ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Thirty-Seven Review Answers 1. Clay particles are packed together so tightly that water is functionally trapped in it. In sand, water percolates more freely through the particles, which are irregularly shaped and therefore not as tightly packed together. Roots in sandy soils, therefore, have more access to water than roots in soils with high clay content. 2. Capillary water is the water held in smaller soil pores that remains in soil and is available to plants. A field capacity is the amount of water held in a given soil after gravity has removed the excess. Wilting point refers to the state when the only water left in a soil is unavailable to plants, so that they will wilt permanently unless water is added. 3. Pressure potential is pressure exerted by the atmosphere on the plant. Solute potential refers to the higher concentration of solutes within the root contributing to the increase of diffusion of water from the surrounding soil into the plant. Pressure potential at the top of the plant is less than the pressure coming from the roots, so water rises up the plant (positive pressure from the atmosphere and negative pressure, or pull, via transpiration from the leaves). 4. Of the water that enters a plant 90% leaves it via transpiration. The leaf structures that help regulate this are the stomata, cuticle, and substomatal spaces. Plants must be exposed to a constant source of water because so much is lost that it must be replenished for metabolic activities to continue. 5. Root hairs are turgid as a result of their greater solute potential. Energy is expended to accumulate minerals in a plant root because their concentration is much greater in the root than in soil so ions must be pumped in at a cost of ATP and against a concentration gradient. Mineral passage is nonselective, except passage through the endodermis. This results because of the presence of waxy Casperian strips which cause minerals to pass selectively through endodermis plasma membrane. 6. Water transport is reduced to near zero when the relative humidity is 100%, i.e., at night. Almost no transpiration occurs under these circumstances. Active transport does continue and causes higher concentrations of ions, which pulls even more water in by osmosis, so that water moves upward in the xylem. The term for this is root pressure. 7. Guttation occurs when water droplets are forced out of special cells at the ends of small veins. It is more likely in very short plants because root pressure can't push water up very great distances. 8. Trees have a smaller diameter in the day because transpiration suction is so great that it causes the xylem walls to pull together. The ultimate source of a plant's water potential is the sun, because sun regulates the transpiration rate through the leaves. 9. Stomatal control requires energy; ATP is used to pump ions into guard cells, water follows passively, and cells stay turgid. Potassium is an integral part of this control mechanism. Guard cells possess chloroplasts to provide an immediate source of ATP to regulate their opening. Leaf tissue produces abscisic acid during water stress. It causes potassium to pass rapidly out of the guard cells, and the stomata to close. 10. Plants can regulate water loss through periods of dormancy during dry weather, with loss of leaves, or simply not existing at all except as seeds. Thick leaves with thick cuticles and few stomata are more resistant to water loss than are other leaves. Trichromes are fuzzy structures on the surface of some leaves that serve to reduce leaf temperature and retain moisture near the surface leaf, which will also retard water loss from the leaf. 11. Translocation is the movement of sugars from where they are produced to where they are used; they are stored as starch and transported as sucrose. The driving force behind translocation is hydrostatic pressure. 12. Carbohydrates move through a plant as follows: sucrose is produced at the source, which is then actively loaded into the veinlet sieve tubes, which increases their solute concentration, then water flows into the tubes via osmosis, sucrose is unloaded at the sink, water follows it and moves toward the sink, where it is pulled out, and sucrose follows it. The process has an active source and sink and is passive in between. The companion cells provide ATP to load and unload sucrose. 13. Macronutrients are inorganic nutrients that plants require in large amounts; they include carbon, hydrogen, oxygen, nitrogen, potassium, calcium, phosphorus, magnesium, and sulfur. They are important because they are components of organic molecules, enzyme activity, the maintenance of cell walls and membranes, and energy production. Micronutrients are inorganic nutrients that plants require in very small amounts; the primary ones are iron, chlorine, copper, manganese, zinc, molybdenum, and boron. Their most common functions are as enzyme activators and in the formation of chlorophyll. 14. Fertilizers are required because cultivation increases erosion and the loss of nutrients, large amounts of nutrients are taken away with the crops, and soils may be naturally poor. Natural communities recycle all available nutrients so that they are used over and over. The three mineral nutrients most commonly found in commercial fertilizers are nitrogen, phosphorus, and potassium. Organic fertilizers may be superior because they help build up the humus content of the soil, enhancing water and nutrient-retaining properties.