Biology Semester II |
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| Sections: |
Introduction | Section 1 | Section 2 | Section 3 |
| Section Three: |
Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 |
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Biology: Plant Hormones, Nutrition, and Transport: Part Eight 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 raise water. In a plant, however, there are no pumps. How water and dissolved mineral nutrients, like iron and calcium, pass from the roots to the tips of the tallest trees depends on both the molecular structure of water and water’s evaporation from leaves. Xylem is the water transporting tissue in plants. One odd feature about xylem is that its cells die when they reach functional maturity. With no cytoplasm, xylem tissue 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. Tracheids are long, tapered cells of xylem. The end plates of these contain many crossbars. Tracheid walls are covered with pits. Vessels, an improved form of tracheid, have no (or very few) obstructions at the ends of the cells. The diameter of vessels is greater than that of tracheids.
Water is pulled up the xylem by the force of transpiration, water loss from leaves. Mature corn plants can each transpire four gallons of water per week. Transpiration rates in arid-region plants can be even higher. Water lost from the leaves causes diffusion of additional water molecules out of the xylem cells in the leaf vein xylem. This creates a tug on water molecules along the water columns within the xylem cells. 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. The Cohesion-Adhesion Theory of Water Movement in Vascular Plants The problem of how water can move from underground roots to the tips of even the tallest trees has plagued plant physiologists for more than one hundred years. In the case of redwoods, the trees are more than 300 feet high. Various hypotheses have been proposed and rejected. Finally, the cohesion-adhesion hypothesis met all the scientific tests. Cohesion is the ability of molecules of the same kind to stick together. Water molecules are polar, meaning they have slightly positive and negative sides. This causes them to form attractions with one another, called hydrogen bonds. These cohesive forces cause water to tend to “stick” to itself. When raindrops fall on a freshly waxed surface (such as a car hood), the water molecules form drops of water instead of spreading out into a thin layer over the surface. Inside the xylem, water molecules form in a long chain extending from the roots to the leaves. When water molecules pass from the leaves through open stomata, water molecules in the xylem are pulled up through the xylem. This same phenomenon happens when you suck water from the end of a soda straw. While you are removing water from the top of the straw, other water molecules are drawn into and up through the straw to replace those lost at the top. However, when you stop sucking and remove your mouth from the straw, the water in the straw sinks back into the glass. Plants do not keep their stomata open all the time, especially in deserts or other arid regions. There must be some other way to keep water in the xylem when the stomata are closed. The other part 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 cellulose in the xylem cell walls keeps the water from sinking back to the roots when a plant closes its stomata. Adhesive forces counteract the force of gravity and aid the rise of water within the xylem. Cohesion-Adhesion Theory Transpiration 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. This creates 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. This combination of cohesive and adhesive forces is referred to as the cohesion-adhesion theory.
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