Investigation 11 T189BigIdeaInteractions4INVESTIGATION 11TRANSPIRATION*What factors, including environmental variables, affect the rate of transpiration in plants?■BACKGROUNDCells and organisms must exchange matter with the environment to grow, reproduce, and maintain organization, and the availability of resources influences responses and activities. For example, water and macronutrients are used to synthesize new molecules, and, in plants, water is essential for photosynthesis. Organisms have evolved various mechanisms for accumulating sufficient quantities of water, ions, and other nutrients and for keeping them properly balanced to maintain homeostasis. In general, animals possess one or more mechanisms, such as those involved in excretion, that let them ingest solutions of nutrients and transport and/or eliminate any excess ions or water. However, plants take a different approach; they absorb and transport water, nutrients, and ions from the surrounding soil via osmosis, diffusion, and active transport. Once water and dissolved nutrients have entered the root xylem, they are transported upward to the stems and leaves as part of the process of transpiration (the evaporation of water from the plant surface). The amount of water needed daily by plants for the growth and maintenance of tissues is small in comparison to the amount that is lost through transpiration. Too much water loss can be detrimental to plants; they can wilt and die.The transport of water upward from roots to shoots in the xylem is governed by differences in water (or osmotic) potential, and these differences account for water movement from cell to cell or over long distances in the plant. Several factors, including environmental pressure and solute concentration, contribute to water potential, with water always moving from an area of high water potential (higher free energy, more water) to lower potential (less free energy, less water). The process is facilitated by osmosis, root pressure, and the physical and chemical properties of water. Transpiration creates a lower osmotic potential in the leaf, and the TACT (transpiration, adhesion, cohesion, and tension) mechanism describes the forces that move water and dissolved nutrients up the xylem, as modeled in Figure 1.* Transitioned from the AP Biology Lab Manual(2001)
T190 Investigation 11
Investigation 11 T191BIG IDEA 4: INTERACTIONSPREPARATIONMaterials and Equipment•Representative plant species that are available in a particular region or season, such as Impatiens(a moisture-loving plant), Coleus, oleander (more drought tolerant), Phaseolus vulgaris(bean seedlings), pea plants, varieties of Lycopersicon(tomato), peppers, ferns, or even Wisconsin Fast Plants (If students plan to investigate transpiration in several different species of plants, you will have to purchase a variety of plants, or students can use cuttings from plants found on campus. Note that the plants can be used to study other biological concepts, such as plant evolution, natural
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