Transport in Plants

• what are the three needs for a transport system in plants?

  surface area:volume

  size

  metabolic rate
• Surface Area : Volume

  plants are multicellular, so have a small SA:V

  cannot solely rely on diffusion across their outer surface to supply their cells
• Size of plant

  some plants are really tall so diffusion distance is too great

  need effective transport systems to move substances up and down from the tips of root to leaves and stems
• Metabolic Demands

  Underground parts of the plant can't photosynthesise but still need glucose transported to them and waste removed

  Hormones made in one part of the plant need to be transported to where they will have an effect

  Mineral ions need to be transported from the root hair cells to all cells in the plant to make proteins
• smaller/single celled plants v larger multicellular plants

  SMALLER/SINGLE CELLED: have a large surface are to volume ratio as well as short diffusion distance, so DO NOT NEED A TRANSPORT SYSTEM

  LARGE/MULTICELLULAR PLANTS: have a small SA:V and long diffusion distance DUE TO SIZE, SO NEED A TRANSPORT SYSTEM
• GAS EXCHANGE IN LARGE PLANTS

  - openings allow trees to get O2 for aerobic respiration, as plants do not have transport system/medium (blood) for oxygen

  - sometimes, the stomata are closed to prevent loss of water/oxygen

  - in winter, deciduous trees lose leaves - LENTICELS PERFORM GAS EXCHANGE
• food store in DICOTS

  x2 cotedylons
• food store in MONOCOTS

  endosperm
• Draw the vascular tissues of a ROOT

  Xylem and phloem are VASCULAR TISSUES involved in MASS FLOW

  • the Vascular tissues are in the centre of the root to help the plant withstand the tugging strains when the leaves/stem when being blown in the wind

  
• What do the phloem and xylem transport and what is this an example of?

  PHLOEM: transports sugars/sucrose as well as other assimilates (amino acids) UP AND DOWN the plant

  XYLEM: transports mineral ions (NO3^-1 and Mg^2+) up the plant

  BOTH AN EXAMPLE OF MASS FLOW
• Draw vascular tissues of a STEM

  Plant stem cells: meristematic tissue found in the growing parts of a leaf that can divide by mitosis and differentiate into many cell types

  CAMBIUM: unspecialised stem cells can cam differentiate into xylem and phloem

  VASCULAR TISSUES are around the edge to strengthen and support the root

  
• Draw the vascular tissues in a LEAF

  The midrib of a dicot leaf is the main vein carrying the vascular tissues and supports the structure of the leaf

  
• Rules for drawing tissues

  no shading

  must take up at least 50% of space provided

  label using pencil and ruler

  only draw as many cells as told

  label lines do not cross over

  no arrow heads

  label on outside of drawing, NOT ON

  no broken lines
• How to observe the position of xylem tissues in the leaf stalk of CELERY

  put leaf stalks/petioles in food colouring/dye

  then cut transversely

  OR

  cut a thin transverse/cross section

  add dye/stain

  OBSERVE UNDER A MICROSCOPE UNDER LOW POWER
• Phloem tissue and PLASMODESMATA


  companion cells and sieve tube elements are linked by microscopic channels through cellulose cell walls called PLASMODESMATA - link the cytoplasm of adjacent cells
• PHLOEM TISSUE

  SIEVE TUBE ELEMENTS:

  • living elongated cells, with little cytoplasm, joined end to end to form a continuous tube/column so solutes can travel long distances
  • can be perforated so solutes can pass from cell to cell
  • have no nucleus and few organelles so more space for solutes to move

  COMPANION CELLS: controls the movement of solutes and MITOCHONDRIA provides ATP for active transport into sieve tube elements

  RIBOSOMES: make enzymes

  STE and CC linked by microscopic channels through the cellulose cell walls (PLASMODESMATA), which link cytoplasm of adjacent cells
• a plants transports sugars/assimilates around a plant via TRANSLOCATION
• SOURCE SITE

  where assimilates are loaded into the phloem (WHERE IT IS MADE)

  • leaf in summer
• SINK SITE


  where the assimilates are unloaded from the phloem (WHERE IT IS USED)

  • growing fruit
  • growing bud
  • growing shoots/roots
• source and sink sites in STORAGE ORGANS SUCH AS TUBERS

  may be referred to as a SOURCE when:

  • they are unloading stores at the beginning of their growth period

  may be referred to as a SINK when:

  • they are laying down their stores
• HOW DO WE KNOW PHLOEM IS BEING USED?

  if the phloem has been removed, sucrose cannot pass down

  this lowers the water potential in this area so water moves into the area via osmosis

  this causes the area to swell
• where is sucrose carried?

  in the phloem sap from source to sink
• why sucrose and not starch?

  starch is INSOLUBLE so won't affect the water potential/osmosis

  can't enter or leave cells

  makes sap more thick and viscous
• why is sucrose a more suitable transport molecule than glucose?

  loading/unloading is controlled by specific transport proteins

  sucrose is less reactive than glucose

  sucrose less likely to be used in the mitochondria

  sucrose is less likely to diffuse out of the sieve tubes as it is larger
• Translocation

  1. H+ ions actively transported (ATP) out of companion cell into source cell creating a conc gradient

  2. H+ ions facilitated diffusion with assimilates back into the CC, via co-transport proteins

  3. Ass diffuse out of the CC through plasmodesmata into the STE, lowering the WP causing H2O to move in via osmosis down a WPG, causing a high HP at top of STE

  4. H2O moves from a high to low HP (down) carrying ass with it (MASS FLOW can occur up or down)

  5. Ass move into CC and sink cell either by diffusion or AT (if high conc in sink cell - GROWING FRUIT)

  6. H2O moved down a WPG out of phloem via osmosis
• Describe how assimilates are loaded into the phloem

  hydrogen ions actively transported out of the companion cell into sink cell using ATP

  this creates a concentration gradient

  hydrogen ions move back into companion cells via facilitated diffusion along with assimilates, via co-transport using co-transport proteins

  Assimilates diffuse out of companion cells through the plasmodesmata, into the sieve tube element of the phloem
• EVIDENCE FOR COMPANION CELLS

  scientists noted that companion cells are more negatively charged than surroundings, and pH increases whilst outside decreases

  • H+ IONS MOVE OUT OF C CELLS

  when CC were treated with CYANIDE (aerobic respiration inhibitor), pH change didn't occur

  • H+ ions move out via AT which requires ATP from AR

  scientists looked at ultrastructure using an electron microscope, and found MITOCHONDRIA, PLASMODESMATA + INTRINSIC PROTEINS in CSM

  • mitochondria - ATP for AT
  • plasmodesmata - diffusion of assimilates into STE
  • intrinsic protein - co-transport of assimilates and H+ ions
• how we know ATP is needed


  companion cells have many mitochondria for ATP production

  if aerobic respiration stops, translocation stops

  PHLOEM MOVES ASSIMILATES 100,000x FASTER THAN DIFFUSION ALONE
• APHID STUDIES

  aphids pierce plant tissue with mouth part/stylet and reach the phloem

  
  

  if the aphid is anaesthetised and removed from the stylet, sap continues to flow out of the stylet due to pressure from phloem contents

  METHOD USED TO MEASURE FLOW RATE AND CONCENTRATIONS OF SUCROSE AT DIFFERENT PARTS OF THE PHLOEM

  
  

  Used to show how pressure and flow rate are lower in the sink than near source

  Used to show that concentration of sucrose is higher near source than sink
• Xylem

  Dead hollow cells with lignified walls and NO organelles
• Xylem structure

  Joined end to end with no end walls to form a continuous tube
• Xylem


  • Open ended: allow continuous transport of water and minerals
• Xylem walls

  • Lignified: strengthens/thickens xylem wall to prevent collapse when water is under tension
  • Water proofs wall to reduce lateral water loss through wall
  • Spiral pattern to allow vessel to stretch to prevent stem breaking
• Xylem walls

  • Adhesion of water molecules increase capillarity
• Xylem walls

  • Walls have bordered pits which allow water to move between vessels
  • Allows water to bypass an air lock/blockage
  • Allows water to be supplied to other tissues/part of the plant
• Why do plants need water?

  raw material of photosynthesis

  mineral ions and products of photosynthesis are transported in aqueous solution

  loss of water by evaporation/water vapour keeps a plant cool

  turgor pressure (hydrostatic pressure) supports the stems and leaves and prevents plant from wilting
• Transpiration definition


  evaporation of water/loss of water vapour from aerial parts of the plant via the stomata
• transpiration STREAM definition

  movement of water up the xylem vessels from roots to leaves and then to air surrounding the leaves
• Transpiration stream

  1. H2O enters xylem by osmosis from a high WP in the soil to a low WP in the root hair cells

  2. Active transport of mineral ions into root cells/xylem creates root pressure/high HP at the bottom of the xylem

  3. Transpiration/evaporation of H2O from the leaf mesophyll cells

  4. Diffusion of water vapour via stomata from a high to low WVP gradient, causing lower HP at top of the xylem

  5. h2o under tension (cohesion between h2o molecules H-Bonds) and (adhesion of h2o molecules to xylem walls)

  6. causes h2o to move up xylem - MASS FLOW - from high to low HP (PG)
• Water under tension

  Cohesion between water molecules (H-BONDS)<|>Adhesion of water molecules and xylem walls