Resource Acquisition and Transport in Vascular Plants

Resource Acquisition and Transport in Vascular Plants What does a plant need? Review: ▪ ▪ ▪ ▪ Selectively permeable membrane: osmosis, transport proteins, selective channels Proton pump: active transport; uses E to pump H+ out of cell → proton gradient Cotransport: couple H+ diffusion with sucrose transport Aquaporin: transport protein which controls H2O uptake/loss a) b) c) d) H+ and membrane potential H+ and cotransport of neutral solutes H+ and contrasport of ions Ion channels Osmosis • **Water potential (ψ): H2O moves from high ψ → low ψ potential, solute conc. & pressure - Water potential equation: ψ = ψS + ψP - Solute potential (ψS) – osmotic potential - Pressure potential (ψP) – physical pressure on solution - Pure water: ψS = 0 Mpa - Ψ is always negative! - Turgor pressure = force on cell wall • Bulk flow: move H2O in plant from regions of high → low pressure ** Review AP Bio Investigation 4 • • • Flaccid: limp (wilting) Plasmolysis: shrink, pull away from cell wall (kills most plant cells) due to H2O loss Turgid: firm (healthy plant) Evolution of Transport in Plants Algal ancestors of land plants absorbed water, minerals, CO2 directly from the surrounding water The evolution of xylem and phloem made possible the long-distance transport in land plants Adaptations represent compromises between enhancing photosynthesis and minimizing water loss Vascular Tissues: conduct molecules Transport of H2O and minerals into xylem: Root epidermis → cortex → [Casparian Strip] → vascular cylinder → xylem tissue → shoot system At Root Epidermis Root hairs: increase surface area of absorption at root tips Mycorrhizae: mutualistic relationship between fungal hyphae + roots Increase H2O/mineral absorption The white mycelium of the fungus ensheathes these roots of a pine tree. Long distance transport Efficient long distance transport of fluid requires bulk flow, fluid movement driven by pressure Efficient movement is possible because mature xylem (tracheids and vessel elements) have no cytoplasm, and phloem (sieve-tube elements) have few organelles in their cytoplasm Transport of H2O Water and minerals can travel through a plant by three routes: Transmembrane route: out of one cell, across a cell wall, and into another cell Symplastic route: via the continuum of cytosol Apoplastic route: via the cell walls and extracellular spaces Transport pathways across Cortex: Apoplast = materials travel between cells Symplast = materials cross cell membrane, move through cytosol & plasmodesmata Entry into Vascular Cylinder: Endodermis (inner layer of cortex) sealed by Casparian strip (waxy material) Blocks passage of H2O and minerals All materials absorbed from roots enter xylem through selectively permeable membrane Symplast entry only! How does material move vertically (against gravity)? Transpiration: loss of H2O via evaporation from leaves into air Root pressure (least important) Diffusion into root pushes sap up Cohesion-tension hypothesis Transpiration provides pull Cohesion of H2O transmits pull from roots→shoots Root pressure At night, when transpiration is very low, root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential Water flows in from the root cortex, generating root pressure Q: Why does guttation form droplets on the leaf edges (margin) instead of under the leaf? A: The water from root pressure is exuded via hydathodes located at the leaf margin. Hydathodes are most likely evolved from modified stomata. Some halophytes possess glandular trichomes that actively secrete salt in order to reduce the concentration of cytotoxic inorganic ions in their cytoplasm; this may lead to the formation of a white powdery substance on the surface of the leaf. Stomata regulate rate of transpiration Stomata – pores in epidermis of leaves/stems, allow gas exchange and transpiration Guard cells – open/close stoma by changing shape Take up K+ → lower ψ → take up H2O → pore opens Lose K+ → lose H2O → cells less bowed → pore closes Stimuli for Stomatal Opening and Closing Generally, stomata open during the day and close at night to minimize water loss Stomatal opening at dawn is triggered by light, CO2 depletion, and an internal “clock” in guard cells All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles Cells stimulated open by: light, loss of CO2 in leaf, circadian rhythms (internal 24 hr clock) Stomata closure: drought, high temperature, wind Effects of Transpiration on Wilting and Leaf Temperature? If the lost water is not replaced by sufficient transport of water, the plant will wilt Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes Xerophytes are plants adapted to arid climates They have leaf modifications that reduce the rate of transpiration Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night SUGAR TRANSPORT IN PLANTS Translocation: transport of sugars into phloem by pressure flow Source → Sink Source = produce sugar (photosynthesis) Sink = consume/store sugar (fruit, roots) Via sieve-tube elements Active transport of sucrose Symplast is dynamic Plasmodesmata allows movement of RNA & proteins between cells Phloem can carry rapid, long-distance electrical signaling Nerve-like function Swift communication Changes in gene expression, respiration, photosynthesis