MCQs

The widely accepted mechanism is the cohesion-tension theory. Transpiration from leaves generates negative pressure (tension), pulling the water column up. Cohesion between water molecules transmits this pull down the entire continuous column from roots to leaves.

A buffer resists change. Thermally, water resists temperature change due to its high specific heat. It absorbs or releases a large amount of heat with a minimal change in its own temperature, stabilizing the internal thermal environment of organisms and ecosystems.

In the autoionization reaction (2H₂O ⇌ H₃O⁺ + OH⁻), one water molecule acts as an acid (proton donor) to form OH⁻, and the other acts as a base (proton acceptor) to form H₃O⁺. This demonstrates its dual acid-base capability.

Water moves from higher water potential to lower water potential. If the external solution has a lower (more negative) water potential (hypertonic), water leaves the cell by exosmosis. The protoplast shrinks and detaches from the rigid cell wall, a process called plasmolysis.

The latent heat of fusion is the heat energy released when water freezes. Because this value is high for water, the freezing process releases heat, slowing the rate of ice crystal formation. This protects cell contents from lethal intracellular freezing in organisms exposed to sub-zero temperatures.

When ions or polar molecules dissolve, water molecules orient themselves according to the charge. The δ⁺ hydrogens face an anion, and the δ⁻ oxygen faces a cation. This layer of tightly bound water is the hydration shell, which isolates and stabilizes the solute in solution.

Specific heat is a measure of thermal inertia. Water's specific heat is defined as 1 calorie per gram per degree Celsius. The high value is due to the energy needed to first disrupt hydrogen bonds before molecular kinetic energy (and thus temperature) can increase.

Water dissociates slightly into H⁺ (actually H₃O⁺) and OH⁻. The concentration of these ions, expressed as pH, is critical. Enzyme activity, protein structure, and nucleic acid stability are all highly sensitive to the hydrogen ion concentration established by water's ionization.

The cohesion-tension theory explains that water molecules are strongly linked by hydrogen bonds (cohesion). When transpiration pulls water from the top of the xylem, the entire continuous column of water is pulled up as a single unit, resisting breakage due to this high tensile strength.

Capillary action is the rise of water against gravity. It results from adhesion (attraction of water to the polar glass walls), which pulls water up the sides, and surface tension (from cohesion), which pulls the entire water column upward to minimize the surface area.

Water has a high dielectric constant (~80 at 20°C). This means it significantly weakens the electrostatic force of attraction between the oppositely charged Na⁺ and Cl⁻ ions in the crystal lattice, allowing them to dissociate and become surrounded by hydration shells.

Anabolic polymer synthesis (e.g., peptide bond, glycosidic bond, phosphodiester bond) involves the removal of a water molecule (dehydration synthesis). The hydroxyl group is removed from one monomer and a hydrogen from another, forming water and a new covalent bond.

Water potential quantifies the tendency of water to move from one area to another. It is the difference between the chemical potential of water in a system and that of pure water at the same temperature and atmospheric pressure. It comprises solute potential (ψs) and pressure potential (ψp).

In liquid water, hydrogen bonds constantly break and reform, allowing close packing. Upon freezing, water molecules lock into a stable, tetrahedral arrangement where each molecule is H-bonded to four others, creating large spaces. This expanded lattice results in lower density.

The reaction CO₂ + H₂O ⇌ H₂CO₃ is catalyzed by carbonic anhydrase. Here, water is not just a solvent but a substrate that chemically participates in the reaction by combining directly with carbon dioxide.

Large bodies of water absorb solar energy during the day and release it slowly at night without large temperature swings. This property, due to the extensive hydrogen bonding network, protects aquatic organisms from thermal shock and provides a stable environment.

In ice, each water molecule forms a maximum of four hydrogen bonds in a rigid, crystalline hexagonal lattice. This structure holds molecules farther apart on average than in the liquid state, reducing the density. This lattice is an open, ordered arrangement, maximizing H-bonding.

Osmosis is the net passive diffusion of water across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). It does not require energy and is central to water relations in cells.

In the Krebs cycle, the enzyme fumarase catalyzes the addition of a water molecule across the double bond of fumarate. This hydration reaction converts fumarate to malate. Water is a direct reactant in this specific step, not just a solvent.