All Free Biology MCQs with Answers

4091. The product of second step of glycolysis:

• A. Glucose 6-phosphate
• B. Fructose 1, 6 Bisphosphate
• C. Fructose 6-phosphate
• D. Glucose

Explanation: Aldohexose (glucose 6 phosphate) is transformed into ketohexose (fructose 6 phosphate).

4092. The product(s) of fourth step of glycolysis is:

• A. G3P
• B. 3PGAL
• C. Dihydroxyacetone phosphate
• D. G.3.P/PGAL and Dihydroxyacetone phosphate

Explanation: As fructose 1, 6 bisphosphate is cleaved to yield two trioses i.e. Glyceraldehyde 3 phosphate and dihydroxyacetone phosphate.

4093. The step of glycolysis in which removal of a water molecule is carried out is:

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

Explanation: Dehydration occurs during the formation of phosphenol pyruvate from 2 phosphoglyceraldehyde.

4094. What is equivalent to half glucose molecule that has been oxidized to the extent of losing two electrons as hydrogen atoms:

• A. G3P
• B. 3PGAL
• C. 3-PG
• D. Pyruvate

Explanation: Pyruvic acid (pyruvate) is the end product of glycolysis and does not enter the Krebs cycle directly. The pyruvate (a 3-carbon molecule) is first changed into a 2-carbon acetic acid molecule. One carbon is released as CO2 coenzyme-A (CoA) to form acetyl CoA (Active acetate). In addition, more hydrogen atoms are transferred to NAD.

4095. During aerobic respiration, the chemical substance that enters the mitochondrion to start Krebs cycle is:

• A. Pyruvic acid
• B. Acetyl CO-A
• C. Acetic acid
• D. Citric acid

Explanation: Acetic acid on entering the mitochondrion unites with Coenzyme A to form acetyl Co-A.

4096. Before start of Krebs cycle, following changes occur, EXCEPT:

• A. Formation of acetyl-Co-A
• B. Oxidation of acetate
• C. Reduction of NAD
• D. Decarboxylation of pyruvate

Explanation: Oxidation of pyruvate takes places not that of acetate.

4097. Krebs cycle is a cyclic series of chemical reactions during which:

• A. Oxidation process is completed
• B. Decarboxylation process is completed
• C. Reduction process is completed
• D. Energy consuming process is completed

Explanation: Oxidative breakdown of organic food is completed.

4098. In first step of Krebs cycle following changes occur, EXCEPT:

• A. Formation of citrate
• B. Condensation of oxaloacetate and acetyl Co-A
• C. Hydration and decondensation of Co-A
• D. Decarboxylation and condensation of Co-A

Explanation: Other three changes given in A), B) and C) occur in first step of Krebs cycle except decarboxylation and condensation of Co-A.

4099. FAD is reduced to get FADH2 in a step of Krebs cycle which involves conversion of:

• A. Succinate to fumarate
• B. Malate to oxaloacetate
• C. Fumarate to malate
• D. α-ketogluterate to succinate

Explanation: The succinate is oxidized to get fumarate in presence of succinic acid dehydrogenase enzyme.

4100. Succinate is converted into fumarate by removal of:

• A. A hydrogen atom
• B. A CO2 atom
• C. Two hydrogen atoms
• D. A water molecule

Explanation: Succinate is converted into fumarate and two hydrogen atoms are removed. The process is catalyzed by succinic acid dehydrogenase.