2.18:
Adhesion
Adhesion occurs when two different types of molecules are attracted to each other.
For instance, adhesion can be observed when a capillary tube is placed in a beaker filled with water.
The water molecules near the sides of the capillary tube adhere to the wall via hydrogen bonds and resist the downward pull of gravity. This upward pull on the liquid at the edges leads to the formation of a concave meniscus.
Since the adhesive force of water to the capillary wall is stronger than the cohesive forces between the water molecules, the water level inside the tube rises via capillary action.
Similarly, plants contain xylem vessels made of polar cellulose molecules.
Due to stronger adhesion forces, the capillary action allows the upward movement of water against the force of gravity.
The diameter of the xylem decreases towards the tops of larger trees, allowing water to continue to ascend from the roots to the leaves and where unused water can exit through transpiration.
2.18:
Adhesion
Adhesion occurs when one type of molecule is attracted to a different molecule. Water exhibits adhesive properties in the presence of polar surfaces, such as glass or cellulose in plants. For instance, when water is poured into a glass, the positively charged hydrogen molecules of water are more attracted to the negatively charged oxygen molecules in the silica than to the oxygen in neighboring water molecules.
Capillary action is a result of water’s adhesive tendencies. When a narrow glass tube is inserted into a beaker filled with water, the water molecules attach to the surface of the tube, and the water level inside the tube rises. The smaller the tube diameter, the farther the water rises as more water molecules are exposed to the glass surface. Capillary action continues as long as the adhesive force is greater than the cohesive forces between the water molecules.
In plants, adhesion and cohesion allow water molecules to move up from the roots to the leaves via transpiration. Plants contain xylem vessels that consist of long, narrow cells called tracheary elements, which transport water. As water molecules exhibit attraction to cellulose, they cling to the xylem cell wall and resist the downward pull of gravity. Cohesive forces between water molecules allow attraction between the water molecules within the tracheal cells. Together, these dynamic forces of adhesion and cohesion create a column of water molecules. The adhesion of water to cellulose is greater than the cohesive forces, enabling the water to move upward from the roots to the leaves.
Suggested Reading
Kim, Hae Koo, Joonghyuk Park, and Ildoo Hwang. “Investigating Water Transport through the Xylem Network in Vascular Plants.” Journal of Experimental Botany 65, no. 7 (April 1, 2014): 1895–1904. [Source]