Chapter 9

• Why do plants need transport systems?

  • metabolic demands are high
  • can grow to big sizes
  • certain parts of plant (e.g. trunks) have small SA:V ratio
• What is the vascular system?

  
  

  • series of transport vessels made up of xylem & phloem
  • these tissues are arranged together in vascular bundles in leaves, stems & roots of herbaceous dicots
• Vascular system in a stem

  in stem vascular bundles are around the edge to give strength & support

  A) epidermis

  B) cortex

  C) phloem

  D) xylem

  E) vascular bundle

  F) parenchyma (packing & supporting tissue)

  
• Vascular systems in roots

  in roots vascular bundles are in middle to withstand the tugging strains that result as stems & leaves are blown in the wind

  A) root hair

  B) exodermis

  C) epidermis

  D) endodermis

  E) xylem

  F) cortex

  G) phloem

  
• Vascular system in leaves

  • in leaves the midrib of a dicot leaf is the main vein carrying vascular tissue through the organ
  • also helps to support structure of the leaf
  • many small branching veins spread through the leaf functioning in both transport & support

  A) palisade mesophyll

  B) xylem

  C) phloem

  D) midrib of leaf

  E) vascular bundle

  
• Key features of xylem

  • dead cells have been fused to form hollow vessels which are strengthened by lignin - prevent collapse
  • pits in xylem wall enable water to leave xylem & move into other cells of plant
  • xylem parenchyma cells store food
  • xylem fibres add strength

  
• Functions of xylem 


  • transport of water & mineral ions up a plant
  • provides support for plant
• Key features of the phloem

  • sieve tube elements - living cells joined end to end, forming tube w sieve plates between cells - allow phloem contents to flow through
  • companion cells carry out all metabolic functions (as sieve tubes lack nuclei) - linked to sieve tubes by plasmodesmata

  
• Function of phloem 


  transport of solutes (e.g. sugars & amino acids) up and down a plant
• How are root hair cells adapted to take up water from soil?

  • small size - able to penetrate between soil particles
  • long & narrow - large SA:V ratio
  • concentration of solutes in roots maintains water potential gradient between soil water & cell
  • thin layer - diffusion & osmosis happen quickly
• How does water move into root hair cells?

  
  

  • water moves into root hair cells by osmosis
  • soil water = high ψ - low conc dissolved minerals
  • root hair cell = low ψ - high conc solvents (sugars, minerals etc)
• Apoplast pathway

  movement of water through root hair cells where water moves through cell walls & intercellular spaces
• Symplast pathway 


  movement of water through root hair cells where water moves through cytoplasm via plasmodesmata
• Movement of water into xylem

  • water reaches endodermis - at this point water in apoplast pathway can go no further so Casparian strip forces it into cytoplasm, joining symplast pathway
  • endodermal cells move mineral ions into xylem by active transport - as result ψ in endodermal cells is higher than in xylem - this inc rate of water moving into xylem by osmosis
  • once inside vascular bundle water returns to apoplast pathway to enter xylem & move up plant
  • active pumping of minerals into xylem to produce movement of water by osmosis results in root pressure which helps force water up a stem
• Endodermis 


  layer of cells surrounding the vascular tissue
• Casparian strip 


  • band of waxy material called suberin that runs around each endodermal cell forming a waterproof layer
  • prevents toxic solutes continuing to move into plant
  • stops water from returning to root cortex from xylem vessels
• Label diagram about movement of water into xylem

  A) xylem

  B) water drawn up transpiration stream

  C) casparian strip - prevent water moving across cell wall

  D) endodermal cells

  E) root cortex cell

  F) root hair cell

  G) apoplastic pathway

  H) symplastic pathway

  
• Evidence for role of active transport in root pressure

  • some poisions (e.g. cyanide) affect mitochondria & prevent production of ATP - if cyanide is applied to root cells so there is no energy supplies, root pressure disappears
  • root pressure increases w a rise in temp & falls w fall in temp - suggesting chemical reactions are involved
  • if levels of oxygen or respiratory substrates fall, root pressure falls
• In most circumstances root pressure is not the major factor in movement of water from roots to leaves
• Transpiration

  evaporation of water from a plant's leave
• What adaptation to leaves have that prevents them losing water rapidly & constantly by evaporation from their surfaces?

  waxy cuticle
• Where does CO2 & O2 move from & to in plants?

  • CO2 moves from air into the leaf
  • O2 moves out of leaf
• How do gases move in & out of a leaf? What does this then cause?

  • stomata open & close by guard cells
  • when stomata are open to allow exchange of gases, water vapour also moves out by diffusion & is lost
  • loss of water = transpiration - consequence of gas exchange
• Transpiration stream - process

  • water leaves plant by transpiration
  • loss of water lowers ψ - so water moves into cell by omosis along apoplast & symplast pathways
  • water molecules cohere to each other (attracted to each other through hydrogen bonding) & adhere to walls of xylem vessels
  • combined effect of adhesion & cohesion results in capillary action - process water can rise up a narrow tube against force of gravity
  • water is drawn up xylem in a continuous stream to replace water lost by evaporation - this is the transportation pull

  
• What does the transpiration pull result in?

  tension in the xylem which in turn helps to move water across the roots from the soil
• Evidence for cohesion-tension theory

  • trees become narrower when they transpire - explained by increased tension in xylem vessels during high rates of transpiration
  • air is sucked up (rather than water leaking out) when a stem is cut
  • water is no longer moved up a broken stem because air that is pulled in breaks the transpiration stream (i.e. no longer continuous chain of water molecules)
• What problems are there with transpiration?

  • in high intensity sunlight when plant is photosynthesising rapidly, there will be a high rate of gas exchange & stomata will all be open
  • plant may lose so much water through transpiration that the supply cannot meet the demand
• When do the stomata close?

  • when water becomes scarce
  • hormonal signals from root triggers turgor loss from the guard cells - closes stomata & conserves water

  
• When do stomata open?

  • allow exchange of CO2 & O2
  • leads to loss of water
  • guard cells pump in solutes by active transport increasing their turgor
  • thickened inner wall of guard cell is less flexible than outer wall - cell becomes bean-shaped & opens the pore

  
• Factors affecting transpiration 


  • light intensity
  • temperature
  • humidity (of air)
  • air movement
  • number of leaves
  • number of stomata
  • thickness of cuticle
• How does light intensity affect transpiration?

  • light required for photosynthesis - stomata open in the light
  • inc light intensity = inc no. open stomata = inc rate of water vapour diffusing out = inc transpiration
• How does air humidity affect transpiration?

  • measure of amount of water vapour in the air
  • high humidity reduces water potential gradient between leaf & air
  • as humidity increases, transpiration rate decreases
  • lower humidity = higher transpiration rate
• How does temperature affect transpiration rate?

  • increases kinetic energy of water molecules - increases evaporation = inc transpiration rate
  • inc in temp also inc conc of water vapour so decreases humidity & its water potential
• How does the air movement affect transpiration rate?


  • each leaf has layer of air around it trapped by shape of leaf & hairs on surface of leaf decrease air movement close to lead
  • water vapour that diffuses out of lead accumulates here & so water vapour potential gradient around stomata increases, in turn inc diffusion gradient
  • air movement will inc rate of transpiration
  • still air reduces rate of transpiration

  
• How does soil-water availability affect rate of transpiration?

  • more water = higher transpiration
  • if v dry plant will be under water stress & rate will be reduced
• How does number of leaves affect rate of transpiration

  • affects surface area available for loss of water vapour
  • more leaves = higher rate
• How does number of stomata affect transpiration rate?

  • affects how much water is able to diffuse from leaves
  • more (& larger) stomata increase rate
• How does thickness of cuticle affect rate of transpiration?

  • waxy cuticles reduce water loss
  • thinner/ no cuticle = transpiration rate
• How do you measure transpiration?

  • measure water uptake using potometer
  • measure distance moved by the air bubble after a set time
  • rate of water uptake = distance moved by air bubble/ time taken for air bubble to move that distance

  
• Considerations when using potometer

  • stem must be cut underwater to avoid introducing air bubbles to the stem
  • care must be taken not to get water onto the leaves