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Chapter 28

Chapter 28 Detailed Notes

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Department
Biology
Course
BI111
Professor
Tristan Long
Semester
Winter

Description
Chapter 28: Transport in Plants 28.1 Principles of Water and Solute Movement in Plants • Long distance transport: o Water and dissolved minerals travel in the xylem from roots to shoots and leaves, and the products of photosynthesis move via the phloem from the leaves and stems into roots and other structures • Examples of short distance transport: water and soluble minerals entering roots by crossing the cell membranes of root hairs, or sugars in the phloem crossing plasma membranes into metabolically active cells 28.1a Short-Distance Transport Mechanisms Move MoleculesAcross Plants Cell Membranes Passive • Osmosis: passive transport of water across a selectively permeable membrane either by simple or facilitated diffusion through water-conducting channel proteins called aquaporin’s (which allow rapid movement of water across a membrane) o Can determine whether water will move into or out of a cell by understanding how the potential energy of water, or water potential is influenced by the amount of solutes in the cell and the pressure that the cell exerts on the cell wall  Simple diffusion  Simplest form of passive transport  Transport of nonpolar molecules and small polar molecules that can readily diffuse across the lipid portion of a membrane  Requires no metabolic energy  Substance moves down concentration or electrochemical gradient  Facilitated diffusion  Transport of polar and charged molecules that move across the membrane via transport proteins  But how efficient is it?  Not very over long distances  10 µm in 0.1 sec; 100 µm in1 sec; 1 mm in 100 sec. Active Transport Mechanisms  Active transport requires metabolic energy (ATP)  Based on H pumps  Moves ions and large molecules across membranes via transport proteins, but because these solutes are being moved against their concentration gradient, cells must expend energy +  H gradient maintained throughATP use or through harnessing the energy is a concentration gradient for secondary active transport  H diffusion into cell powers uptake of solutes Active Transport Mechanisms  Symport  Material transported in same direction as movement of H and solute, organic uptake  Antiport +  Mat+rial transported in opposite direction to movement of H and solute, Na export 28.1b The Relationship Between Osmosis and Water Potential • Solute potential: effect of dissolved solutes on water potential o When solutes are added to water, they disrupt hydrogen bonding between water molecules  Polar water molecules interact with solutes, forming a hydration shell that surrounds the solute molecules  Water molecules in a hydration shell are constrained from moving, and thus addition of solutes decreases the free energy of the water in the solution. Thus, water potential is lower in a solution with more solutes than in pure water • Solute potential and pressure potential determine a cell’s water potential: the equation to express this relationship: Ww = Ws + Wp Diffusion and Osmosis  Diffusion: the spontaneous movement of molecules or particles along a concentration gradient  Osmosis: Special case of diffusion  Water molecules diffuse across a selectively permeable membrane to an area of higher water concentration (low solute concentration), to a lower water concentration (high solute concentration).  Water potential is symbolized by the Greek letter psi and is measured in units of megapascals (MPa)  Water potential is a relative value that is defined in reference to pure water at atmospheric pressure (such as water in an open container which has a value of 0 MPa)  Two factors that determine that are presence of solutes and physical pressure Consequences of Gradients • Hypertonic: higher concentration of solutes outside the cell • Isotonic: equal concentration of solutes inside and outside the cell • Hypotonic: lower concentration of solutes outside the cell • Pressure potential (generated by cell walls): • Increases physical turgor pressure inside cells halts net osmosis across membrane 28.1c Osmosis in Plant Cells Creates Turgor Pressure, Which is Necessary for Plant  Support • When animal cells are placed in hypotonic solution, they may swell to bursting • In plants, bursting is relented by the cell wall o Causes turgor pressure • As long as the Ww of soil is higher than that of the root epidermal cells, water  will follow the gradient and flow into root cells, making the turgid Experiment: Osmotic Environment Flaccid Cell • Central vacuole o Tonoplast membrane (contains a dilute solution of sugar, proteins, other  organic molecules, and salts)  Many solutes that enter a plant cell are actively transported from  the cytoplasm into the central vacuole through ion channels in the  tonoplast, as they accumulate in the vacuole water follows by  osmosis o Maintains turgor pressure 28.2a Water Travels across the Root to the Root Xylem by Two Pathways • 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  symplastic pathway • Continuous network of cell walls and spaces between cells make up nonliving  areas of root (apopplastic) • Each endodermal cell also has a ribbon like Casparian strip in its radical and  transverse walls, positioned somewhat like a ribbon of packaging tape around a  rectangular package o Impregnated with suberin (blocks apoplastic movement of water at  endodermis) o Water is forced to detour from apoplast, moving across plasma membrane  of endodermal cells into the symplastic pathway. Water and solutes then  pass through Plasmodesmata to the cells in outer layer of the stele o Allows endodermis to control which substances enter and leave a plants  vascular tissue Transport Routes in Plants Pathways of Water into Roots  Apoplastic pathway: Water does not cross cell membrane, diffuses through  nonlving regions including cell walls and air spaces  Symplastic pathway: Water crosses membrane, often uses plasmodesmata,  diffuses through cytoplasm  Transmembrane pathway: Water crosses plasma membranes and perhaps  tonoplasts (vacuolar membrane) Casparian strip in root  endodermis forces apoplastic  water to symplast Active transport of minerals into symplast Active transport at Casparian strip across  membrane Allows cell membranes to regulate solute  movement Stomata  Transpiration losses of water must be regulated to prevent rapid dessication  Cuticle limits H2O loss but also prevents CO  2ptake  Water is lost when stomata open for photosynthesis + +  Stomatal opening controlled by symport of H /K  Water follows K  by osmosis  Turgid stomata open, flaccid closed  Physiology  Stomata must balance H O 2oss and CO upta2  by responding to many  signals, biological clock  Stomata open to increase photosynthesis  Increasing light (blue)  Decreasing CO  c2ncentration in leaf  Stomata close under water stress  Abscisic acid is hormonal signal for closure, synthesized by roots   Mesophyll cells take up Abscisic acid from xylem and release it 28.3 Long­Distance Transport of Water and Minerals in the Xylem • Bulk flow: mass movements of molecules in response to a difference in reassure  between two locations, like water in a closed plumbing system gushing from an  open faucet • Xylem sap: dilute solution of water and ions that flows in the xylem, that moves  by bulk flow from roots to shoots • Transpiration: driving force for the upward movement of xylem sap from root to  shoot is the evaporation of water from leaves and other above ground parts of land  plants o Through cohesion of water molecule and tension created by the  evaporation of water from plant surfaces 28.3a The Properties of Water Play a Key Role in its Transport • Cohesion o Tend to form hydrogen bonds with one another  Hydrogen bonding pulls drop of water closer together, and therefore has a better  chance of fighting gravity • Adhesion o Form hydrogen bonds with molecules of other substances Mechanical Properties of Water
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