transport in plants

• xylem

  • non living tissue (lignin kills cells)
  • transports water and minerals upwards
  • supports the plant
• xylem parenchyma

  • packs around xylem vessels storing food
  • contains chemical tannin which protects plant tissue from herbivore attack
• lignin

  • lignified walls provide extra mechanical strength
  • come in forms of rings, spirals or solid tubes with unlignified areas
• phloem

  • living tissue
  • transports sugars and amino acids up and down the plant
• sieve tube elements

  • in areas between cells wall becomes perforated to form sieve plates
  • sieve plates allow contents to flow through
  • organelles break down filling phloem will phloem sap
• companion cells

  • linked to sieve tube elements by many plasmodesmata
  • function as a 'life support system' for sieve tube cells
  • nucleus and organelles are maintained
• importance of water in plants

  • raw material for photosynthesis
  • transports mineral ions
  • evaporation keeps plants cool
  • turgor pressure drives cell expansion
  • turgor pressure provides support to stems and leaves
• root hair adaptions

  • microscopic size
  • concentration gradient maintained
  • thin layer
  • large SA:V
• symplast pathway

  -continuous cytoplasm of the living plant cells (connected by plasmodesmata)

  1. root hair cell has higher water potential than the next one along
  2. continuous flow until xylem is reached
• apoplast pathway

  -cell walls and intercellular space

  1. water molecules move into xylem
  2. water pulls more molecules behind them (cohesion)
  3. tension is created by cohesion, this causes continuous flow
• movement of water into xylem

  casparian strip- waterproof layer that runs around endodermal cell

  1. water in apoplast cant go through casparian strip so its forced into the cytoplasm (joining the symplast pathway)
  2. water potential is lower in the xylem than the endodermal cells so the rate of osmosis increases
  3. once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem
  4. root pressure gives water a push up the xylem
• evidence for active transport in root pressure

  • if poisen applied to roots, root pressure disappears (cyanide affects ATP production)
  • temperature adjusts to root pressure- suggesting chemical reactions are involved (warm- high pressure, cold- low pressure)
  • if oxygen levels fall, root pressure falls
  • xylem sap may ooze from stems
• transpirtation

  • the loss of water vapour from the leaves and stems as a consequence of gas exchange
• transpirtation stream

  • responsible for moving water from the roots up through the plant
• transpiration process

  1. water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and then out the stomata
  2. water loss lowers cell water potential
  3. water moves in from adjacent cells by osmosis (along apoplast and symplast pathways)
  4. repeated across the leaf to xylem
• cohesion

  • hydrogen bonds within water molecules
• adhesion

  • hydrogen bonds with carbohydrates
• cohesion tension theory

  1. combined action of cohesion and adhesion causes capillary action
  2. the continuous water stream is the result of cohesion (transpiration pull)
  3. the pull results in tension that helps water move across the roots
• capillary action

  • the process of water rising up a narrow tube against gravity
• evidence of cohesion tension theory

  • when xylem vessels are broken, air is drawn in rather than water rushing out
  • when the xylem is broken the water can no longer move up the stem (continuous water stream has been broken)
• stomatal control

  • stomata opening/ closing is controlled by the turgor of guard cells

  opening

  • inner walls of guard cells less flexible than outer wall.
  • cells become bean shaped
  • this opens the pore

  closing

  • signals from roots can trigger turgor loss from guard cells
  • this closes the pore
• factors affecting rate of transpiration

  light

  • dark- close
  • light- open

  air movement

  • wind increases rate

  humidity

  • high- decreases rate
  • low- increases rate

  soil water availability

  • dry soil- decreases rate

  temperature

  • high- increases rate
  • low- decreases rate
• translocation

  • plants transport organic compounds in the phloem from sources to sinks
• assimilates

  • products of photosynthesis that are transported around a plant
• source

  • regions of a plant that produces assimilates
• sink

  • regions of the plant that require assimilates to supply their metabolic rate
• phloem loading by the apoplast route

  1. sucrose from the source travels through the cell wall, inner cell spaces to the companion cells, sieve tube elements by diffusion
  2. in companion cells, sucrose moves across cell membrane into cytoplasm
  3. hydrogen ions actively pumped out of companion cells into surrounding tissue ATP
  4. hydrogen ions return to companion cells down a concentration gradient to transport proteins
  5. water moves in by osmosis, increasing turgor pressure. water carrying assimilates moves into sieve tube elements and moves up/ down by mass flow
• phloem unloading

  1. sucrose is unloaded at any point needed
  2. main mechanism of movement is diffusion
  3. once sucrose moves into cells, it rapidly diffuses into surrounding cells to maintain concentration gradient
  4. loss of sucrose from phloem leads to an increase in phloem water potential
  5. water moves out to surrounding cells by osmosis
• evidence for translocation

  • companion cells adapted for active transport
  • if mitochondria of the companion cells are poisened, translocation stops
  • flow of sugars is faster than it would be by diffusion alone (suggests active processes drives mass flow)
• xerophytes

  • plants adapted to dry conditions
  • cacti
• hydrophytes

  • plants adapted for aquatic conditions
  • water lilies
• xerophyte adaptations

  • reduced number of stomata (reduces water loss by transpiration)
  • reduced leaves (reduced SA:V minimises water loss by transpiration)
  • long roots (widespread roots absorb rain water before evaporation/ access water below surface)
  • succulents (store water in parenchyma tissue in their stem and roots)
  • sunken stomata (microclimate of humid moist air reduces water gradient which reduces rate of transpiration)
• hydrophytes adaptations

  • many 'aways open' stomata (maximises gas exchange)
  • small roots (water can diffuse directly into stem and leaves)
  • air sacs (enables leaves and flowers to float on water surface)
  • wide flat leaves (spread on the surface to capture sunlight)
  • thin/ no waxy cuticle (no need to conserve water)