MCQs

The formation of ice crystals and the 9% volume expansion upon freezing can physically rupture cell membranes and delicate tissue structures. This is the basis of frostbite and why cryoprotective agents are needed to preserve cells.

Surrounding exposed non-polar groups, water forms highly ordered, low-entropy cages. When these groups aggregate in the protein's core, this caged water is released into the bulk solution, significantly increasing its entropy. This increase in the entropy of water is a major driving force for protein folding.

As surface water cools to 4°C, it sinks, displacing warmer water until the entire lake is near 4°C. Further surface cooling creates less dense, near-freezing water that stays on top and forms ice. This insulates the dense, liquid 4°C water layer at the bottom, allowing aquatic life to survive.

All metabolic reactions occur in an aqueous medium. Freezing locks water into a solid crystalline state, drastically reducing the water available as a solvent and reactant. This halts enzyme activity and prevents microbial growth, preserving the food.

Because of its high specific heat, water can store a large amount of thermal energy for a given mass and temperature change. As it cools to body temperature, it slowly releases this stored heat, providing sustained thermal therapy.

The high surface tension of the water-based fluid lining the alveoli would cause their collapse. Type II alveolar cells secrete pulmonary surfactant, a mixture of phospholipids and proteins that intersperses between water molecules, disrupting cohesion and dramatically lowering surface tension.

A hypertonic solution has a higher solute concentration (lower water concentration/water potential) than the cell's interior. Water moves out of the cell by osmosis towards the lower water potential, causing the cell to shrink and its surface to become scalloped (crenation).

The phospholipid head groups are charged and highly polar. Water interacts favorably with these groups, forming hydrogen bonds with the oxygen atoms of the phosphate and electrostatic interactions with the charged nitrogenous base (e.g., choline). This hydration stabilizes the bilayer surface.

Water adheres to the polar -OH groups of cellulose/glass. This adhesion pulls water up the sides, creating a concave meniscus and generating the upward force for capillary action. If cohesion were dominant (like mercury), a convex meniscus and depression would result.

Water molecules experience a net inward pull due to unbalanced cohesive forces at the surface. This surface tension forces the droplet to assume the shape with the smallest possible surface area-to-volume ratio, which is a perfect sphere in the absence of gravity.

Antidiuretic hormone (ADH) increases the water permeability of the collecting duct by inserting aquaporins. Water then moves passively by osmosis down the osmotic gradient created by the counter-current multiplier system in the hypertonic medullary interstitium.

The dissolution process involves energy input to break the crystal lattice (lattice energy) and energy release from forming hydration shells (hydration energy). If the lattice energy is greater than the hydration energy, the net process is endothermic, absorbing heat from the surroundings and making the solution cold.

Viscosity is the internal resistance to flow. Water has a relatively low viscosity compared to other liquids like oils. This property allows blood (a water-based fluid) to be pumped efficiently through the cardiovascular system with minimal energy loss due to friction.

When a plant cell is in a hypotonic environment, water enters by osmosis, causing the protoplast to swell and press against the rigid cell wall. This hydrostatic pressure, called turgor pressure, provides structural support to non-woody plants.

In hydrolytic enzymes, a water molecule, often activated by a base in the active site, acts as a nucleophile. It attacks an electrophilic carbon in the peptide or glycosidic bond, leading to bond cleavage. The enzyme precisely orients this catalytic water molecule.

H₂S cannot form significant hydrogen bonds due to sulfur's lower electronegativity. Water's ability to form a 3D network of strong intermolecular H-bonds requires considerably more thermal energy to separate the molecules into a gaseous state, thus resulting in a liquid state at room temperature.

The descending limb is permeable to water. The medullary interstitium has a high solute concentration (low water potential). Water moves out of the descending limb by osmosis down this water potential gradient, concentrating the urine.

The ocean absorbs vast amounts of solar heat during the day/summer with a small temperature rise and releases it slowly at night/winter. This large thermal inertia moderates the temperature of the adjacent land, keeping coastal areas cooler in summer and warmer in winter.

Water molecules form highly ordered, cage-like structures (clathrates) around non-polar solutes to maintain hydrogen bonding. This organization represents a local decrease in entropy (ΔS < 0), which is thermodynamically unfavorable and drives the hydrophobic effect.