Biology: Semester II

Section:

Section 3

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Xylem and Transport of Water and Minerals

Xylem and Transport of Water and Minerals

Once water has entered the root, it will be taken upward through the cells of the xylem tissue. In one way, this is like water flowing into the bottom of a well from the surrounding dirt. Once the water is in the well it must be raised upward where it will be used. In a well, pumps assist in the raising of the, but in a plant there are no pumps. How water and dissolved mineral nutrients, such as iron and calcium, pass from the roots to the tips of the tallest trees involves the atomic structure of water as well as evaporation from leaves. The path that water and dissolved nutrients pass through is the xylem.

Xylem is the water transporting tissue in plants. One odd feature about xylem is its cells die when they reach maturity. Xylem is like a long series of soda straws linked end-to-end. There are two types of xylem cells that conduct water: tracheids and vessels. Vessels are better at transporting water than tracheids because they are wider and have less obstructions.


Structure of xylem: Vessels are large cells, tracheids are much smaller.

Water is pulled up the xylem by the process of transpiration, which is water loss from leaves. All water molecules are attracted to each other by hydrogen bonds. Water lost from the leaves causes diffusion of new water molecules out of the xylem and into the leaf, creating a “tug” on other water molecules within the xylem cells (since they are all bonded together). This "tug" causes water molecules to rise up from the roots, eventually reaching the leaves. The loss of water from the root xylem allows additional water to pass through the endodermis into the root xylem cells of the root. Think of people riding a roller coaster. As the ride ends, people (water molecules) get out of the cars and new people from the line (xylem) get into the cars. Because many of the people who were waiting in the line are now on the roller coaster, the line is shorter, so more people can get in at the end (roots) of the line.

Adhesion

The other feature of the hypothesis is adhesion, the tendency of molecules of different kinds to stick together. When water falls on an un-waxed automobile hood, the water adheres to the paint and does not form the drops we see on a freshly waxed surface. Adhesion of water to the cell walls in xylem keeps gravity from pulling the water back to the roots when a plant closes its stomata.

Cohesion-Adhesion Theory

Transpiration (loss of water) at the leaves exerts a pull on the water column within the xylem of the leaf. The lost water molecules are replaced by water from the xylem of the leaf veins, causing a “tug” on water in the xylem. Adhesion of water to the cell walls of the xylem facilitates movement of water upward within the xylem. Water is pulled from the stems to replace the water in the leaves. This combination of cohesive and adhesive forces is referred to as the Cohesion-Adhesion Theory.


The cohesion-adhesion theory explains the movement of water through the xylem cells from root to leaf.

What is the difference between cohesion and adhesion? solution

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