MCQs

The organization of water molecules in a specific, ordered pattern around macromolecules (like the spine of hydration in DNA) is a form of structural water. These water molecules are not just a passive background solvent but are integral to the maintenance and function of the 3D structure.

Root hair cells actively accumulate ions and sugars, making their solute potential very negative and thus their total water potential lower (more negative) than the surrounding soil water. Water moves passively down this water potential gradient by osmosis.

Intracellular enzymes have a narrow, optimal temperature range. The high water content of cytoplasm buffers the cell against sudden, localized heat release from exothermic reactions, stabilizing the temperature and protecting enzymes from thermal denaturation.

Cohesion is the attraction between like molecules (water to water). Adhesion is the attraction between unlike molecules (water to glucose). The polar -OH groups on glucose form hydrogen bonds with water molecules, which is the molecular basis for its solubility.

Transpiration is the evaporation of water from mesophyll cell walls. This phase change from liquid to gas is endothermic, absorbing energy (latent heat of vaporization). This energy is taken from the leaf tissue, effectively cooling it and preventing heat damage from solar radiation.

By convention, the water potential of pure water at ambient pressure and temperature is defined as zero. Any addition of solute lowers the solute potential (making it negative), and any positive pressure increases the pressure potential, so most biological solutions have a negative total water potential.

As extracellular water begins to freeze, the phase transition from liquid to solid releases the latent heat of fusion. This local release of heat warms the immediate surroundings, slowing the rate of cooling and delaying the freezing of intracellular water, which is lethal.

The solvent evaporates quickly. This phase change from liquid to vapor requires the absorption of its latent heat of vaporization. This heat is drawn from the skin, causing a rapid and intense cooling sensation, even more pronounced than with water due to its higher volatility.

In ice, a rigid tetrahedral lattice yields exactly 4 H-bonds per molecule. In liquid water, thermal energy causes the bonds to constantly flicker, break, and re-form. This results in a dynamic, fluctuating network where the average number is closer to 3.4 rather than the maximum 4.

In condensation (dehydration) synthesis, the new covalent bond (e.g., C-N peptide bond) is formed by removing a hydroxyl group from one monomer and a hydrogen from another. The byproduct of this bond formation is a single water molecule.

The curvature of water menisci in the cell walls of the leaf mesophyll generates the tension that pulls the water column. As cells lose water during wilting, these menisci recede, reducing the curvature and thus the tension. This feedback reduces the pulling force, limiting further water loss.

The dissolution process disrupts the local, ordered structure of both the solute and the water, distributing the solute molecules randomly throughout the solvent. This increase in randomness represents an increase in the overall entropy (ΔS > 0) of the system, which is a driving force for dissolution.

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.