Transport Systems in Plants
27.1 Complex Multicellular Organisms Require Complex Transport Systems
1. The first life-forms were simple enough to directly exchange materials with the environment.
2. More complex multicellular organisms developed folds and transport systems, which maximized surface area and contact with the environment.
3. In plants, xylem transports water and dissolved minerals, and phloem distributes photosynthate. Both tissue types are bundled together in roots to form the stele.
27.2 Plants Require Water and Nutrition
4. In all plants, the macronutrients are carbon, hydrogen, oxygen, nitrogen, potassium, calcium, magnesium, phosphorus, and sulfur. The micronutrients are chlorine, iron, boron, manganese, zinc, copper, and molybdenum. Certain plants assimilate other elements.
5. Plants get carbon and oxygen from CO2 in the atmosphere, and hydrogen from water. The other elements come from soil.
6. Soil consists of rock and mineral particles mixed with decaying organic molecules and organisms. Soil layers are called horizons. Topsoil contains humus and is a major source of water and nutrients for plants.
7. Root branches, root hairs, mycorrhizae, and root cortical cells provide abundant surface area to absorb water and dissolved minerals. Some plants secrete proteins to enhance mineral uptake.
8. Several types of bacteria fix nitrogen into forms that plants can use.
27.3 Water and Dissolved Minerals Are Pulled Up to Leaves
9. Water and dissolved minerals (xylem sap) are pulled up through xylem to replace water lost through transpiration in leaves. This is called the cohesion-tension theory.
10. Xylem consists of microscopic tubes made from dead tracheids and vessel elements.
11. Water’s properties of cohesion and adhesion make xylem transport possible.
12. A break in the flow of water through xylem is called cavitation.
13. Water enters roots by osmosis because the solute concentration in the soil is less than that of cells of the root.
14. The endodermis with its impermeable Casparian strip controls which minerals enter the nearby xylem. Water and dissolved minerals move to xylem by the apoplastic pathway (extracellularly) and by the symplastic pathway (through cells).
27.4 Photosynthate Is Pushed to Heterotrophic Cells
15. Photosynthate may be used or stored in its cell of origin, or moved in phloem to heterotrophic cells in roots, flowers, or fruits.
16. Phloem sap includes photosynthate and water and minerals from xylem.
17. Phloem sap flows through sieve tubes from a source to a sink, with pressure generated by continual influx of water from xylem. This mechanism is called the pressure flow theory.