transport in plants

• distribution of vascular tissue

  • xylem - transport of water
  • phloem - transport of assimilates
  • endodermis - sheath of cells surrounding the vascular bundle
  • pericycle - layer of meristem cells
• distribution of vascular tissue in young stem

  • xylem
  • phloem
  • cambium - layer of meristem tissue
  • parenchyma - packing and support tissue capable of cell division
  • collenchyma - cells provide structural support in growing shoots and leaves
• distribution of vascular tissues in the leaf

  • vascular bundle
  • branching network of veins help support and transport
  • palisade and spongy mesophyll are adapted for photosynthesis
  • within each vein the xylem is above the phloem
• structure of xylem

  • vessels - columns of fused dead cells
  • fibres - long dead cells with thick cell walls
  • parenchyma - living cells, forms packing tissues and stores food and tannins
• structure of xylem vessels

  • column of live cells with waterproof lignin
  • cell contents die and cell wall breaks down
  • lignin arranged in spirals, annular, reticulate patterns
• adaptations of xylem vessels

  • no end walls to impede flow
  • lignin prevents collapsing of walls
  • diameter most effective to maintain capillary action
  • lignin allows adhesion of water molecules
  • pits allow for sideways movement
  • arrangement of lignin allows for movement of the stem
• structure of phloem tissue

  • sieve tube elements - columns of cells that transport the assimilates
  • companion cells - linked to the sieve tube, contain dense cytoplasm and carry out metabolic reactions
  • parenchyma - packing tissue
  • fibres - thick lignin walls, dead and hollow
  • sclereids - structure and support, lignified
• sieve elements

  • thin layer of cytoplasm
  • thin cell walls
  • end walls are perforated forming sieve plates
  • no nucleus
• companion cells

  • linked to sieve elements by plasmodesmata
  • dense cytoplasm
  • large nucleus
  • more mitozhondira
• water pontential - the tendency of water to move out of a solution. the more solute in a solution the lower the water potential. water moves from a higher to low water potential.
• movement of water into the root

  • soil water potential is high due to the absence of the minerals
  • vacuolar sap and cytoplasm have low water potential
  • water moves in by osmosis
  • the cell becomes turgid
  • the adjacent cell will have a lower water potential
• apoplast pathway across the root

  1. water moves through the fluid filled space of cellulose cell walls which are fully permeable
  2. not osmosis as water not through membranes
  3. water moves up the xylem with cohesion
  4. stops at the endodermis - the casparian strip
• symplast pathway across the root

  1. water travels through the cytoplasm
  2. cells are connected by plasmodesmata
  3. osmosis present
• vacuolar pathway across the root - when water is not just confined to the cytoplasm
• the casparian strip

  cells in the endodermis contain a waxy layer of Suberin

  blocking the apoplast pathway and forcing water into the symplast pathway.

  mineral ions are actively transported into the xylem, water flows down the water potential gradient into the xylem
• root pressure - movement of water up the stem

  movement of mineral ions by active transport into the xylem at the roots can force water through the stem

  root pressure will be affected by metabolic poisons

  this cannot reach the top of tall trees
• transpiration pull - movement of water up the stem

  1. water molecules are attracted to each other by cohesion
  2. long column of water formed
  3. as water is lost at the top via transpiration the column is pulled up through the xylem
  4. lignin prevents the walls from collapsing
  5. if a column is broken water can flow into adjacent xylem vessels by the pits
• capillary action - movement of water up the stem

  1. adhesion between water molecules and the lignified xylem walls
  2. water can rise against the force of gravity
• mass flow - the overall movement of water and mineral ions from high hydrostatic pressure at the roots to a lower hydrostatic pressure at the leaves
• transpiration the loss of water vapour from the upper parts of the plant
• transpiration process

  1. water enters the leaves in the xylem, it is then passed into the mesophyll cells by osmosis (either symplast or apoplast)
  2. water evaporates from the surface of the mesophyll, water vapour collects in the air spaces, raising the water potential
  3. once the water vapour potential is higher inside the leaf than outside, water diffuses out of the leaf via the stomata
• factors affecting transpiration rate

  • number of leaves
  • number and size of stomata
  • presence of waxy cuticle
  • light
  • temperature
  • humidity
  • wind
  • water availability
• adaptations of xerophytes

  • leaves reduced to spines - conserve water
  • hairs - increase humidity around leaves
  • dense spongy mesophyll - less water to evaporate
  • rolled leaves - reduces the exposure of stomata to the air
  • sunken stomata - creates humid microclimate
  • thick waxy cuticle
  • succulents - water stored for later
• adaptions of hydrophytes

  • wide flat leaves
  • aerenchyma - large air spaces
  • stomata on upper surface
  • no waxy cuticle
  • small roots
  • air sacs
  • large sa of leaves and roots
• translocation - the movement of assimilates up and down the plant
• sources - assimilated loaded into the phloem

  • photosynthesising leaves
  • storage organs at the state of growth season
  • seeds when they germinate
• sinks - assimilates are removed from the phloem

  • roots when growing
  • any actively dividing cells
  • storage organs
  • fruit and flowers
• source - mass flow

  1. sucrose is actively loaded into the sieve tube elements
  2. reduces water potential of the sap
  3. water follows the sucrose into the sieve elements
  4. increases hydrostatic pressure
• mass flow - at sink

  1. sucrose diffuses out of sieve tube elements
  2. increases water potential of the sap
  3. water follows sucrose out of the phloem
• at the source - sucrose is moved across the leaf by the symplast or apoplast pathways. the sucrose then needs to be loaded into the phloem
• loading sucrose into the phloem - symplast route

  1.sucrose moves form photosyntheising mesophyll into companion cells by plasmodesmata
• loading sucrose into the phloem - apoplast route

  1. hydrogen ions are pumped out of the companion cells using atp
  2. h ions move down concentration gradient into companion cell through facilitated diffusion bringing with them sucrose molecules
• evidence for translocation

  • radioactively labelled carbon and tree ringing tells us the phloem is used
  • comapnion cells have many mitochondria and are affected by metabolic poisons