Ch. 36 Transport in Plants Occurs at three levels: Uptake by cell Cell to cell Long distance from root to leaves and reverse. Cellular uptake Passive transport-movement across membranes slow without: Transport proteins-in cell membrane. May bind to molecule and transport.(carrier protein) May create passageway (channel protein) Active transport Moving solutes against conc. Gradient. Uses carrier proteins and energy (ATP) Ex. Proton pump-uses energy released to move Hydrogen ions to establish high conc. (remember electron transport/chemiosmosis) Water potential Water moves by osmosis Moves from hypotonic to hypertonic In plants, cell wall has an effect as well as solute conc. Combined, they equal water potential. Water potential basics Movement from high to low. Called water potential because moving water can do work-potential energy. Water potential of pure water open in atmosphere =zero. Solutes lower water potential. Increase in physical pressure increases water potential. Water potential cont. As solute increases, water potential decreases. As pressure increases, so does water potential. Explains water movement across membranes. See p.751 and 752 Transport across membranes Water is polar; inside cell membrane is nonpolar. Selective channels called aquaporins aid in passive transport of water across membrane. Cell walls of plant cells allow continuous movement across. Absorption by root Root hairs-grow from epidermal cells near tip; Soil sticks to hairs; water and minerals stick to soil. Mycorrhizae-fungus fibers; symbiotic relationship with root hairs. Soil solution moves into epidermal walls (hydrophilic) and moves into root cortex. Connections between cytoplasm area of plant cells allows continuous flow. (symplast) Lateral transport Movement of materials from outer to inner cells. Transmembrane movement-across plasma membrane after plasma membrane. Apoplastic-movement through cell walls. Symplastic-movement through cytoplasm connected by plasmodesmata. Root structure and water movement Water moves through cortex Reaches inner layer of cortex known as endodermis. Protects inner area of root known as stele. Stele contains xylem and phloem Endodermal cells have waxy layer called Casparin strip. Cont. Water moving through symplast goes into endodermis. Water moving apoplastically has barrier. Water must move across endodermis plasma membrane and then in symplastically. Water moves into tracheids and vessel element cells of xylem. Transpiration Loss of water vapor through leaves and aerial parts of plants. Water moves out through pores called stomata. Stomata are connected to air spaces in spongy mesophyll of leaf tissue. Water found in air spaces because space in contact with moist cells. Root pressure Xylem contains minerals as well as water. Transpiration low at night; minerals still pumped in causing decrease in water potential. Water moves in creating a pressure or push. Root pressure causes guttation-water droplets on leaf margins. Root pressure NOT major force moving water upward. Transpiration-Cohesion Pull Water molecules exhibit hydrogen bonding. Hydrogen bonding results in cohesion and adhesion. Transpiration causes negative pressure in xylem; water sticks together. Air dry--- water exits stomata—water from cells moves into air space as replacement--- pulls on water in xylem. Transpiration-Cohesion Cont. Water sticks to hydrophilic cell walls of tracheids and vessel elements. Water chain must remain continuous. Bulk flow of water due to solar energy; no energy of plant used. Controlling transpiration Stomata open to allow CO2 in. Water can escape. Oxygen from photosynthesis leaves through stomata. Stoma surrounded by two guard cells. Guard cells change shape to control size of opening. Guard cell function Guard cells take in or lose water due to K+ conc. As this ion conc. increases, water moves in; guard cells become turgid;cells are thicker along outer edge so they bow. (p.760) Stomata typically open during day and closed at night. Guard cells cont. K+ can be stimulated by light; depletion of CO2 can cause stoma to open; internal clock-circadian rhythms-can control opening/closing. High temp. can cause closing of stomata along with high transpiration. Abscisic acid made due to low water can cause guard cells to shrink. Translocation-transport of food in plants Occurs in phloem. Phloem sap may move up or down. Movement of food from “source” (high conc.) to “sink” (low conc.) Fruit and areas of plant growth are sinks. Cells of phloem tissue called sieve tubes. Translocation cont. Phloem also has companion cells-have nucleus and organelles-help control what occurs in sieve tube cells. Food moves from source to sink due to pressure flow. High solute conc. causes water to move into sieve tube; creates pressure. As solutes leave sink, water follows which causes loss of pressure. Sugar movement into sieve tubes and companion cells accomplished by active transport; mechanism too fast by simple diffusion. Sucrose too big to cross membranes on own. In spring, roots of trees and other perennials are source and stem/leaf cells coming out of dormancy are sink area. Sap moves up.
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