A1.1 Water

The question describes water moving along the interior surfaces of plant cell walls — this is movement of water over a solid polar surface, not through open vessels.

Adhesion is the attraction between water molecules and other polar or charged surfaces. Cell walls are composed of cellulose, a polar polysaccharide, so water molecules form hydrogen bonds with cellulose fibres.

Cohesion refers to water–water attraction, which keeps the water column intact but does not explain why water clings to the cell wall surface itself. The movement along the wall is driven by adhesion.

The correct answer is adhesion of water molecules to polar cellulose fibres in cell walls, because the δ+ and δ− regions of water are attracted to polar groups on cellulose, drawing water along the wall surface.

The question asks which property explains water moving along cell wall surfaces — a surface-contact phenomenon.

Cohesion explains why water molecules stick to each other, maintaining a continuous column. It does not explain why water is attracted to the cellulose surface itself, so this option is incorrect.

High latent heat relates to thermal/evaporative properties of water — how much energy is needed to evaporate it. This has no direct role in making water move along a solid surface.

Surface tension arises at the air–water interface due to cohesion, producing an elastic-like skin. It does not describe water moving along a solid polar surface inside the cell wall.

By elimination, adhesion of water molecules to polar cellulose fibres is correct. Water's polarity allows it to bond to cellulose, pulling it along the wall surface toward evaporating sites.

Water is a polar solvent. Substances that dissolve readily are polar or ionic (hydrophilic). Non-polar (hydrophobic) substances do not dissolve in water.

A dipeptide and glucose-6-phosphate both contain polar functional groups (–OH, –NH₂, –COOH, phosphate) and dissolve in water. ${\text{K}}^{+}$ is an ion and dissolves readily. A triglyceride consists of a glycerol backbone esterified to three non-polar fatty acid chains — it is largely non-polar and hydrophobic.

Because triglycerides are non-polar, water molecules cannot form a hydration shell around them. They do not dissolve in the aqueous cytoplasm and therefore cannot freely participate as dissolved substrates in cytoplasmic reactions.

The correct answer is a triglyceride (fat molecule), as it is non-polar and has essentially zero solubility in the aqueous cytoplasm.

We need the substance least soluble in water — i.e., the most hydrophobic molecule.

Amino acids carry polar and ionisable groups (–NH₂, –COOH, R-groups). Dipeptides are hydrophilic and dissolve readily in water, so this is not the answer.

Glucose-6-phosphate contains multiple –OH groups and a negatively charged phosphate group. It is highly polar and very soluble in water.

Ions are charged and form hydration shells with water molecules. ${\text{K}}^{+}$ is extremely soluble in water.

A triglyceride (fat molecule) is the only non-polar option. Its long hydrocarbon fatty acid chains cannot interact with water, making it essentially insoluble in aqueous solution.