Practice Questions

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.