MCQs

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.

A competitive inhibitor competes for the active site, effectively making it harder for the enzyme to bind its substrate. More substrate is required to reach half the maximum velocity. Therefore, the apparent Km (substrate concentration at 1/2 Vmax) is increased in the presence of a competitive inhibitor.

Amylose is a linear polymer of glucose with α-1,4 linkages. Amylopectin is a much larger, branched polymer that has both α-1,4 linkages in the straight chain and α-1,6 glycosidic bonds at the branch points approximately every 24-30 glucose units.

Kinases are a class of transferase enzymes that catalyze the transfer of a γ-phosphate group from a high-energy donor molecule like ATP to a specific substrate. Protein kinases phosphorylate specific serine, threonine, or tyrosine residues on target enzymes, regulating their activity. Phosphatases reverse this.

The amino-terminal ends of both the light (VL) and heavy (VH) chains form the antigen-binding site. These variable domains have highly diverse amino acid sequences from one antibody clone to another, creating a unique 3D surface that is specific for a single epitope.

The definition of competitive inhibition is a "competition" for the active site. At a high enough concentration, the substrate out-competes the inhibitor for the active site, so all enzyme molecules can still bind substrate and reach Vmax. The apparent Km is increased, but Vmax is ultimately unchanged.

Allosteric regulation is mediated by modulator molecules that bind to a site (allosteric site) physically distinct from the active site. This binding causes a conformational change that can either increase (allosteric activator) or decrease (allosteric inhibitor) the activity of the enzyme at its active site.

In covalent catalysis, a powerful nucleophilic R-group in the active site (e.g., the -SH of cysteine or -OH of serine) forms a transient covalent bond with the substrate. This acyl-enzyme intermediate is then resolved by another step, releasing the product and regenerating the free enzyme.