Advanced Placement (AP) Biology Practice Exam

The term for water molecules attracting to other substances is adhesion. This is a crucial property that allows water to stick to other materials, which is especially significant in various biological and physical processes, such as capillary action. In capillary action, adhesion enables water to climb up thin tubes or within the small pores of materials, counteracting the force of gravity.

Adhesion occurs because water is a polar molecule, meaning it has a partial positive charge on one end and a partial negative charge on the other. This polarity allows water to form hydrogen bonds with other polar substances. For example, when water interacts with the cellulose fibers in plant xylem, it adheres to those surfaces, helping facilitate the movement of water from roots to leaves.

Recognizing the role of adhesion in biological systems is vital for understanding how plants transport water and nutrients, as well as other phenomena such as the behavior of water in soils and its interactions with various surfaces. This property differentiates adhesion from cohesion, which refers specifically to the attraction between water molecules themselves. In contrast, hydrophobicity describes a lack of affinity for water, and solubility pertains to the ability of substances to dissolve in water.