Going with the Flow: Water Transport
Plants transport water through their xylem which is mostly made up of dead cells whose strong secondary cell walls form tubes throughout the plant. The xylem itself isn’t actively moving water through the plants; instead, it’s like the pipes in the plumbing of your home--hollowed out tubes through which water can move. Water Transport
Water moves through plants by a combination of two factors: The adhesive and cohesive properties of water. The pull of water evaporation from the leaves called transpiration. Water Transport
The explanation for how water moves through plants is called the cohesion-tension theory. The cohesion of water
Water molecules stick together due to the hydrogen bonds that form between them. Although each individual hydrogen bond is just a weak electrical attraction, when you combine lots of hydrogen bonds together, the attraction becomes very powerful-- so powerful for example that a person can put on a pair of water skis and slide across the surface of a lake. If water molecules didn’t stick together to create a resistance called surface tension, the skier would fall right in. The cohesion of water
Cohesion describes the attraction between water molecules due to their hydrogen bonds. Adhesion describes the attraction between water and other substances. The cohesion of water
One way to visualize the importance of cohesion and adhesion in water movement through xylem is to look at how water moves through narrow tubes called capillary tubes. Capillary action occurs when a narrow tube pulls a liquid upward against the force of gravity. The cohesion of water
When narrow tubes are placed in water, water will automatically creep upward in the tube due to adhesive forces between water molecules and the molecules that make up the tube. The cohesion of water
The narrower the tube, the higher the water will move.
Gravity also acts on the water molecules, pulling them downward.
So, the height of the column of water depends on the balance of forces between the attraction to move upward in the tube and the downward pull of gravity. The cohesion of water
In a plant, capillary action occurs when water moves through the narrow tubes of xylem. Cohesive forces between the water molecules themselves act as the glue that holds the column of water together. Adhesive forces between the water molecules and the cellulose microfibrils that make up the cell walls of the xylem draw the water column up the xylem. The cohesion of water
The water column will rise until the adhesive forces are balanced by the downward pull of gravity. Xylem tubes are very narrow, so lots of contact occurs between the water and the cell walls, helping the upward movement of the water column. The pull from above: Transpiration
Capillary action explains how a column of water would rise upward and then remain hanging in the narrow tubes of xylem in a plant, but it’s not enough to explain what makes water keep flowing upward from the roots to the leaves. Water moves upward through the xylem because water that evaporates from the leaves is replaced by water from the continuous column in the xylem. The evaporation of water from the leaves is called transpiration. The pull from above: Transpiration
Water moves through a plant and out into the atmosphere because of the gradient of water potential created by transpiration. Remember: Water always moves from an area of higher water potential to an area of lower water potential. The pull from above: Transpiration
1. The atmosphere has a very negative water potential, more negative than the water potential of the air in the leaf. So, water moves from the leaf air into the atmosphere. 2. The leaf air has a more negative water potential than the leaf cells, so the water moves from the leaf cells into the leaf air. The pull from above: Transpiration
3. The leaf cells have a more negative water potential than the column of water in the xylem, so water moves from the xylem into the leaf cells. The pull from above: Transpiration
4. The column of water has a more negative water potential than the root cells, so water moves from the root cells into the xylem. The pull from above: Transpiration
5. The root cells have a more negative water potential than the soil, so water moves from the soil into the root cells. The pull from above: Transpiration
The rate of water movement through the plant depends upon the rate of transpiration. Anything that increases the rate of transpiration will increase the rate of water movement through the plant. The pull from above: Transpiration
Several environmental factors can increase the rate of transpiration. 1. When the air around a plant is dry, its water potential is lower, increasing the rate of transpiration. 2. If the wind is blowing, water vapor surrounding the leaf will be swept away, which lowers the water potential of the air around the leaf, increasing transpiration. The pull from above: Transpiration
3. Increases in temperature increase the vapor pressure of water, which increases the rate of transpiration. 4. Anything that causes stomata to open has the potential to increase transpiration. The pull from above: Transpiration
Transpiration essentially pulls the water column up through the plant. The pulling force of transpiration is the opposite of the pushing force of pressure, so botanists refer to it as negative pressure or tension. Cavitation: Fixing a break in the lines
In order for water columns to move up plants, the water molecules need to stick to each other. If anything happens to introduce an air bubble into the column of water, the column of water below the air bubble will drop, and water movement in the column will stop. The sudden formation of gas bubbles in a liquid is called cavitation. Cavitation: Fixing a break in the lines
Several factors can cause cavitation in the xylem of plants. The origin of sounds in water-stressed trees Cavitation: Fixing a break in the lines
On a hot dry day, the tension on water columns due to rapid transpiration may exceed the adhesive forces that hold the water column together. The water molecules get pulled apart and separated by a gas bubble. Dissolved gases in the water column may come out of solution, creating a gas bubble. Freezing and thawing cause changes in the arrangements of the water molecules that can lead to the introduction of gas bubbles. Cavitation: Fixing a break in the lines
Because plants rely upon cohesion of water molecules to move water through the plant, cavitation is potentially a serious problem. But, as is typical of living things, where there’s a problem, there’s a solution. 1. Woody plants form new xylem every year. One solution to xylem that doesn’t work anymore because of air bubbles is tojust ignore it and make new xylem. Cavitation: Fixing a break in the lines
2. Plants that experience high levels of cavitation may rely upon tracheids rather than vessels for water transport. Vessel elements are wide open tubes, whereas tracheids have more enclosed ends. Also, tracheids have pits that allow for lateral water transport as well as vertical water transport. So, if an air bubble occurs in a vessel element,the whole water column is blocked. But, if an air bubble occurs in a tracheid, the bubble may be contained to a single tracheid cell. Water may be able to find a path around the blocked cell by traveling through the pits of the surrounding tracheids. Cavitation: Fixing a break in the lines
3. At night, plants use root pressure to force water upward and repair broken water columns. At night, root cells release ions into the xylem, lowering its solute potential and thus its water potential. As a result, water flows into the xylem. If the incoming water pushes a broken water column up through an air gap until it touches the rest of the column, then the water column is restored. Root pressure is most effective in restoring water in shorter plants. Maximum height a water column can be forced upward by this method is about 60 feet.