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UT BIO 311D - Transport in Vascular Plants
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BIO 311D 1st Edition Lecture 29Outline of Last Lecture I. Plants have a hierarchical organization consisting of organs, tissues, and cellsII. The Three Basic Plant Organs: Roots, Stems, and LeavesIII. RootsIV. StemsV. LeavesVI. Dermal, Vascular, and Ground TissuesVI. Common Types of Plant CellsOutline of Current Lecture I. Adaptations for acquiring resources were key steps in the evolution of vascular plantsII. Shoot Architecture and Light CaptureIII. Root Architecture and Acquisition of Water and MineralsIV. Different mechanisms transport substances over short or long distancesV. Short-Distance Transport of Solutes Across Plasma MembranesVI. How Solutes and Pressure Affect Water PotentialVII. Water Movement Across Plant Cell MembranesVIII. Aquaporins: Facilitating Diffusion of WaterIX. Long-Distance Transport: The Role of Bulk FlowCurrent LectureAdaptations for acquiring resources were key steps in the evolution of vascular plants• The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water• Early nonvascular land plants lived in shallow water and had aerial shoots• Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport• The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis• Xylem transports water and minerals from roots to shoots• Phloem transports photosynthetic products from sources to sinks• Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water lossShoot Architecture and Light Capture• Stems serve as conduits for water and nutrients and as supporting structures for leaves• There is generally a positive correlation between water availability and leaf size• Stems serve as conduits for water and nutrients and as supporting structures for leaves• There is generally a positive correlation between water availability and leaf size• Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows• Self-pruning is the shedding of lower shaded leaves when they respire more than photosynthesize• Leaf orientation affects light absorption• In low-light conditions, horizontal leaves capture more sunlight• In sunny conditions, vertical leaves are less damaged by sun and allow light to reach lower leaves• Shoot height and branching pattern also affect light capture• There is a trade-off between growing tall and branchingRoot Architecture and Acquisition of Water and Minerals• Soil is a resource mined by the root system• Taproot systems anchor plants and are characteristic of gymnosperms and eudicots• Root growth can adjust to local conditions– For example, roots branch more in a pocket of high nitrate than low nitrate• Roots are less competitive with other roots from the same plant than with roots from different plants• Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae• Mutualisms with fungi helped plants colonize land• Mycorrhizal fungi increase the surface area for absorbing water and minerals, especially phosphateDifferent mechanisms transport substances over short or long distances• There are two major pathways through plants– The apoplast– The symplast• The apoplast consists of everything external to the plasma membrane• It includes cell walls, extracellular spaces, and the interior of vessel elements and tracheids• The symplast consists of the cytosol of the living cells in a plant, as well as the plasmodesmata• Three transport routes for water and solutes are– The apoplastic route, through cell walls and extracellular spaces– The symplastic route, through the cytosol– The transmembrane route, across cell wallsShort-Distance Transport of Solutes Across Plasma Membranes• Plasma membrane permeability controls short-distance movement of substances• Both active and passive transport occur in plants• In plants, membrane potential is established through pumping H+ by proton pumps• In animals, membrane potential is established through pumping Na+ by sodium-potassium pumps• Plant cells use the energy of H+ gradients to cotransport other solutes by active transport• Plant cell membranes have ion channels that allow only certain ions to pass• To survive, plants must balance water uptake and loss• Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure• Water potential is a measurement that combines the effects of solute concentration andpressure• 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• Water potential is abbreviated as Ψ and measured in a unit of pressure called the megapascal (MPa)• Ψ = 0 MPa for pure water at sea level and at room temperatureHow Solutes and Pressure Affect Water Potential• Both pressure and solute concentration affect water potential• This is expressed by the water potential equation: Ψ = ΨS + ΨP• The solute potential (ΨS) of a solution is directly proportional to its molarity• Solute potential is also called osmotic potential• Pressure potential (ΨP) is the physical pressure on a solution• Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast• The protoplast is the living part of the cell, which also includes the plasma membrane• Consider a U-shaped tube where the two arms are separated by a membrane permeableonly to water• Water moves in the direction from higher water potential to lower water potentialWater Movement Across Plant Cell Membranes• Water potential affects uptake and loss of water by plant cells• If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis• Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall• If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid• Turgor loss in plants causes wilting, which can be reversed when the plant is wateredAquaporins: Facilitating Diffusion of Water• Aquaporins are


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