Fact Recall

• How are plant sections viewed under the microscope?

  dissect a thin layer with a sharp blade

  stain with toluidine blue O for 1 min to stain xylem blue green

  view by mounting a section on the slide
• Describe root tissue

  epidermis cells extended into root hairs

  from there, outside to in = cortex, endodermis, casparian strip (Suberin) , pericycle.

  xylem forms a cross shape in the centre with phloem cylinders around the outside
• Describe stem tissue

  From the outside - epidermis, cortex ( made up of collenchyma and parenchyma ) , vascular bundles in a ring around an internal pith

  vascular bundles, outside to in - phloem , cabrium, xylem
• Describe function of xylem

  Transport of water and mineral ions by cohesion and tension mechanism

  water needed for metabolic reactions, transport of solutes and to create a pressure potential to support the plant
• Describe water uptake at the root

  Root hairs have a large SA to Volume ratio

  water enters down a concentration gradient by osmosis and crosses the root xylem via apoplast pathway ( non living spaces between cells and cell walls ) , symplast pathway ( living cytoplasm, plasmodesmata ) and vacuolar pathway
• Significance of the casparian strip

  Thickening of the Suberin which repels water and moves water from the apoplast to the symplast - all solutes must go through plasma membrane at this point which controls what substances get through
• Describe water uptake in xylem

  Cohesion tension mechanism - unidirectional flow up stem in xylem ( lignified for strength )

  water evaporates at leaf and column is pulled up due to cohesion between water molecules ( H bonds ) and adhesion between water molecules and xylem vessels which creates tension so column moves upwards from less to more negative potential
• How is water lost at the leaf?

  leaves have stomata which are pores that open for gas exchange - water evaporates from each stomata

  most plants have stomata under leaves to reduce water loss by transpiration
• How do stomata open and close

  Each stomata surrounded by two guard cells, when water is plentiful they are turgid. thicker inner walls bend less than the thinner inner walls which open the pores

  when water is scarce the guard cells are flaccid and stoma closes
• How does light effect the rate of transpiration?

  brighter the light the faster the rate of transpiration because light intensity increases , stomata opens for photosynthesis, water evaporates out of pores
• How does temperature affect rate of transpiration?

  Higher the temperature the faster the transpiration rate as increased heat energy gives particles more kinetic energy so water molecules diffuse more rapidly out of stomata and evaporate
• How does humidity affect rate of transpiration?

  drier the air around leaf the the faster the rate of transpiration because concentration gradient of water between inside and outside of the leaf is greater
• How does wind effect the rate of transpiration?

  Stronger the air flow around a plant the greater the transpiration rate because air which may be humid around the plant is moved away - maintaining a low concentration gradient of water next to the plant and increasing diffusion gradient
• How is the rate of transpiration measured?


  using a potometer — cut shoot at angle to increase SA for water uptake , underwater to prevent entry of air bubble, attach to tube underwater. keep air and watertight. dry leaves leave to acclimate and introduce air bubble and measure how this moves along a graduated as water evaporates from leaves
• What are the xerophytic adaptations of marram grass?

  stomata sunk into pits and spines or hairs on epidermis to trap moist air which reduces concentration gradient of water - reducing evaporation

  curled leaves, stomata inside, protected from wind which decreases rate of diffusion and evaporation

  thick waxy waterproof cuticle
• Xerophytic adaptations of cacti

  Spines not leaves so reduced SA for water loss

  close stomata at hottest parts of day when transpiration rate highest
• What is a CAM plant?

  open stomata at night to absorb carbon dioxide when less water is lost

  carbon dioxide stored as malic acid and broken down to release carbon dioxide for photosynthesis in the day when stomata shut to conserve water
• Hydrophytic adaptations of water lilies

  • Stomata on upper surface of floating leaves for gas exchange - lower surface submerged
  • flexible leaves/stems to prevent damage for water currents - water supports plant
  • air spaces in tissue to help them float and store oxygen for respiration - low oxygen levels under water
• Function of phloem

  Soluble products of photosynthesis called assimilates ( glucose changed into sucrose ) moved around the plant via phloem sieve tubes — translocation
• Structure of phloem

  Made up of sieve tubes ( no nuclei - few organelles ) joined end to end with perforated end walls called sieve plates and companion cells - have lots of mitochondria - energy for active transport

  plasmodesmata connect sieve tubes and companion cells
• Mass flow hypothesis

  Sucrose loaded into phloem sieve tubes via active transport using ATP from mitochondria in the companion cells at the source - causes water potential decrease - water enters via osmosis down concentration gradient which increases volume and hydrostatic pressure

  drives bulk movement to sink where sucrose unloaded - increases water potential so water leaves - decreasing hydrostatic pressure
• Describe mechanism of active loading

  ATP is used to actively transport H+ ions out of companion cells, this sets up a concentration gradient so that H+ ions re-enter passively via a co-transporter protein

  sucrose binds to this at the same time and is moved into the phloem against its concentration gradient
• Example of source and sinks

  If sucrose produced after photosynthesis in leaves - this is the source

  transported to roots (sink) doe storage of starch

  transported to new leaf to be turned into cellulose - root is now the source and leaf is the sink

  movement unidirectional in the phloem
• Evidence for mass flow

  Pressure in phloem seen as fluid leaves if a cut is made

  radioactive tracers been used to track movement from source to sink
• Similarities between xylem and phloem

  Both vascular tissue used for bulk transport - substances flow through cells
• Differences between xylem and phloem

  • xylem transports water and mineral ions - phloem transports organic solutes
  • phloem just used for transport - xylem is used for transport and support