Practice Questions

The Grotthuss mechanism allows a proton to move extremely rapidly through a network of water molecules. A proton attaches to one end of an H-bonded chain, and a different proton is simultaneously released at the other end, without a single proton traversing the entire distance.

Water plays a dual, active role: it forms hydrogen bonds with the phosphate head groups (hydration), stabilizing them, and it exerts the hydrophobic effect, forcing the fatty acid tails to aggregate to minimize their exposure to the aqueous phase.

Blood (which is ~92% water) absorbs excess heat from metabolically active tissues (like muscle and liver) with a minimal rise in its temperature. It then circulates, distributing this heat to cooler peripheral tissues, effectively acting as a conveyor belt for thermal energy.

The organization of water molecules in a specific, ordered pattern around macromolecules (like the spine of hydration in DNA) is a form of structural water. These water molecules are not just a passive background solvent but are integral to the maintenance and function of the 3D structure.

Root hair cells actively accumulate ions and sugars, making their solute potential very negative and thus their total water potential lower (more negative) than the surrounding soil water. Water moves passively down this water potential gradient by osmosis.

Intracellular enzymes have a narrow, optimal temperature range. The high water content of cytoplasm buffers the cell against sudden, localized heat release from exothermic reactions, stabilizing the temperature and protecting enzymes from thermal denaturation.

Cohesion is the attraction between like molecules (water to water). Adhesion is the attraction between unlike molecules (water to glucose). The polar -OH groups on glucose form hydrogen bonds with water molecules, which is the molecular basis for its solubility.

Transpiration is the evaporation of water from mesophyll cell walls. This phase change from liquid to gas is endothermic, absorbing energy (latent heat of vaporization). This energy is taken from the leaf tissue, effectively cooling it and preventing heat damage from solar radiation.

By convention, the water potential of pure water at ambient pressure and temperature is defined as zero. Any addition of solute lowers the solute potential (making it negative), and any positive pressure increases the pressure potential, so most biological solutions have a negative total water potential.

As extracellular water begins to freeze, the phase transition from liquid to solid releases the latent heat of fusion. This local release of heat warms the immediate surroundings, slowing the rate of cooling and delaying the freezing of intracellular water, which is lethal.