BIO 151 Exam 2 Study Guide (Ch. 36) Questions And Answers

Define osmosis and water potential. Explain how water potential is measured. - ANS Osmosis = diffusion of water through a selectively permeable membrane Water potential: a measurement that combines the effects of solute concentration and pressure. • Water potential determines the direction of movement of water. • Water flows from regions of higher water potential to regions of lower water potential. • Potential refers to water's capacity to perform work. Measured in megapascals (MPa), a unit of pressure Explain how solutes and pressure affect water potential. - ANS • As the solute concentration increases, water potential decreases. - When solutes are added, they bind to water molecules. As a result, there are fewer free water molecules, reducing the capacity of water to move and do work. • Turgor pressure- helps maintain the stiffness of plant tissues and serves as the driving force for plant elongation. Define the terms flaccid, plasmolyze, turgor pressure, and turgid. - ANS Flaccid- limp as a result of losing water Plasmolyze- shrinks and pulls away from cell wall Turgor Pressure- cell contents press plasma membrane against cell wall Turgid- very firm Define bulk flow and describe the forces that generate pressure in the vascular tissue of plants. - ANS Bulk flow- the movement of liquid in response to a pressure gradient. LONG DISTANCE TRANSPORT - Unlike osmosis, bulk flow is independent of solute concentration - In phloem, the loading of sugars generates a high positive pressure forcing sap to the opposite end. Inxylem, negative pressure drives long distance transportation. Transpiration reduces xylem pressure and increases plant xylem tension. Explain what routes are available to water and minerals moving into the vascular cylinder of the root. - ANS Water and mineral salts from the soil enter the plant through the epidermis of roots, cross the root cortex into the vascular cylinder, and flow up to the shoot system. Relate the structure of sieve tube cells, vessel cells, and tracheids to their functions in bulk flow. - ANS Sieve tube cells are dead and contain no cytoplasm making it easier for the bulk flow of water to flow through. Vessel cells in leaves lower their pressure by transpiration causing bulk flow upwards. Tracheids aid in xylem bulk flow by holding the sap Explain how the endodermis functions as a selective barrier between the root cortex and vascular cylinder. - ANS The endodermis is the innermost layer of cells in the root cortex and surrounds the vascular cylinder and functions in selective passage of minerals from the cortex to the vascular tissue. Describe the potential and limits of root pressure to move xylem sap. - ANS When transpiration is low, the accumulation of minerals lowers the water potential within the vascular cylinder. Water flows in from the root cortex as an upward push of xylem sap. Define the terms transpiration and guttation. - ANS Transpiration is the evaporative loss of water from a plant while guttation is the exudation of water droplets, caused by root pressure in certain plants. Explain how transpirational pull moves xylem sap up from the root tips to the leaves. - ANS Water exits the leaves by the stomata when air outside the leaf is dry. Negative pressure causes water to move upward in mesophyll cell walls. Water is brought to the leaves by the xylem. As more water is lost to the air, water from moister parts of the leaf are moved to the dry spot, lowering the tension. This causes pulling in xylem and a negative water potential. Explain this statement: "The ascent of xylem sap is ultimately solar powered. - ANS The sun and its heat causes water from leaves to evaporate and without the transpiration, the upward pull of the xylem wouldn't exist. Define and describe the process of translocation. Trace the path of phloem sap from a primary sugar source to a sugar sink. - ANS Translocation is the transport of organic nutrients in the plant. The sugar source produces the sugar by photosynthesis or breakdown of starch. Then the sugar is consumed or stored by a sugar sink. After stockpiling carbs in the summer, it is a source as its starch are broken down to sugar which is carried to the tips of the plant Describe the process of sugar loading and unloading. - ANS Sugar must be loaded to sieve tube members before being exported to sinks. It passes through mesophyll cells to sieve tube members, passing through the plasmodesmata. The sucrose is then unloaded by phloem at the sink end of the sieve tube.Sugar disperses from the phloem to sink tissues and is followed by water Define pressure flow. Explain the significance of this process in angiosperms. - ANS In angiosperms, sap moves through a sieve tube by bulk flow driven by positive pressure. The building of pressure at the source and reduction at the sink causes water to flow from source to sink with the sugar.