2. Water transport in multicellular plants

• WATER TRANSPORT IN PLANTS

  • Water is key in structure + metabolism of plants:

  1. Turgor pressure [hydrostatic pressure] = due to osmosis in plants it provides a hydrostatic skeleton to support the stems + leaves
  2. Turgor also drives cell expansion = force that enables plant roots to force their way through concrete
  3. Loss of water by = evaporation keeps plant cool
  4. Mineral ions + products of photosynthesis = transported in aqueous solutions
  5. Water = raw material for photosynthesis
• Movement of water into root

  • Root hair cells: are the exchange surface in plants where water is taken into plant from soil
  • Root hair: long + thin extension from root hair cell [a specialised epidermal cell found near growing root tip]
• Root hairs well adapted as exchange surfaces:

  1. Microscopic size = penetrate easily between soil particles
  2. Each microscopic hair = has a large SA:V ratio + there are thousands one ach root tip
  3. Each hair has a thin surface layer = diffusion + osmosis can happen quickly
  4. Concentration of solutes in cytoplasm of root hair cells = maintains water potential gradient between soil water + cell

  • Soil water has a low conc of dissolved minerals = high water potential + root hair cell contains sugars, mineral ions, aminos etc so has a low water potential
  • So water moves into root hair by osmosis
• Movement of water across root

  • Once water has moved into root hair cell = it continues to move across root to xylem in 1 of 2 different pathways:
  • The symplast pathway
  • The apoplast pathway
• THE SYMPLAST PATHWAY:

  • Water moves through symplast = continuous cytoplasm of plant cells that’s connected through plasmodesmata by osmosis
  • Root hair cell has a higher water potential than next cell along as water has diffused from soil
  • Water moves from root hair cell --> to next door cell --> until xylem reached by osmosis
  • As water leaves root hair cell by osmosis = steep water potential gradient maintained so water moves in from soil
• THE APOPLAST PATHWAY:

  • Water moves through apoplast = cell wall + intercellular spaces
  • Water fills spaces between network of fibres in cellulose cell walls
  • As water moves int xylem --> more water pulled through apoplast behind them due to waters cohesive forces
  • Pull from water moving into xylem and plant along with cohesive forces between water molecules = creates tension meaning there's a continuous flow of water through cell wall = offers no resistance
• MOVEMENT OF WATER INTO XYLEM

  • Water moves across root in apoplast + symplast pathways until it reaches endodermis
  • Endodermis: layer of cells surrounding vascular tissue of roots
  • Endodermis is particularly noticeable in roots due to effect of Casparian strip
  • Casparian strip: a band of waxy material called suberin that surrounds endodermal cells forming waterproof layer 
  • Water in apoplast pathway due to this is forced into cytoplasm of cell joining water in symplast pathway
• MOVEMENT OF WATER INTO XYLEM 2

  • Endodermal cells = have a low solute conc compared to xylem + they move mineral ions into xylem through active transport
  • xylem have lower water potential than endodermal cells
  • this increases rate at which water osmoses into xylem down WP gradient from endodermis through symplast pathway
  • Once inside vascular bundle = water returns to apoplast = to enter xylem + move up plant
  • Active pumping of minerals into xylem to produce water movement by osmosis causes = root pressure [not affected by transpiration]
  • Root pressure gives water push up xylem [not a major factor]
• Evidence for role of active transport in root pressure

  • Poisons [cyanide] affect mitochondria + prevent ATP production = if cyanide applied to root cells = no energy supply = no root pressure
  • Root pressure = increases with rise in temp = chemical reactions involved
  • If levels of o2/respiratory substrates fall = root pressure falls
  • Xylem sap forced out of pores in some conditions = guttation