All Free Biology MCQs with Answers

3231. 3-Phosphoglycerate in the presence of ATP and reduced NADP (from the light-independent stage) is reduced to:

• A. Ribulose bisphosphate
• B. 6 carbon compound
• C. 5 carbon compound
• D. 3 carbon compound

Explanation: In the presence of ATP and reduced NADP (produced during the light-dependent reactions), 3-phosphoglycerate (3-PGA) is reduced to glyceraldehyde-3-phosphate (G3P) during the light-independent reactions of photosynthesis, also known as the Calvin Cycle.

3232. How does chemosynthesis differ from photosynthesis?

• A. Source of energy to carry out the reaction
• B. Production of organic compounds
• C. Reduction of CO2
• D. Carried out by bacteria

Explanation: Chemosynthesis and photosynthesis are both processes that produce organic compounds, but they differ in the source of energy used.Photosynthesis: Uses light energy to convert carbon dioxide and water into glucose and oxygen. Light-dependent reactions, occurring in the thylakoid membranes of chloroplasts, capture energy from sunlight.Chemosynthesis: Uses chemical energy to convert carbon dioxide and water into glucose and sulfur compounds (or other chemical compounds). Energy is obtained from the oxidation of inorganic substances, such as hydrogen sulfide or methane, rather than from sunlight.

3233. Chlorophyll molecule resemble the haem group of hemoglobin, the main difference between these two molecule is that chlorophyll has _ ion.

• A. Fe
• B. Mg
• C. Mn
• D. Ca

Explanation: The main difference between the chlorophyll molecule and the heme group of hemoglobin is that chlorophyll contains a magnesium ion (Mg2+), whereas the heme group of hemoglobin contains an iron ion (Fe2+). The central metal ion in chlorophyll is magnesium, while in hemoglobin, it is iron. This difference in metal ions is crucial for the distinct functions of these molecules.

3234. In the Calvin cycle, the reducing power from NADPH is used during:

• A. Formation of 1, 3- Bisphosphoglycerate
• B. Conversion of 1, 3- Bisphosphoglycerate to Glyceraldehyde 3- phosphate
• C. Formation of fructose 1, 6- bisphosphate from 1, 3- bisphosphoglycerate
• D. Formation of glucose from fructose bisphosphate

Explanation: In the Calvin Cycle, the reducing power from NADPH is used during the reduction phase of the cycle. The Calvin Cycle, also known as the light-independent reactions of photosynthesis, takes place in the stroma of chloroplasts and is responsible for the synthesis of carbohydrates, primarily glucose. During the Calvin Cycle, the reducing power in the form of NADPH, which was generated during the light-dependent reactions (photophosphorylation), is utilized to reduce 3-phosphoglycerate (3-PGA) to glyceraldehyde-3-phosphate (G3P). This reduction phase involves a series of enzyme-catalyzed reactions, and the NADPH donates electrons to convert 3-PGA into G3P.

3235. The oxidation of PGAL during glycolysis yields _ molecules.

• A. ATP
• B. NADH
• C. FADH
• D. ADP

Explanation: During glycolysis, the oxidation of one molecule of 3-phosphoglyceraldehyde (PGAL or G3P) leads to the conversion of NAD+ to NADH. This occurs in the step where PGAL is converted to 1,3-bisphosphoglycerate. This process yields NADH, an important electron carrier for the electron transport chain, making NADH the correct answer. Other options such as ATP and ADP are involved in different steps of glycolysis, but they are not products of PGAL oxidation. FADH2 is not involved in glycolysis.

3236. Lactic Acid Fermentation is a type of anaerobic respiration that occurs in human muscle cells. It yields only _ % of the energy present in a glucose molecule.

• A. 2
• B. 7
• C. 10
• D. 20

Explanation: Lactic acid fermentation is a type of anaerobic respiration that occurs in human muscle cells when oxygen availability is limited, such as during intense physical exercise. In lactic acid fermentation, glucose is partially oxidized to lactic acid, and it yields only about 2% of the energy present in a glucose molecule.

3237. Formation of _ will be greater with the faster breakdown of glucose and glycogen to compensate for energy requirements in anaerobic respiration.

• A. Enzymes
• B. Hormones
• C. Lactic acid
• D. Fat

Explanation: In anaerobic respiration, when oxygen is scarce, glucose and glycogen are broken down to meet energy needs, resulting in lactic acid as a byproduct. This process does not produce enzymes or hormones as byproducts. Fat metabolism mainly occurs in the presence of oxygen and is not involved in this type of respiration.

3238. Plant pigments responsible for red, yellow, and orange colors in many fruits and vegetables are:

• A. Chlorophyll a
• B. Chlorophyll b
• C. Carotenoids
• D. Cellulose

Explanation: The correct answer is Carotenoids. These pigments impart red, yellow, and orange hues to many fruits and vegetables. They play a significant role in plant health and offer antioxidant benefits to humans. Chlorophyll a and b are primarily green pigments associated with photosynthesis, not with the vibrant colors in question. Cellulose, while important for structural integrity in plants, does not contribute to coloration.

3239. The complete aerobic oxidation of glucose results in the synthesis of as many _ molecules of ATP.

• A. 16
• B. 26
• C. 36
• D. 46

Explanation: The complete aerobic oxidation of one molecule of glucose results in the synthesis of approximately 30-38 molecules of ATP. This includes ATP generated through glycolysis (2 ATP), the citric acid cycle (2 ATP), and oxidative phosphorylation (electron transport chain and chemiosmosis, which produce the majority of ATP). The number often cited is 36, but it can vary based on cellular conditions such as the efficiency of electron transport and the shuttle systems used for transporting electrons into the mitochondria. Therefore, the correct answer is approximately 36 ATP, while the other options do not accurately reflect the typical ATP yield from aerobic respiration.

3240. If a molecule is reduced it gains:

• A. Energy
• B. Electrons
• C. Hydrogen protons
• D. All of the above

Explanation: Reduction is a chemical reaction in which a molecule gains electrons. This gain of electrons can be accompanied by the gain of hydrogen protons due to the nature of biological redox reactions. The energy increase is a result of the electron transfer process. Hence, the correct answer is option D: 'All of the above.' Although options A, B, and C each describe an aspect of reduction, only option D encompasses all the elements typically associated with reduction reactions.