Transport In Plants

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Vascular Tissue

  • vascular bundle: 2 tissues: xylem & phloem
  • xylem in roots is central- resists pull so helps anchor root
  • xylem in stems is arranged in bundles-flexible support but resistant to strain of bending
  • vascular tissues in leaves is a network of veins for flexible strength and resistance to tearing
  •   (http://www.meritnation.com/img/study_content/curr/1/11/14/213/3426/NS_10-10-08_Bhuwan_11_Biology_6_12_NTH_LVN_html_m1c8fe96f.jpg)
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Structure Of Xylem

  • cells: vessels & tracheids
  • form a system of tubes through which water can travel
  • cellulose walls add lignin- strong hard substance
  • lignin builds up and cell contents dies leaving a hollow tube
  • lignin is impermeable to water and solutes
  • xylem transport and give mechanical strength
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Water Uptake by the Roots

  • large amounts of water lost through leaves in transpiration
  • greatest uptake happens in the roots, where surface area is increased by root hairs
  • soil water contains weak solution of mineral salts & has high water potential
  • vacuole of root has strong solution of dissolved solutes so it has a lower water potential
  • this allows water to move in down a concentration gradient via osmosis
  • then they travel across the cortex in 3 pathways
    • apoplast- cell wall
    • symplast- cytoplasm & plasmdesmata
    • vacuolar- vacuoles
  • apoplast is the fastest
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Root Pressure

  • xylem in centre surrounded by endodermis
  • endodermis has suberin which makes a casparian strip
  • suberin is waterproof and prevents use of apoplast pathway
  • water must travel through the symplast pathway
  • active transport of salts lowers the water potential in the xylem to allow water to move in via osmosis down a concentration gradient
  • the water potential gradient produced makes a force called root pressure
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Uptake Of Minerals

  • taken up via active transport through the roots from the soil
  • minerals move along apoplast pathway in solution 
  • enter cytoplasm when reach casparian strip 
  • ions must then be taken up by active trasport past the casparian band 
  • plant can be selective 
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Movement of Water from Roots to Leaves

  • transpiration pulls water up the stem
  • passive process 
  • water travels up stem xylem to leaves where it evaporates out as water vapour
  • transpiration draws up water molecules beneath it along same pathways as roots
  • transpiration pull= large cohesive forces between molecules and adhesive forces between water molecules and hydrophilic lining of the vessels 
  • maintain column of water in the xylem
  • cohesion tension theory
  • capillarity contributes to rise of water in xylem- water rises up narrow tubes by capillary action
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Transpiration

  • continuous losing of water to the atmosphere
  • 99% of water absorbed is lost through the leave
  • evaporation of water from the inside of leaves through stomata is transpiration
  • balance water uptake with water loss
  • loses too much water- wilts, cannot regain turgor and dies
  • stomata need to be open in the day for photosynthesis and gaseous exchange
  • most of the water is lost through stomata
  • 5% through epidermis but this is reduced by waxy cuticle
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Factors Affecting Transpiration

  • temperature
    • rise in temp provides additional kinetic energy for the movement of water molecules 
    • additional energy speeds up evaporation and rate of diffusion of watervapour through the stomata to the atmosphere
    • WP of atmosphere is lowered as its temperature rises- can hold more moisture
  • humidity
    • air inside leaf is saturated with water vapour
    • humidity outside the leaf varies
    • water potential gradient between leaf and atmosphere is great and when stomata are open water vapour rapidly diffuses from leaf
  • air movement 
    • transpiration in still air makes a layer of saturated air around leaf 
    • resistance to diffusion of water vapour 
    • reduces rate of transpiration
    • movement of air increases transpiration 
  • light intensity
    • controls degree of stomatal opening
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Potometry

  • potometers actually measure water uptake rates but since this water is mainly lost through transpiration anyway the rates are classed as the same
  • potometer can be used to measure water uptake by a leafy shoot under different conditions or used to compare different types of shoots
  • set up
    • cut leaf shoot underwater
    • fill apparatus completely with water so no air bubbles
    • use rubber tubing underwater to connect shoot to potometer
    • lift up potometer/shoot seal joints with waterproof jelly
    • dry leaves
    • introduce air bubble
    • measure air bubble travel in set intervals
    • use water reservoir to reset bubble
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Adaptations of Flowering Plants Mesophytes

mesophytes

  • habitats of adequate water supply
  • mainly crops of temperate regions
  • grow best in well drained soils and moderate dry air
  • loss of water prevented by closing stomata
  • water uptake at night makes up for losses during the day
  • many shed their leaves before winter
  • aerial parts can die off non wood plants but underground organs survive
  • annual mesophytes survive winter as dormant seeds
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Adaptations of Flowering Plants Xerophytes

xerophytes

  • low water availabilit
  • hot dry arid cold conditions
  • marram grass colonised sand dunes, with no soil, rapid drainage high wind speeds salt spray and lack of shade
  • marram grass adaptations
    • rolled leaves- thin walled epidermal cells in grooves shrink in lack of water causing leaf to roll into itself reducing the surface area from which transpiration can occur
    • sunken stomata- found in grooves on inner side of leaf in deep pits trapping humid air reducing wpg reducing rate of diffusion of water
    • hairs- stick and interlocking to trap water vapour reducing wpg
    • thick cuticle- waxy covering on leaf surface reducing water loss lowering cuticlar transpiration
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Adaptations of Flowering Plants Hydrophytes

hydrophytes

  • submerged or partially submerged in water
  • water lilly
  • rooted to mud at bottom of pond with floating leaves on the surface of water
  • water supports them- no need for lignified tissues
  • little need for transport tissues so xylem is poor 
  • little to no cuticle
  • stomata on upper surface
  • stems and leaves have large air spaces for bouyancy and reservoir of co2 and o2
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Translocation

  • products of photosynthesis transported in phloem away from synthesis site (source) to other parts where they are used- growth /storage (sinks)
  • transport of soluble organic materials eg sucrose/ amino acids 

phloem

  • sieve tubes adapted for longitudinal flow of material, formed from sieve elements placed end to end
  • ends perforated by pores= sieve plates
  • cytoplasmic filaments containing phloem proteins extedn through the pores between sieve tubes
  • sieve tubes have no nucleus or organelles so are accompanied by companion cells with dense cytoplasm, large central nucleus and an abundance of mitochondria 
  • connected to sieve tube by plasmodesmata
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Transport In Phloem

  • ringing experiments
    • cylinder sof outer bark removed from tree tissue (therefore phloem) contents above and below analysed later
  • radioactive tracing
    • aphids mouth stylet removed from head when feeding from sap
    • high pressure sap leaks through tube and is collected and analysed
    • found transocation to be too rapid a process to be explained by diffusion
  • radioisotope labelling
    • CO2 with radioactive carbon supplied to plant leaf 
    • carbon then fixed in a sugar in photosynthesis and its translocation can be traced using autoradiography
    • source and sink tissues placed on photgraphic film and developed
    • presence of radioactivity shows as fogging
    • found sugars are transported in both upwards and downwards
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Theories Of Translocation

mass flow hypothesis

  • passive mass flow of sugars from phloem in leaf where there is highest concentration to other areas of lower concentration such as growing tissues
  • sugar made at leaves causes WP to become more negative and water flows in, building hydrostatic pressure and forcing the sugars into the phloem joined to the sink
  • mass flow of solutes into the sink forced water out into xylem bringing water back to th leaves

arguments

  • rate of transport is way more faster than diffusion
  • doesnt explain the existence of sieve plates which in this theory would act as a barrier
  • sucrose and amino acids move at different rates and directions in the same tissue
  • phloem has high rate of O2 consumption, translocation is stopped in the presence of respiratory inhibitors
  • companion cells have load of mitochondria for energy but MFH has no explanation for this
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New Theories

  • active processes are involved
  • observed streaming in cytoplasm of individual sieve tubes= bi-directional movements
  • different solutes transported in different routes= protein filaments pass through sieve pores
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