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Wilfrid Laurier University
Joanne Lee

28.1: In long distance transport, water, and dissolved minerals travel in the cylem from roots to shoots and leaves and the products of photosynthesis move via the phloem from the leaves and stem into roots and other structures. 28.1a) short distance transport mechanisms move molecules across plant cell membranes In Passive transport: substances move with their concentration gradient or, if the substance is an ion, with its electrochemical gradient. ^ meaning that cells do not need to expend energy. Passive transport includes both simple diffusion (transport of nonpolar molecules and smarll polar molecules that can readily diffuse across the lipid portion of a membrane) and facilitated diffusion (transport of polar and charged molecules that move across the membrane via transport proteins) ACTIVE TRANSPORT: moves ions and large molecules across membrane via transport proteins, but because these solutes are being moved against their concentration gradient, cells must expend energy. ^ energy can be provided by the ATP hydrolysis ( for primary active transport) or by harnessing the energy in a concentration gradient ( for secondary active transport) Individual cells gain ad lose water by OSMOSIS, the passive transport of water across ta selectively permeable membrane either by simple diffusion or by facilitated diffusion throughwater-conducting channel proteins called AQUAPORINS, which allow rapid movement of water across a membrane. -Osmosis occurs in response to solute concentration gradients, a pressure gradient or both. - WATER POTENTIAL is influenced by the amount of solutes in the cell and the pressure that the cell exerts on the cell wall. 28.1b) the relationship between osmosis and water potential: The movement of water across a membrane is strongly influenced by the concentration of solutes on either side of the membrane. -Effect of dissolved solutes on water potential is called SOLUTE POTENTIAL -When solutes are added to water, they disrupt some of the hydrogen bonding between water molecules and the polar water molecules interact with the solutes, forming a hydration shell that surrounds the solute molecules. -Water Potential is LOWER in a solution with more solutes than in pure water; Pure water has a solute potential of 0 MPa, while solutions containing solutes will have solute potential values of less than zero. -The water potential is higher outside plant cells than inside them, so water tends to enter the cells by osmosis; this is the process by which soil water is drown into a plants roots. =together, solute potential and pressure potential determine a cells water potential: the equation to express this relationship is =solute potential +pressure; in all cases water will be moving from a solution of higher (less negative) potential to a solution of lower(more negative) potential. 28.1C Osmosis in plant cells creates turgor pressure, which is necessary for plant support: Most of the volume of a mature plant cell is occupied by a large CENTRAL VACUOLE, which is surrounded by a vacuolar membrane (TONOPLAST) and contains dilute solutions of sugars, proteins, other organic molecules and salts. In plants, bursting of cells due to water is prevented by the cell walls, which resists further inward movement of water. The pressure o the water filled vacuole and cytoplasm against the wall keeps the cell firm or TURGID, so we refer to the pressure of the cytoplasm on the wall as TURGOR PRESSURE. TURGOR PRESSURE develops as a result of osmosis and increases until it is high enough to prevent more water from entering a cell by Osmosis. As long as the water potential of soil is higher than that of the root epidermal cells, water will follow the water potential gradient and flow into root cells, making them turgid. Disruption in water uptake from the soil can lead to the drooping of leaves and stems called WILTING, which occurs when turgor pressure in the cells of leaves and stems drops to very low levels. 29.2a) water travels across the root to the root xylem by two pathways: The living cells make up the SYMPLAST and are interconnected by plasmodesmata, allowing water to flow from the cytoplasm of one cell to the next via the SYMPASTRIC PATHWATH. The continuous network of cell walls and spaces between cells makes up the nonliving areas of the root or the APOPLAST. -water moves through the apoplastic pathway as it flows through these non-living spaces without crossing plasma membrane. - This apoplastic water travels until it encounters the ENDODERMIS, innermost layer of the cortex. The endodermal cells are tightly packed. Each endodermal cell also has a ribbonlike CASPARIAN STRIP in its radial and transverse walls, positioned somewhat like a ribbon of packing tape around a rectangular package. The casparian strip is impregnated with SUBERIN, a waxy substance that is impermeable to water and blocks the apoplastic movement of water at the endodermis. - The endodermis prevents needed substances in the XYLEM from leaking out, back into the root cortex. - Thus, the casparian strips allow the endodermis to control which substances enter and leave a plant’s vascular tissue. 28.2b Roots take up ions by active transport Mineral ions dissolved in soil water also enter roots through the epidermis. Some enter the apoplast along with water, but most ions important for plan nutrition tend to be much more concentrated in roots. Mechanisms that control which solutes will be absorbed by root cells ultimately determine which solutes will be distributed through the plant Once an ion reaches the stele, it diffuses from cell to cell until it is “loaded” into the xylem. Once in the xylem, water and mineral ions can move laterally to and from tissues or travel upward in the conducting elements. Minerals are distributed to living cells and take up by active transport. 29.3a) the properties of water play a key role in its transport: First, water molecules are strongly cohesive: they tend to form hydrogen bonds with one another. Second, water molecules are adhesive: they form hydrogen bonds with molecules of other substances, including the carbohydrates in plant cell walls. Water transport begins as water evaporates from the walls of mesophyll calls inside leave and into the intercellular spaces. This water vapor escapes by transpiration through open stomata, the pores in the leaf surface. As water molecules exit the lead, they are replaced by others from the cytoplasm of mesophyll cells. In the xylem, water molecules are confined in narrow, tubular xylem cells. The water molecules form a long chain, like a string of weak magnets, held together by hydrogen bonds between individual molecules. 28.3b) leaf anatomy contributes to cohesion-tension forces: Leaf anatomy is key to the process that moves water upward in plants. As much as two-thirds of a leaf’s volume consists of air spaces. Leaves may also have thousands to millions of stomata through which water vapor escapes. Both of these factors increase transpiration. Also, every square centimeter of a leaf contains thousands of tiny xylem veins. This close proximity supplies water to cells and the spaces between them, from which the water can readily evaporate. 28.3C) In the tallest tree, the cohesion-tension mechanism May reach its physical limit The theory predicts tht xylem sap
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