33.6. Water Movement

Vascular plants have a conducting system, as humans do, for transporting fluids and nutrients from one part to another. Functionally, a plant is essentially a bundle of tubes with its base embedded in the ground. At the base of the tubes are roots, and at their tops are leaves. For a plant to function, two kinds of transport processes must occur: First, the carbohydrate molecules produced in the leaves by photosynthesis must be carried to all of the other living plant cells. To accomplish this, liquid, with these carbohydrate molecules dissolved in it, must move both up and down the tubes. Second, minerals and water in the soil must be taken up by the roots and ferried to the leaves and other plant cells. In this process, liquid moves up the tubes. Plants accomplish these two processes by using chains of specialized cells. Cells of the phloem transport photosynthetically produced carbohydrates up and down the plant (red arrows in figure 33.11), and those of the xylem carry water and minerals upward (blue arrows in figure 33.11).

Figure 33.11. The flow of materials into, out of, and within a plant.

Water and minerals enter through the roots of a plant and are transported through the xylem to all parts of the plant body (blue arrows). Water leaves the plant through the stomata in the leaves. Carbohydrates synthesized in the leaves are circulated throughout the plant by the phloem (red arrows).

Cohesion-Adhesion-Tension Theory

Many of the leaves of a large tree may be more than 10 stories off the ground. How does a tree manage to raise water so high? Several factors are at work to move water up the height of a plant. The initial movement of water into the roots of a plant involves osmosis. Water moves into the cells of the root because the fluid in the xylem contains more solutes than the surroundings—recall from chapter 4 that water will move across a membrane from an area of lower solute concentration to an area of higher solute concentration. However, this force, called root pressure, is not by itself strong enough to “push” water up a plant’s stem.

Capillary action adds a “pull.” Capillary action results from the tiny electrical attractions of polar water molecules to surfaces that carry an electrical charge, a process called adhesion. In the laboratory, a column of water rises up a tube of glass because the attraction of the water molecules to the charged molecules on the interior surface of the glass tube “pulls” the water up in the tube. In figure 33.12, which illustrates this process, why does the water travel higher up in the narrower tube? The water molecules are attracted to the glass molecules, and the water travels up farther in the narrower tube because the amount of surface area available for adhesion is greater than in the larger-diameter tube.

Figure 33.12. Capillary action.

Capillary action causes the water within a narrow tube to rise above the surrounding water; the attraction of the water molecules to the glass surface, which draws water upwards, is stronger than the force of gravity, which tends to draw it down. The narrower the tube, the greater the surface area available for adhesion for a given volume of water, and the higher the water rises in the tube.

However, although capillary action can produce enough force to raise water a meter or two, it cannot account for the movement of water to the tops of tall trees. A second very strong “pull” accomplishes this, provided by transpiration, to be discussed later. Opening up the tube and blowing air across its upper end demonstrates how transpiration draws water up a plant stem. The stream of relatively dry air causes water molecules at the water column’s exposed top surface to evaporate from the tube. The water level in the tube does not fall, because as water molecules are drawn from the top, they are replenished by new water molecules pulled up from the bottom. This, in essence, is what happens in plants. The passage of air across leaf surfaces results in the loss of water by evaporation, creating a “pull” at the open upper end of the plant. New water molecules that enter the roots are pulled up the plant. Adhesion of water molecules to the walls of the narrow vessels in plants also helps to maintain water flow to the tops of plants.

A column of water in a tall tree does not collapse simply due to its weight because water molecules have an inherent strength that arises from their tendency to form hydrogen bonds with one another. These hydrogen bonds cause cohesion of the water molecules (see chapter 2); in other words, a column of water resists separation. The beading of water droplets illustrates the property of cohesion. This resistance, called tensile strength, varies inversely with the diameter of the column; that is, the smaller the diameter of the column, the greater the tensile strength. Therefore, plants must have very narrow transporting vessels to take advantage of tensile strength.

How the combination of gravity, adhesion, and tensile strength due to cohesion affects water movement in plants is called the cohesion-adhesion-tension theory. It is important to note that the movement of water up through a plant is a passive process and requires no expenditure of energy on the part of the plant.

Transpiration

The process by which water leaves a plant is called transpiration. More than 90% of the water taken in by plant roots is ultimately lost to the atmosphere, almost all of it from the leaves. It passes out primarily through the stomata in the evaporation of water vapor, as you can see in panel 1 of the Key Biological Process illustration below. On its journey from the plant’s interior to the outside, a molecule of water first diffuses from the xylem into the spongy mesophyll cells of the leaf. Then, water passes into the pockets of air within the leaf by evaporating from the walls of the spongy mesophyll that line the intercellular spaces. These intercellular spaces open to the outside of the leaf by way of the stomata. The water that evaporates from these surfaces of the spongy mesophyll cells is continuously replenished from the tips of the veinlets in the leaves. Molecules of water diffusing from the xylem replace evaporating water molecules. Because the strands of xylem conduct water within the plant in an unbroken stream all the way from the roots to the leaves, when a portion of the water vapor in the intercellular spaces passes out through the stomata, the supply of water vapor in these spaces is continually renewed from lower down in the column (panel 2) and ultimately from the roots (panel 3). Because the process of transpiration is dependent upon evaporation, factors that affect evaporation also affect transpiration. In addition to the movement of air across the stomata, mentioned earlier, humidity levels in the air will affect the rate of evaporation—high humidity reducing it and low humidity increasing it. Temperature will also affect the rate of evaporation—high temperatures increase it and lower temperatures reduce it. This temperature effect is especially important because evaporation also acts to cool plant tissues.

Structural features such as the stomata, the cuticle, and the intercellular spaces in leaves have evolved in response to two contradictory requirements: minimizing the loss of water to the atmosphere, on the one hand, and admitting carbon dioxide, which is essential for photosynthesis, on the other. How plants resolve this problem is discussed next.