All Free Biology MCQs with Answers

1891. Sebum produced from sebaceous glands in a mammal helps in___________?

• A. Protection against micro organisms
• B. Temperature regulation
• C. Excretion
• D. All of these

Explanation: Sebum forms an oily coating that reduces water loss and contains substances that inhibit the growth of microorganisms on the skin, so its key protective role is antimicrobial protection. The likely mistake is temperature regulation, because skin helps regulate temperature mainly through sweat and changes in blood flow, not sebum.

1892. Amylase becomes denatured at an alkaline pH of:__________?

• A. 6
• B. 7
• C. 8
• D. 9

Explanation: Salivary amylase is suited to the nearly neutral pH of the mouth and loses its functional shape in strongly alkaline conditions, such as around pH 9. Students may choose pH 8 because it is also alkaline, but pH 9 is the standard textbook value given for denaturation of salivary amylase.

1893. If enzyme concentration is low than substrate pH and temperature values are equal to requirement then which of the following will increase rate of reaction____________________?

• A. increase in concentration of enzyme
• B. increase in concentration of substrate
• C. increase in pH
• D. increase in temperature

Explanation: When pH and temperature are already optimal and enzyme concentration is limiting, adding more enzyme provides more active sites and increases the reaction rate. Increasing substrate cannot solve the shortage of enzyme once the available active sites are being used. The likely mistake is choosing b by assuming that more substrate always produces a faster reaction.

1894. Excessive increase in temperature of medium causes the enzyme molecule to____________?

• A. Activate
• B. Unaffected
• C. Denatured
• D. None of these

Explanation: Excessive heat disrupts the weak bonds that maintain an enzyme's three-dimensional shape, changing the active site and causing denaturation. A common mistake is choosing activate because moderate warming increases molecular motion, but temperatures beyond the optimum damage the enzyme.

1895. If more substrate to already occurring enzymatic reaction is added and there is no effect on the rate of the reaction what is the form given to this situation___________?

• A. Saturation
• B. Denaturation
• C. Composition
• D. Inhibition

Explanation: When adding more substrate no longer increases the reaction rate, the enzyme's active sites are saturated and are already occupied by substrate molecules. The likely mistake is choosing inhibition, but inhibition reduces enzyme activity through an inhibitor rather than through simple exhaustion of available active sites.

1896. If the concentration of enzyme is kept constant and amount of substrate is increased a point is reached where increase in substrates concentration does not affect the reaction rate because of____________________?

• A. Enzymes get denatured at higher substrate conc
• B. Rate of reaction is indirectly proportional to substrate concentration at this point
• C. All the active sites on enzyme molecule are occupied
• D. None of these

Explanation: With a fixed enzyme concentration, increasing substrate eventually occupies all available active sites, so the enzymes are working at their maximum rate. The likely mistake is choosing a, but high substrate concentration does not normally denature enzymes; it produces saturation instead.

1897. The rate of reaction is directly proportional to the concentration of an enzyme which statement is incorrect in this respect_________________?

• A. Increase in enzyme molecule increases the available active sites.
• B. This relation is for unlimited time period with unlimited enzyme concentration
• C. If the concentration is doubled the rate will become two fold.
• D. None of these.Chemistry

Explanation: The rate is approximately directly proportional to enzyme concentration only when sufficient substrate is available and other conditions remain suitable; it is not valid for unlimited time and unlimited enzyme concentration. A likely mistake is choosing c, but doubling enzyme can double the rate only while substrate and other limiting factors do not restrict the reaction.

1898. If more substrate to already occurring enzymatic reaction is added more enzyme activity is seen because_____________?

• A. There is probably more substrate present than there is enzyme.
• B. There is probably more enzayme available than there is substrate.
• C. There is probably more product present than there is either substrate or enzyme.
• D. The enzyme substrate complex is probably failing to form during the reaction

Explanation: If adding substrate increases the rate, the original reaction had substrate below the level needed to occupy all available active sites, so some enzyme was still available. The likely mistake is choosing a, but that would describe a situation where substrate was already in excess and adding more had little or no effect.

1899. The number of alveoli in human lungs is estimated to be around:_________?

• A. 150 millions
• B. 500 millions
• C. 800 millions
• D. 1 billion

Explanation: Human lungs contain roughly 300 to 500 million alveoli, so 500 million is the closest estimate given. Alveoli greatly increase the surface area for diffusion of oxygen and carbon dioxide. Students may choose 800 million because estimates vary, but 500 million is the standard approximate figure in many textbooks.

1900. The higher animals have special organs for exchange of gases as their skin is ___________ ?

• A. Thin
• B. Thick
• C. Soft
• D. Hard

Explanation: In higher animals, the skin is relatively thick and is not sufficient for rapid diffusion over the whole body, so specialised respiratory organs are needed. Students may choose thin because gases diffuse through thin surfaces, but that describes an efficient respiratory surface rather than the general skin of higher animals.