MCQs

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.

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.

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 insect's mass is distributed over its long hydrophobic legs so that the force per unit area is less than the surface tension. The cohesive hydrogen bonds at the water-air interface create a strong film that resists being broken by the light insect.

Water is relatively transparent to wavelengths of visible light, the spectrum used for photosynthesis. This allows aquatic plants and phytoplankton to carry out photosynthesis in the upper layers (photic zone), forming the base of the aquatic food web.

Adhesion is the attraction of water to the xylem walls (cellulose is polar with many -OH groups). This adhesion helps to counteract gravity and, combined with cohesion, allows for a continuous capillary column. The transpiration-cohesion-tension mechanism relies on both adhesion and cohesion.

Most liquids contract upon cooling, becoming densest at their freezing point. Water behaves anomalously; it reaches its maximum density at 4°C. Below 4°C, it expands. This is due to the formation of transient, expanded ice-like clusters of hydrogen bonds as it approaches the freezing point.

The reaction center P680, upon excitation by light, donates an electron to the primary electron acceptor and becomes a strong oxidant (P680⁺). It extracts electrons from water molecules via the oxygen-evolving complex, splitting water and returning P680 to its ground state.

Water drives membrane formation. The hydrophobic fatty acid tails are excluded from water to minimize the ordering of water molecules. This entropic force causes the tails to aggregate, while the polar heads interact favorably with water, self-assembling into a bilayer.

The dielectric constant is a measure of a solvent's ability to insulate opposite charges from each other. Water's high value (~80) means it weakens the electrostatic attraction between dissolved ions, enabling their dissociation and hydration. This is central to its role as a solvent for salts.

Wax is a non-polar, hydrophobic surface. The polar water molecules are more strongly attracted to each other via hydrogen bonding (cohesion) than they are to the wax surface (adhesion). This causes the water to minimize its contact with the wax and form spherical beads.

Imbibition is the physical adsorption of water onto the hydrophilic surfaces of macromolecules (proteins, polysaccharides) and cell walls inside the seed. This matric potential is an extremely negative component of water potential, creating a massive driving force for water uptake.

Evaporation is a physical change, not a chemical one. The covalent bonds within the molecule are not broken. The energy (latent heat of vaporization) is used to overcome the attractive intermolecular hydrogen bonds holding the water molecules together in the liquid phase.