MCQs

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.

When sweat evaporates, the phase change from liquid to gas requires a large amount of heat energy (latent heat of vaporization). This heat is absorbed from the skin's surface, effectively cooling the body. The cooling is due to heat removal, not heat release.

During the light-dependent reactions, water undergoes photolysis. Water is split (oxidized) by the oxygen-evolving complex, providing replacement electrons to P680 (Photosystem II) and releasing protons (H⁺) and molecular oxygen (O₂) as a byproduct.

Transpiration pull creates negative pressure in xylem. Due to strong cohesion (H-bonds between water molecules), the continuous water column is pulled upwards. Adhesion to xylem walls also assists, but the tensile strength of the water column is a direct result of cohesion.

At 4°C, water is densest. Below 4°C, it expands, and ice (0°C) is ~9% less dense, so it floats. This surface ice layer insulates the liquid water below, maintaining a temperature above freezing and allowing aquatic life to survive winter.

Cohesion creates a strong network of hydrogen bonds at the water-air interface, generating surface tension. This film-like layer is resistant to external force, supporting objects denser than water if they do not break the surface layer.

Enzymatic digestion of starch is a hydrolysis reaction. Water molecules are used to break the α-1,4 glycosidic bonds between glucose monomers. The H from water attaches to one glucose, and the OH attaches to the adjacent glucose.

Water's high specific heat (1 cal/g°C) means it absorbs considerable heat energy for a small temperature increase. This property, due to hydrogen bonding, provides thermal stability to organisms and large bodies of water, protecting protoplasm from drastic temperature shifts.

Hydrolysis (hydro = water, lysis = splitting) uses water as a reactant. The bond in the polymer is broken, and the components of water (H and OH) are added to the resulting monomers. Enzymes catalyze this reaction, but water is a substrate, not a catalyst.

Water dissolves polar and ionic solutes by forming hydration shells. The partial charges of water molecules interact electrostatically with ions or polar groups, and hydrogen bonding stabilizes the dissolved state. Low molecular weight and high specific heat are separate properties.

Water has a V-shaped bent geometry (104.5° bond angle). Oxygen's higher electronegativity pulls shared electrons closer, creating a partial negative charge (δ⁻) on oxygen and partial positive charges (δ⁺) on hydrogens. The bonds are polar covalent, not ionic, and the unequal sharing creates a molecular dipole.

The 5' cap and the 3' poly-A tail are not translated. Their primary roles are to increase the stability of the mRNA by protecting its ends from ribonucleases, and to facilitate the initiation of translation by interacting with translation initiation factors.

The glycosidic bond in sucrose is formed between the anomeric carbon (C1) of glucose and the anomeric carbon (C2) of fructose. Since neither carbonyl group is free to open into an aldehyde or ketone form, sucrose cannot reduce Cu²⁺ and is thus a non-reducing sugar.