MCQs

Water is the solvent in which buffers operate, and its ionization constant (Kw) is fundamental to the pH scale. A buffer's equilibrium (e.g., H₂CO₃ ⇌ H⁺ + HCO₃⁻) involves the release or absorption of H⁺ ions produced by water's interaction with the buffer components, resisting pH change.

Wilting occurs when transpiration exceeds water uptake. Cells lose water, decreasing their water potential and turgor pressure. The protoplast no longer presses firmly against the cell wall, and the non-woody tissues lose their rigidity, causing the plant to droop.

The hydration of CO₂ (CO₂ + H₂O → H₂CO₃) produces carbonic acid, which dissociates into H⁺ and HCO₃⁻. The release of H⁺ lowers the pH. This is a major source of acidity, not just the dissolution of pre-formed lactic acid.

Water, as a major component of synovial fluid and serous fluid, forms a cohesive, slippery film. The high cohesion of water molecules allows the fluid to resist being squeezed out from between surfaces under pressure, providing an excellent lubricating layer.

An increase in plasma solute (NaCl) concentration decreases its water potential. Water moves by osmosis from the area of higher water potential (inside cells) to the area of lower water potential (blood plasma), leading to cellular dehydration and increased blood volume/pressure.

Water molecules form highly ordered "cage-like" structures around exposed non-polar groups, which decreases entropy. To minimize this, water forces non-polar groups to aggregate, freeing the caged water and increasing overall entropy. This entropic force is the hydrophobic effect.

While water's polarity and geometry are contributing factors, the direct reason for its abnormally high boiling point is the strength and number of intermolecular hydrogen bonds. A large amount of thermal energy is required to disrupt these attractive forces to allow molecules to escape as vapor.

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.