BIO NMDCAT

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.

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.

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.

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 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.

In a glass cylinder, the adhesive force between polar water and the glass is stronger than the cohesive force between water molecules. Water climbs the glass wall, creating a concave meniscus. In contrast, mercury (non-polar) has stronger cohesion than adhesion to glass, forming a convex meniscus.

The high heat of vaporization (latent heat) means a large amount of thermal energy is required to convert liquid sweat to vapor. This energy is absorbed from the skin, lowering its temperature. This is a highly effective cooling mechanism.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.