All Free Biology MCQs with Answers

3811. Pick up the anaerobic one:

• A. Fermentation
• B. Oxidative phosphorylation
• C. Respiratory chain
• D. Krebs cycle

Explanation: Fermentation is the correct answer because it is an anaerobic process, meaning it occurs without oxygen. It allows cells to produce energy in oxygen-deprived environments by converting carbohydrates into alcohol or acids. The other options, such as oxidative phosphorylation, the respiratory chain, and the Krebs cycle, are aerobic processes that require oxygen to function. These processes occur as part of cellular respiration, where oxygen is utilized to extract energy from organic molecules.

3812. Glucose is completely broken down only in the:

• A. Aerobic respiration
• B. Cellular respiration
• C. Internal respiration
• D. External respiration

Explanation: Aerobic respiration is the correct answer because it is the cellular process where glucose is completely oxidized in the presence of oxygen, yielding maximum ATP. This process includes glycolysis, the citric acid cycle, and oxidative phosphorylation. Cellular respiration, as a term, includes both aerobic and anaerobic processes, and not all of them completely break down glucose. Internal and external respiration refer to gas exchange processes rather than the biochemical breakdown of glucose.

3813. During aerobic respiration, glucose is oxidized to:

• A. Lactic acid
• B. CO2 and water
• C. Ethanol and CO₂
• D. Pyruvate

Explanation: During aerobic respiration, glucose is completely oxidized in the presence of oxygen to produce carbon dioxide, water, and energy in the form of ATP. Other possible products, such as lactic acid, ethanol, or pyruvate, occur under different conditions: lactic acid forms during anaerobic respiration in muscles, ethanol and carbon dioxide form during fermentation in yeast, and pyruvate is an intermediate produced during glycolysis before further oxidation. Only carbon dioxide and water represent the final products of complete aerobic glucose oxidation.

3814. During aerobic respiration, glucose is oxidized to carbon dioxide and water; in this process energy is

• A. Stored as ATP
• B. Released as heat
• C. Stored as glycogen
• D. Stored as fat

Explanation: During aerobic respiration, glucose is completely oxidized to carbon dioxide and water, and the energy released in this process is primarily captured and stored as ATP. This ATP provides the cell with usable energy for various functions, while some energy is also lost as heat. Other forms, like glycogen or fat, are not direct products of energy capture during respiration.

3815. This form of anaerobic respiration occurs in muscle cells of humans and other animals during extreme physical activities, such as sprinting:

• A. Alcoholic fermentation
• B. Aerobic respiration
• C. Anaerobic respiration
• D. Lactic acid fermentation

Explanation: The correct answer is lactic acid fermentation. During intense physical activities like sprinting, the oxygen supply to muscle cells is insufficient to meet energy demands through aerobic respiration. As a result, muscle cells switch to lactic acid fermentation, an anaerobic process, to produce ATP. This process converts glucose into lactic acid, allowing continued energy production without oxygen. Alcoholic fermentation is incorrect because it occurs in yeast and some bacteria, not in human muscle cells. Aerobic respiration is not applicable, as it requires oxygen. The general term 'anaerobic respiration' is too broad and does not specify the process in muscle cells.

3816. Which molecule serves as the primary energy carrier in cells?

• A. ATP
• B. NADH
• C. Glucose
• D. FADH₂

Explanation: ATP is the primary energy carrier in cells. It stores energy in its high-energy phosphate bonds and releases it to power essential cellular activities such as muscle contraction, active transport, and biosynthesis. Other molecules like glucose, NADH, and FADH₂ play roles in storing or transferring energy, but ATP is the form that cells directly use.

3817. A compound found in every living cell, it is one of the essential chemicals of life. It plays a key role in most biological energy transformations. It is:

• A. NADH
• B. FADH₂
• C. ATP
• D. Glucose

Explanation: ATP is considered the energy currency of the cell because it directly provides energy for most cellular processes. It is formed during cellular respiration and used in various biochemical reactions. While NADH and FADH₂ are crucial in energy metabolism, they primarily function as electron carriers that help produce ATP. Glucose serves as a primary energy source but must be converted into ATP to be used as energy within cells.

3818. The maintenance of a living system requires a:

• A. Continuous supply of free energy
• B. Continual supply of free energy
• C. Continuously increasing supply of free energy
• D. Continuously decreasing supply of free energy

Explanation: The correct answer is Option B: continual supply of free energy. Living organisms require a steady and ongoing influx of energy to support vital processes like growth, reproduction, and environmental response. Without this continuous energy supply, these life-sustaining processes would halt, leading to the organism's demise. Option A fails to emphasize the necessity for constancy in the energy supply. Option C is incorrect because it suggests a need for progressively more energy, which is not typical for living organisms. Option D is incorrect, as a decreasing energy supply would lead to detrimental effects on the organism's health.

3819. Cellular respiration is essentially:

• A. Oxidation process
• B. Redox process
• C. Reduction process
• D. Decarboxylation process

Explanation: The correct answer is Option A) Oxidation process. Cellular respiration is fundamentally an oxidation process, where organic molecules are broken down to produce ATP, primarily through the loss of electrons. Option B is partially correct because cellular respiration is a redox process, but the primary focus here is on oxidation. Option C is incorrect because the process is not solely about reduction; rather, it includes reduction as part of the redox reaction. Option D is incorrect, as decarboxylation is just one aspect of cellular respiration and does not define the entire process.

3820. Cellular respiration is also called:

• A. Internal respiration
• B. Biological oxidation
• C. Organismic respiration
• D. Both A and B

Explanation: Cellular respiration is best described as biological oxidation. This process involves the breakdown of organic compounds, such as glucose, in the presence of oxygen to produce ATP, which is the main energy source for cellular activities. Internal respiration is not appropriate because it refers to the exchange of gases between the blood and body cells, not the energy-producing biochemical reactions within cells. "Organismic respiration" is a broader term that includes all aspects of breathing but does not specifically focus on cellular-level processes. Therefore, the correct term for cellular respiration is biological oxidation.