Ch. 36 Transport in Vascular Plants Feb 4­1:32 PM 1 Essential Question: How does a tall tree get the water from its roots to the top of the tree? Feb 4­1:38 PM 2 Shoot architecture and Light Capture: Phyllotaxy ­ arrangement of leaves on a stem to maximize light capture, reduce self shading ­ determined by shoot apical meristem and specific to each species ­alternate = one leaf per node ­opposite = two leaves per node ­whorled = more than two leaves per node Norway spruce ­ 1 is youngest leaf Apr 14­7:00 AM 3 leaf area index = ratio of total upper leaf surface of a single plant divided by surface area of land, normal value ~ 7 if above 7 ­ leaves,branches undergo self pruning ­ programmed cell death Mar 28­2:46 PM 4 Leaf orientations: horizontal leaf orientation ­ for low­light, capture sunlight more effectively vertical leaf orientation ­ for high light, grasses, light rays coming in parallel to leaf so not too much light Apr 14­7:05 AM 5 Root architecture: mychorrhizae ­ mutualistic relationship between fungi and roots ­80% of land plants have this ­increases surface area for water and mineral absorption Mar 28­2:49 PM 6 Overview of transport in trees Feb 6­9:37 AM 7 Three types of transport in vascular plants: 1. transport of water and solutes by individual cells a. passive transport (osmosis) through aquaporins ­ transport proteins water potential ­ combined effects of solute concentration and physical pressure (esp. in plants due to cell wall) ­determines direction of movement of water ­free water moves high to low [ ] ­measured in megapascals (MPa) ­water potential = "0" in an open container (at sea level and rm. temp.) Feb 4­1:39 PM 8 = water potential = solute potential ­ proportional to # of dissolved solute molecules (adding solutes lowers the water potential, so is always negative) = pressure potential ­physical pressure on a solution ( can be + or ­ relative to atmospheric pressure) turgor pressure= force against the cell wall after cell swells with water Feb 4­2:05 PM 9 Water potential and water movement in an artificial model (keep in mind water goes from high to low water potential) Feb 4­2:12 PM 10 Water relations in plant cells Feb 4­2:14 PM 11 Feb 6­9:43 AM 12 b. active transport a. proton pump ­ uses ATP to pump Hydrogen ions out of cell ­makes proton gradient, membrane potential (voltage) outside cell = positive, inside = negative Feb 4­1:58 PM 13 proton pumps can help: 1. get potassium (K+) inside cell _ Feb 4­1:46 PM 14 2. cotransport of anions ­ transport protein couples H+ downhill with uphill ­ of nitrate (NO )3 Mar 28­2:53 PM 15 3. Cotransport of neutral solute (ex. sugar) Mar 28­2:53 PM 16 Three compartments of plant cell that regulate transport: 1. Cell wall ­ barrier between extracellular contents and plasma membrane 2. plasma membrane ­ barrier between cell wall and cytosol 3. tonoplast (wall of vacuole) ­ barrier between cytosol and vacuole (holds cell sap) Feb 4­1:51 PM 17 between cells plasmodesmata link the cytosol so molecules can pass easily ­ called the symplast continuous cell walls and extracellular spaces is called apoplast vacuoles are not shared between cells Feb 4­2:25 PM 18 2. short distance transport cell to cell also known as lateral transport three ways: a. transmembrane­ crossing of cell wall and plasma membrane from cell to cell b. via the symplast­ via plasmodesmata c. along the apoplast ­ pathway of cell walls and extracellular spaces Feb 4­2:27 PM 19 happens in root tips/root hairs (increase surface area) soil particles coated with water and dissolved minerals adhere to root hairs ­can then go by apoplastic or symplastic routes Feb 4­2:46 PM 20 ­water and minerals then have to pass endodermis (innermost layer of root cortex) ­minerals already in symplast can go right through ­if via apoplast ­ reach dead end [Casparian strip made of suberin (waxy material)] ­to get past Casparian strip ­ need to pass through plasma \ membrane and enter via symplast ­end of pathway is back through the apoplast by diffusion and active transport ­ then can go into tracheids and vessels and go up plant Feb 4­2:53 PM 21 Casparin strip Feb 6­9:50 AM 22 3. long distance transport ­ Bulk Flow movement of fluid driven by pressure ­through tracheids in xylem and sieve tubes in phloem ­transpiration (evaporation of water from a leaf) reduces pressure in leaf xylem, creates tension that pulls xylem sap up from the roots Feb 4­2:34 PM 23 Factors affecting ascent of xylem sap: 1. pushing xylem sap water flows in from root cortex ­generates root pressure to push sap upward ­if more water enters leaves than transpired = guttation water droplets that can be seen on tips of leaves ­minor mechanism Feb 4­3:07 PM 24 2. Pulling xylem sap (pulled by negative pressure)­transpirational pull ­normally air outside leaf is drier than inside leaf so water potential is higher inside leaf than outside so water leaves the leaf via stomata ­also involved­ water adhesion to cellulose microfibrils and cohesion of the water molecules to each other ­negative water potential of leaves provides the "pull" ­only works if there is an unbroken chain of water molecules Feb 4­3:08 PM 25 in winter when sap freezes = can get cavitation (formation of water vapor pocket) so breaks chain ­ causes clicking noises in tree due to rapid expansion of bubbles when tree warms ­water can find an alternate route to go ­youngest, outermost secondary tissue transports water Mar 25­3:08 PM 26 Ascent of xylem sap Feb 4­3:19 PM 27 Stomata regulate the rate of transpiration a leaf has high surface to volume ratio and large surface areas for photosynthesis ­due to this ­ has a large area to lose water stomata help reduce the water loss transpiration is high on sunny, warm, dry and windy days because these increase evaporation ­stomata close to decrease water loss ­prolonged drought = wilting ­lose turgor pressure Feb 4­3:20 PM 28 transpiration also causes evaporative cooling of the leaf, prevents enzymes from denaturing cacti can survive low rates of transpiration and high leaf temperatures Mar 25­3:08 PM 29 typical stomata Feb 4­3:28 PM 30 mechanism of stomatal opening and closing Transport of potassium across the plasma membrane and vacuolar membrane causes the turgor changes in guard cells. Feb 4­3:29 PM 31 stomata open in day and close at night triggers for opening: 1. light ­ blue­light receptors in guard cells trigger accumulation of potassium ions and become turgid 2. low carbon dioxide due to photosynthesis 3. "internal clock" of guard cells triggers for closure in daylight: 1. low water ­ guard cells lose turgor 2. hormone ­ abscisic acid signals closure of guard cells Feb 4­3:35 PM 32 xerophytes ­ plants adapted to arid environments have leaf modifications­ small, thick leaves, thick cuticle ­stomata on lower side of leaf, located in depressions ­some desert plants shed leaves during dry months ­have fleshy stems that store water Feb 4­3:36 PM 33 phloem transport = translocation (transport of organic nutrients) use sieve tube members ­phloem sap movement is variable, but always from a sugar source to a sugar sink sugar source = plant organ this produces sugar by photosynthesis or breakdown of starch ex. mature leaves sugar sink = an organ that consumes or stores sugar ex growing roots, buds, stems or fruits tubers or bulbs may be source or sink depending on season ­sugar sink receives sugar from nearest source ex. upper leaves provide stem with sugar lower leaves provide roots with sugar ­neighboring sieve tubes may flow in opposite directions Feb 4­3:40 PM 34 loading of sucrose into phloem Feb 4­3:48 PM 35 ­sap flows through sieve tube by bulk flow moved by positive pressure ­pressure at source end and low pressure at sink end cause water to flow from source to sink, carries sugar along with it ­xylem recycles the water Feb 4­3:49 PM 36 tapping phloem sap with the help of an aphid Feb 6­9:56 AM 37 Apr 14­8:03 AM 38.