All Free Biology MCQs with Answers

1511. The one which is present in all aerobic species is _________?

• A. Cytohrome c
• B. Cytochrome a
• C. Cytochrome a3
• D. Cytochrome b

Explanation: Cytochrome c is a mobile electron carrier in the electron transport chain and transfers electrons between respiratory complexes, making it a standard component of aerobic respiration. Cytochromes a, a3 and b occur in particular respiratory complexes rather than being the general component identified across aerobic organisms. Students may choose d because cytochrome b is also part of the electron transport chain, but it is not the intended universal marker here.

1512. Interconversion of carbohydrates proteins and lipids in living cells are an example of____________?

• A. Coordinated catabolic activities
• B. Coordinated anabolic activities
• C. Both A and B
• D. Any one

Explanation: Cells interconvert carbohydrates, proteins and lipids through linked metabolic pathways, so both breakdown reactions and synthesis reactions are involved. Catabolic pathways release smaller molecules and energy, while anabolic pathways use smaller molecules to build cellular substances. Students may choose only coordinated catabolism or only coordinated anabolism by considering one direction of interconversion and ignoring the other.

1513. The trioses which are intermediate in respiration and photosynthesis are_______________?

• A. Glyceraldehydes
• B. Dihydroxy acetone
• C. Both of these
• D. None of these

Explanation: Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are three-carbon intermediates found in pathways of both photosynthesis and respiration, so both listed trioses qualify. The likely mistake is choosing glyceraldehydes alone by remembering only glyceraldehyde-3-phosphate and overlooking dihydroxyacetone phosphate.

1514. ATP is a an important molecule for its major function___________?

• A. As an energy currency of the cell
• B. As a coenzyme
• C. Both a and b
• D. None of these

Explanation: ATP stores usable chemical energy in its phosphate bonds and transfers that energy to cellular processes, so it is called the energy currency of the cell. It can participate in some enzyme reactions, but its major biological role is energy transfer rather than acting as a coenzyme. Students may choose c by treating every molecule involved in an enzyme reaction as a coenzyme.

1515. Which of the following occurs in anaerobic respiration but not in aerobic respiration?

• A. Release of CO2
• B. Reduction of NAD
• C. Formation of ATP
• D. Production of ethanol from acetaldehyde

Explanation: In alcoholic fermentation, acetaldehyde accepts hydrogen from reduced NAD and is converted into ethanol, regenerating NAD for glycolysis. Aerobic respiration also forms ATP and reduces NAD during earlier stages, whereas ethanol production is specific to this anaerobic pathway. Students may choose c because ATP formation is associated with respiration generally, but it is not unique to anaerobic respiration.

1516. How many moles of carbon dioxide are produced by complete oxidation of one mole of puruvic acid?

• A. 1
• B. 2
• C. 3
• D. 6

Explanation: One pyruvic acid molecule has three carbon atoms, and complete oxidation releases all three as carbon dioxide: one during the link reaction and two during the Krebs cycle. Thus, 1 mole of pyruvic acid produces 3 moles of carbon dioxide. The common mistake is choosing 2 moles by counting only the two Krebs-cycle decarboxylations and ignoring the link reaction.

1517. Reactions in which simple substances are combined to form complex substances are called_____________?

• A. Metabolic reactions
• B. Catabolic reactions
• C. Anabolic reactions
• D. None of these

Explanation: Anabolic reactions build complex molecules from simpler substances and usually require an input of energy, such as the formation of proteins from amino acids. Catabolic reactions do the opposite by breaking complex molecules down and often releasing energy. Students most likely choose metabolic reactions, but metabolism includes both anabolic and catabolic reactions.

1518. In which of the following location enzymes controlling cellular respiration are present____________?

• A. Nucleus
• B. Chlorophast
• C. Milochondria
• D. Ribosome

Explanation: In eukaryotic cells, the major enzymes controlling aerobic cellular respiration are located in mitochondria, especially the matrix and inner mitochondrial membrane. A student may choose chloroplasts because they also contain energy-related enzymes, but chloroplasts perform photosynthesis rather than cellular respiration.

1519. The stent energy in the form of ADP is regenerated by mitochondria into__________?

• A. AMP
• B. ADP
• C. ATP
• D. All of these

Explanation: Mitochondria regenerate ATP from ADP by adding inorganic phosphate during oxidative phosphorylation: ADP + phosphate forms ATP. ATP is the immediately usable energy-carrying molecule, whereas AMP has fewer phosphate groups and is not the normal product of this regeneration. The likely mistake is choosing ADP because it appears in the question, even though ADP is the starting molecule.

1520. ___________ is the potential surce of chemical energy for cellular activities?

• A. C-H bond
• B. C-O bond
• C. C-N bond
• D. P-O-C bond

Explanation: C-H bonds in organic food molecules contain potential chemical energy, which is released stepwise during cellular respiration and transferred to ATP. Students may choose a P-O-C bond because ATP contains phosphate bonds, but the question is referring to the energy stored in food molecules rather than ATP.