Free Biological Importance of Water MCQs with Answers

1. Which property of water allows it to act as a temperature buffer for living cells?

• A. Its low surface tension
• B. Its high specific heat capacity
• C. Its low density in the liquid state
• D. Its inability to form hydrogen bonds

Explanation: Water has an unusually high specific heat capacity because a large amount of energy is absorbed breaking hydrogen bonds before the temperature rises. A cell can therefore take in or lose considerable heat with only a small change in internal temperature, which protects enzymes from denaturation. Surface tension in water is high, not low, and water is denser as a liquid than as ice, so the other options invert real properties.

2. Water is described as a universal solvent primarily because its molecules are

• A. non-polar
• B. polar
• C. ionised at all temperatures
• D. held together by covalent bonds between molecules

Explanation: The oxygen atom pulls the shared electrons away from the two hydrogens, leaving a partial negative charge on the oxygen and partial positive charges on the hydrogens. This polarity lets water surround and separate ions and other polar solutes, which is why almost every reaction in a cell happens in aqueous solution. The bonds between water molecules are hydrogen bonds, not covalent bonds.

3. Water makes up approximately what proportion of the fresh mass of a typical living cell?

• A. about 10 per cent
• B. about 30 per cent
• C. about 50 per cent
• D. 70 to 90 per cent

Explanation: Most cells are between seventy and ninety per cent water, which is why it is treated as the medium of life rather than as one nutrient among others. Every metabolic reaction occurs in aqueous solution, water is itself a reactant in hydrolysis and a product of condensation, and it transports dissolved substances. Seeds and spores are the low water exceptions, and their metabolism is correspondingly almost stopped.

4. Sweating cools the body efficiently because water has a high

• A. specific heat capacity
• B. latent heat of vaporisation
• C. surface tension
• D. density

Explanation: A large amount of energy is needed to break the hydrogen bonds holding liquid water together and convert it to vapour, and that energy is drawn from the skin, so a small volume of sweat removes a great deal of heat. High specific heat capacity is a different property, the one that lets a cell resist temperature change in the first place. Confusing the two is the usual error in this question.

5. Ice floats on water, and this is biologically important because

• A. the ice layer insulates the water below, letting aquatic organisms survive the winter
• B. it allows water to evaporate faster in cold weather
• C. it increases the amount of oxygen dissolved in the lake
• D. it lowers the freezing point of the remaining water

Explanation: Water is unusual in becoming less dense as it freezes, because the hydrogen bonds lock the molecules into an open lattice, so ice forms at the surface instead of sinking. The layer then insulates the water beneath, which stays liquid and allows fish and other organisms to survive. If ice sank, a pond would freeze solid from the bottom upwards and life in it would be destroyed.

6. The rise of water in the xylem as an unbroken column depends mainly on

• A. the low viscosity of water
• B. the high density of water
• C. cohesion between water molecules caused by hydrogen bonding
• D. the fact that water is a poor solvent for gases

Explanation: Hydrogen bonds make water molecules stick to one another, so a column pulled by evaporation at the leaves does not break, and adhesion to the walls of the narrow xylem vessels supports the column as well. The same cohesion gives water its high surface tension, which lets small insects rest on a pond. Viscosity and density play no part in maintaining the transpiration stream.

7. Which property of water helps in moderating Earth's climate and maintaining stable temperature in aquatic environment?

• A. Low viscosity
• B. High specific heat capacity
• C. High surface tension
• D. High polarity

Explanation: A great deal of energy goes into breaking hydrogen bonds before the temperature of water rises, so oceans and lakes absorb and release heat with only small temperature changes and coastal climates stay mild. Polarity is the underlying cause of the hydrogen bonding, but the property that directly buffers temperature is the specific heat capacity. Surface tension matters for insects walking on water rather than for climate.