All Free Biology MCQs with Answers

2111. Which step causes activation of catalytic site of an enzyme__________________?

• A. Change in pH of the surroundings.
• B. Formation of Enzyme Susstrate complex.
• C. Change in the charge of the active site.
• D. Change in temperature

Explanation: When an enzyme-substrate complex forms, interactions between the substrate and active site can produce induced fit, positioning catalytic groups correctly and activating the catalytic site. A student may choose a change in pH because pH alters enzyme charge, but pH change is an external factor and is not the normal activation step itself.

2112. Lock and Key model was proposed by______________?

• A. Emil Fischer
• B. Koshland
• C. Robin Williams
• D. Rudolph Virchow

Explanation: Emil Fischer proposed the lock-and-key model, in which the substrate has a shape complementary to the enzyme's relatively rigid active site. The likely alternative is Koshland, who proposed the induced-fit model in which the enzyme changes shape as the substrate binds.

2113. Muscle fatigue occurs due to accumulation of___________?

• A. Acidic acid
• B. Butyric acid
• C. Citric acid
• D. Lactic acid

Explanation: During intense exercise, oxygen delivery may not meet demand, so muscle cells rely more on anaerobic glycolysis and convert pyruvate to lactate, traditionally described in basic biology as lactic acid accumulation. Students may choose citric acid because it is associated with cellular respiration, but it is not the product linked with short-term muscle fatigue in this context.

2114. Tetany is caused by__________?

• A. Hypocalcemia
• B. Hypercalcemia
• C. Clostridum tetani.
• D. Low O2 level

Explanation: Tetany is caused by hypocalcemia, or a reduced concentration of calcium ions in the blood, which increases nerve and muscle excitability and produces spasms. The likely distractor is Clostridium tetani, which causes infectious tetanus, a different condition from hypocalcemic tetany.

2115. Which of the following statement about fatigue is correct?

• A. The major event in it is accumulation of lactic acid.
• B. Lactic acid accumulation turns pH in acidic range leading to muscle ache.
• C. Ionic imbalance may also cause it.
• D. All of these

Explanation: Muscle fatigue may involve lactate and hydrogen ion accumulation, changes in pH, and ionic disturbances that interfere with excitation and contraction, so all the listed mechanisms are accepted at this level. The likely distractor is option a or b if a student treats one mechanism as the only cause of fatigue.

2116. Rigor mortis i.e, stiffening of body after death results from__________?

• A. Accunulation of rigid proteins molecules in sarcoplam.
• B. Unavailability of ATP, which is necessary to break the link between actin and myosin.
• C. Decrease in body temperature after death.
• D. Death of tissue due to unavailability of O2.

Explanation: After death, ATP production stops, so myosin heads cannot detach from actin because ATP is required for their release. Students may choose decrease in body temperature, but cooling can contribute to stiffness while ATP depletion is the direct cause of rigor mortis.

2117. What happens during muscle contraction to the length of each myosin and action filament?

• A. Z- lines get closer
• B. A band becomes short
• C. 1 band elongates
• D. There is no change in sarcomere

Explanation: During contraction, actin slides over myosin without either filament changing length, bringing the two Z-lines closer together. Students may choose A band becomes short, but the A band represents the length of thick filaments and remains constant; the I band and H zone shorten.

2118. If a cross section of a sarcomere is seen, each myosin is surrounded by how many actin molecules:

• A. 9
• B. 5
• C. 6
• D. 7

Explanation: In a cross-section of the sarcomere, each thick filament lies at the centre of a hexagonal arrangement and is surrounded by six thin actin filaments. The common error is five or seven, which results from not recognizing the regular hexagonal packing pattern.

2119. Diameter of thick filament is____________?

• A. 7 nm.
• B. 8 nm
• C. 16nm
• D. 10-100 ?m.

Explanation: A thick myosin filament has a diameter of about 15 to 16 nm, so 16 nm is the appropriate value. Students may choose 7 nm because thin actin filaments are about 7 nm in diameter, while 10 to 100 micrometres is far too large for a filament.

2120. A smallest contractile unit of muscle contraction called sarcomere is the area between two:

• A. H- zone
• B. M- Line
• C. Z- Line
• D. A band

Explanation: A sarcomere extends from one Z-line to the next Z-line, and these boundaries move closer during contraction. Students may choose the M-line because it lies at the centre of the sarcomere, but it is a central landmark rather than a boundary.