3.3 Transport in Plants

• Transpiration


  The process by which plants release water vapor into the air through their leaves, involving xylem transport and stomatal opening.
• Function and features of the xylem

  • Transports mineral ions and water from the roots to the leaves
  • dead
  • no cytoplasm, no organelles and no cell wall at the ends
  • lignin in its cell wall
• what is lignification and what are the benefits?

  • kills cells and ensures maximum flow of water through a hollow vessel
  • waterproof - ensures all the water stays inside the xylem but preventing any leaving
• How are xylem vessels formed

  • cell begins to die
  • organelles are lost
  • cell wall begins to degrade
  • and lignin is deposited
• adaptation of xylem

  • narrow which generates a pressure to ensure the water only flows upwards
• Sclerenchyma 


  dead cells that provide support when plant mature - acts as a secondary wall of lignin
• Collenchyma

  living cells with uneven thick walls that allow the plant to bend without breaking
• Characteristics of Parenchyma cells


  Thin cell walls, living cells, and starch storage abilities.
• Cambium


  layer of partially undifferentiated cells between the phloem and xylem that supports growth
• adaptations of root hair cells

  • extends far into soil; large SA:V
  • no chloroplasts
  • lots of mitochondria
• symplast pathway 


  • only transports water
  • water from soil enters cell cytoplasm, leaves via the plasma membrane and travels to the next cell via the plasmodesmata
• Vacuolar Pathway 


  • water from soil enters cell cytoplasm and can enter the next cell via the plasmodesmata OR vacuoles
  • only water
• Apoplast pathway

  • water with dissolved minerals travels between the water filled holes in the cellulose cell wall
  • mass flow - mineral ions can be transported to the cortex
• brief outline of the transpiration stream

  root hair cell --> endodermis --> caspariain strip --> medulla --> pulled up the stem --> spongy mesophyll --> air space --> stomata --> out
• how does the RHC move water and minerals into the cell and across to the cortex to the medulla

  • actively pumps mineral ions into the cell which lowers the ψ
  • water follows by osmosis due a  ψ gradient
  • travels via the symplast/vacuolar pathway
  • if mineral ions dissolved then will enter the apoplast pathway
• Endodermis across Casparian strip?

  transporter proteins which actively pump in mineral ions across the casparian strip and lowers the  ψ

  water follows via osmosis

  even though new water potential has been created, water will not flow out due to casparian strip
• Medulla --> up the stem

  1. capillary action - adhesion between water molecules and xylem walls
  2. transpiration pull - cohesion and adhesion
  3. root pressure - pressure gradient created by the lost of water vapour at the leaves
• at the leaves what happens to the water

  • enters the spongy mesophyll via osmosis or apoplast pathway
  • evaporates
  • enters air space
  • Water vapour potential gradient created, moves from high (inside) to low (outside plant) exists via stomata
• What is a Xerophyte


  plant that is adapted to live in extremely dry conditions
• What is a hydrophyte

  plant that is adapted to live on top of water or in extremely wet conditions
• Adaptations of Marram Grass

  • rolled leaves - trap air - high water vapour potential outside the plant so little diffuse out, more water preserved
  • stomata in pits- creates moistures
  • hairs on the lower surface - decrease the movement of air
• Adaptations of Cacti 


  • store water in the stem and can extend to store more
  • spines instead of leaves - reduces surface area
  • Widespread, extensive network of roots - maximises water uptake and when it rains innit
• Adaptations of water lilies

  • large air spaces in the leaves - keeps plant afloat, allows for more absorption of sunlight
  • large air space in The stem - Buoyancy, oxygen can diffuse quickly to the roots for aerobic respiration (ATP)
  • stomata on epidermis allows contact with the air - gas exchange
• Adaptation of companion cell

  cytoplasm dense with mitochondria which produce ATP for aerobic respiration
• Describe translocation at the sources

  ATP pumps protons OUT of the companion cell into other cells - creates a proton gradient. so they diffuse back in but with a sucrose/amino acid (aided by a co transporter protein). Concentration of sucrose/ amino acid in the companion, assimilates diffuses out
• Translocation at the sinks

  • cells use sucrose and concentration of it will decrease
  • sucrose moves out of the phloem into the cell
  •  ψ in phloem increases so water moves into phloem
  • Hydrostatic pressure is low
• Explain how mass flow of phloem sap occurs in plants (3)


  • sucrose in the STE enters the phloem and decreases the  ψ
  • water enters STE, hydrostatic pressure increases
  • sucrose leaves the STE and increases the  ψ, hydrostatic pressure decreases
  • sap moves down a pressure gradient
• Translocation at the sources

  H+ pumped out of companion cell and diffuse back in due to a concentration gradient. They bring with them sucrose or amino acid with the help of co-transporter protein. Concentration of sucrose increases
• Translocation at the sinks

  Cells use sucrose and so the concentration of sucrose decreases.

  Sucrose moves out of the phloem. ψ in phloem increases. water moves out due to a ψ gradient. hydrostatic pressure decreases