Pyruvate
• C. G3P → 1,3-BPG and PEP → Pyruvate
• D. 2-PG → PEP and PEP → Pyruvate

Explanation: ATP is produced at two substrate-level phosphorylation steps:1) 1,3-BPG → 3-PG (via phosphoglycerate kinase)2) PEP → Pyruvate (via pyruvate kinase).

3202. In anaerobic conditions in muscles, pyruvate is converted into:

• A. Acetyl-CoA
• B. Lactic acid
• C. Ethanol
• D. CO2 + H2O

Explanation: In oxygen deficiency, pyruvate is converted to lactic acid by lactate dehydrogenase, regenerating NAD⁺ needed for glycolysis to proceed.

3203. Acetyl-CoA combines with oxaloacetate to form:

• A. Succinyl-CoA
• B. Citrate
• C. α-Ketoglutarate
• D. Malate

Explanation: Acetyl-CoA (2C) combines with oxaloacetate (4C) via citrate synthase to form citrate (6C), the first step of the Krebs cycle.

3204. The dark reactions of photosynthesis are characterized by:

• A. Synthesis of ATP, O2 and NADH
• B. Utilization of ATP, CO2 and NADPH
• C. Electron transfer from NADPH to RuBP
• D. Carbon dioxide transfer from RuBP to glucose

Explanation: Dark reaction is also called carbon-fixing reaction. It is a light-independent process in which sugar molecules are formed from carbon dioxide, water molecules, ATP, and NADPH(products from the light-dependent reaction). The dark reaction occurs in the stroma of the chloroplast, where they utilize the products of the light reaction.

3205. The end product in anaerobic respiration is:

• A. Ethanol and Carbon dioxide
• B. Lactate
• C. Pyruvate
• D. Both A and B

Explanation: The end products of anaerobic respiration depend on the specific type of anaerobic process involved. There are two main types of anaerobic respiration: lactic acid fermentation and alcoholic fermentation. In lactic acid fermentation, the end product is lactic acid.This process occurs in various microorganisms and in muscle cells during strenuous exercise when oxygen is not available in sufficient amounts to support aerobic respiration. The overall reaction is:Glucose → 2 Lactic Acid + Energy (in the form of ATP) In alcoholic fermentation, the end products are ethanol and carbon dioxide. This process is commonly carried out by yeast and some bacteria in the absence of oxygen.The overall reaction is:Glucose → 2 Ethanol + 2 Carbon Dioxide + Energy (in the form of ATP).

3206. Which of the following photosystems is involved in cyclic photophosphorylation?

• A. PS II
• B. PS I
• C. PS I and PS II
• D. PS III

Explanation: Cyclic photophosphorylation employs the use of only photosystem I (PS1). this occurs to overcome the ATP deficiency and the use of only PSI results in the formation of only ATP without the formation of NADPH.

3207. The photosynthetic pigments of plants are arranged as clusters in thylakoid membranes. The reaction centers of these clusters consist of _ molecules.

• A. Chlorophyll
• B. ATP
• C. Carotenoids
• D. Glucose

Explanation: Chlorophyll, is the correct answer because they are the photosynthetic pigment that comprises the reaction centers of these clusters, or photosystems 1 and 2. Light energy collected by the photosystem is transferred to the reaction center, which consists of chlorophyll molecules, after which an electron is excited and the subsequent steps of the light-dependent reaction of photosynthesis occur.

3208. Which of the following processes does NOT need Pyruvic Acid as a substrate?

• A. Alcoholic fermentation
• B. Calvin cycle
• C. Aerobic respiration
• D. Lactic Acid fermentation

Explanation: The Calvin Cycle, also known as the light-independent reactions of photosynthesis, does not directly use pyruvic acid as a substrate. The Calvin Cycle operates in the stroma of the chloroplast and is responsible for converting carbon dioxide into glucose and other carbohydrates. It fixes CO2 with RuBP in the presence of RuBPC/O

3209. The light dependent reaction of photosynthesis occurs in:

• A. Stroma of chloroplast
• B. Guard cells of stomata
• C. Thylakoid membrane of chloroplast
• D. Cytoplasm of leaf cell

Explanation: The correct answer is the thylakoid membrane of the chloroplast. The light-dependent reactions of photosynthesis occur in these membranes, where chlorophyll absorbs light energy. This energy is then converted into chemical energy in the form of ATP and NADPH, which are essential for the subsequent light-independent reactions (Calvin cycle) in the stroma.

3210. The rate of photosynthesis and respiration may equal one another at what time?

• A. Night
• B. Noon
• C. Dusk
• D. Dawn and Dusk

Explanation: At dawn and dusk, when light intensity is low, the rate of photosynthesis and respiration may, for a short time, equal one another. Thus the oxygen released from photosynthesis is just the amount required for cellular respiration. Also, the carbon dioxide released by respiration just 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 compensation point.