MCQs

Dehydration alters the precise ionic concentrations of Na⁺, K⁺, and Ca²⁺ outside and inside neurons. This disrupts the membrane potential, action potential generation, and neurotransmitter release, leading to impaired cognitive function. The brain is highly sensitive to osmotic shifts.

Water molecules are integral to DNA structure. They form a "spine of hydration" in the minor groove and interact with the charged phosphate backbone, shielding negative charges and stabilizing the B-form of DNA. The hydrophobic effect also drives base stacking.

Protons (H⁺) do not diffuse as free ions. Instead, they "hop" along a chain of hydrogen-bonded water molecules (Grotthuss mechanism). A water molecule accepts a proton on one side, and a different proton is released from the other side. This facilitates extremely rapid proton transfer in biological systems.

Water moves from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). It is the water potential gradient, not the solute concentration gradient per se, that provides the driving force.

A high dielectric constant, by definition, weakens the force of attraction between charges. This stabilizes dissolved ions in solution, preventing their precipitation and making them available for reactions. It also affects the pKa of acids by stabilizing their conjugate bases.

Evaporation is a phase transition, not a chemical reaction. The molecules are the same; they are just farther apart. The energy required (latent heat) is used exclusively to overcome the attractive forces—primarily hydrogen bonds—between the water molecules.

The dissolution of non-polar molecules in water would require water to form highly organized clathrate cages around them, causing a significant decrease in the system's entropy (ΔS < 0). This makes the process thermodynamically unfavorable, resulting in the hydrophobic effect and phase separation.

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.