Transport in plants:

• Needs:

  Metabolic demands:

  • internal and underground parts of the plant don't photosynthesis but still need glucose and oxygen therefore it needs to be transported from photosynthetic areas and waste must be removed

  SIZE:

  • Can be very large so need effective transport systems to move substances over large distances

  SA:V :

  • when considering organisms SA:V is small so cannot rely on diffusion alone to supply cells with all they need
• Xylem:

  Transports water and mineral ions from root up the plant

  • made of dead cells
  • cell walls strengthen from lignin (waterproof) arranged in spirals so is flexible
  • some areas don't have lignin forming a non-lignified pit
  • pits in 2 adjacent vessels are aligned so water can move across from one to another allowing water to leave vessels and go to where it is needed
  • Has parenchyma acts as packaging tissue to separate and support xylem
• Phloem:


  Made of living tissue

  • transports assimilates up and down plants
  • provides cells with sugar and amino acids
  • made of sieve tube elements joined end to end forming long tubes
  • sieve plates allow movement from one element to another
  • Sieve plates support tube and keep lumen open
  • have companion cells attached via plasmodesmata
  • companion cells have loads of mitochondria to produce ATP for active processes
• Movement of water:

  water enters via root cell.

  root cell has a:

  • large SA: V meaning increases SA so more water is absorbed
  • many of them so increasing water absorbed
  • Thin membrane leading to a short diffusion pathway
• Movement across the root:

  Symplast:

  • water moves through cytoplasm through plasmodesmata by osmosis
  • Root hair cell takes in water so has a higher water potential than the next cell along

  So water moves by osmosis to the next cell along, increasing its water potential. The root hairs water potential decreases so takes in more water from the soil

  • Repeats until xylem is reached
• Movement of water across the root:

  Apoplast:

  • Moves through cell wall and intercellular spaces through the open structures of cellulose fibres
  • Water moves into xylem, more water follows due to cohesive forces
  • Creates tension and continuous flow of water
• Movement of water into xylem:

  • Water that has moved through the root meets the endodermis (tissue surrounding vascular bundle)
  • The casparian strip surrounds endodermis tissue. This is waxy and waterproof
  • H20 in the apoplast pathway is forced into the symplast pathway (cytoplasm) via selectively permeable cell membranes- any toxic substances from soil stopped.Solute concentration in the cytoplasm of endodermal cells is lower than that in the xylem. The endodermal cells actively transport mineral ions into the xylem. Endodermal cells have a higher water potential that the xylem.
• Movement of water into xylem:

  • The active pumping of mineral ions into xylem moves water in by osmosis, resulting in root pressure. Gives the water a push up the xylem
• Evidence of active transport in root pressure:

  • Poison the mitochondria with cyanide, root pressure disappears
  • Root pressure increases with temperature, so chemical reactions are improved
  • If oxygen levels fall/ respiratory substrates fall, root pressure falls. Therefore respiration involved (active)
• Transpiration:

  • CO2 moves from air into leaf and 02 moves from leaf to air through pores-stomata (mostly on underside of leaf, open and closed by guard cells)
  • Stomata open, gas is exchanged, water vapour also moves out by diffusion and is lost (water loss from stems and leaf as a result of evaporation from cell surfaces- transpiration)Stomata open and close to control water loss, however, some stomata always need to be open for respiration and photosynthesis so water is lost
• Evidence for cohesion tension theory:

  • Changes in diameter of treesTranspiration at its highest (day) tension in xylem is highest, so tree shrinks in diameter.
  • At night, less transpiration, lower tension in xylem, so tree diameter increases
  • Break xylem vessel
  • When you cut flower stems to put them in water, air is drawn up, rather than water leaking out - shows that there is a pull upwards so capillary action draws water upwardsOnce xylem broken, plant no longer pulls water up as the continuous stream of water molecules is broken
• Factors affecting transpiration:

  1. Light

  • Increase light intensity, more stomata open, so more water diffuses out, so more evaporation from leaf surfaces- increased rate of transpiration
  • 2. Humidity
  • High humidity, lower rate of transpiration- reduced water potential gradient between inside of leaf and outside of air
• Factors affecting transpiration:

  1. Temperature

  • Increase temp, increase KE of water molecules, so increases rate of evaporation from spongy mesophyll cells into the air spaces of the leaf

  • Increase temp, increases the volume of water vapour that air can hold, therefore decreases relative humidity- increase rate of transpiration

  2. Air movement

  • Water vapour close to leaf is quickly moved away, so maintains water potential gradient- increases rate of transpiration

  3. Soil water availability

  • V dry, decrease rate of transpiration
• How sucrose entered phloem

  APOPLAST ROUTE:

  1. Companion cells pump

  H+ out of cytoplasm and into cell walls using proton pump (using ATP)

  2. Large [H+] in companion cells walls, so H+ diffuses back into the companion cell down concentration gradient

  3. To do this, it diffuses through a cotransporter protein. As H+ diffuses back into companion cell, sucrose is co-transported against its concentration gradient back into the companion cell

  4. This sucrose then moves out of companion cells, through plasmodesmata, into sieve tubes of phloem
• Mass flow - moves assimilated to sinks:

  Sucrose builds up in companion cells and sieve tube elements, so they have a low water potential

  Therefore water diffuses in from an area of high water potential

  • This causes a build up of turgor pressure
  • So water (carrying the sucrose) moves into the phloem, reducing the pressure in the companion cells
  • The water then moves up/down plant by mass flow to areas of low pressure (sinks)
• Phloem unloading:

  Sucrose diffuses from phloem to surrounding cells

  Sucrose is quickly: moved into another cell by diffusion, or converted into another substance (glucose for respiration, starch for storage)

  1,°

  • Therefore, sucrose concentration is maintained, sinks always have lower sucrose concentration than phloem

  > When the sucrose leaves the phloem, the phloems water potential rises

  So water diffuses out of phloem into surrounding cells/ pulled into transpiration stream
• Xerophytes:

  • Thick waxy cuticleSunken stomata
  • Reduce air movement, create microclimate of still, humid air so reduces water potential gradient, so reduces transpiration

  Reduced number of stomata

  • Reduce transpiration, however also reduced gas exchange capabilities

  Reduced leaves

  • Smaller SA:V so reduce transpiration

  Hairy leaves

  • Trap air, creates microclimate of still, humid air, so reduces water potential gradient, so minimising transpiration
• Xerophytes:

  Curled leaves

  • Microclimate

  Succulents

  • Store water in times of plentiful supply and use in drought

  Leaf loss

  • Lose leaves when water not available, decrease metabolic needs

  Root adaptations

  • Long tap roots- reach deep water stores
  • Widespread, shallow roots- get water as it rains
• Phloem:


  Sieve tube elements are living cells have no nucleus and have very few organelles

  have perforated bed walls to assist in mass transport of fluids

  As they lack organelles they depend on the companion cells (next to them) to provide ATP needed for active transport of organic substances into the sieve tube elements
• Transport of water:


  from soil to root hair cell

  • have thin walls to reduce the diffusion pathway
  • have large surface area to absorb as much water as they can