transportation in plants

• Why do plants need a transportation system?

  • All cells within the plant, need access to water/minerals/nutrients
  • Without transportation, these molecules are unable to move through the plant and reuces efficiency of plant growth/development
  • Sugars are produced easily by leaf (palisade) cells
  • Water/Minerals are absorbed easily by root cells
• Two adjacent systems within plants

  • Xylem tissue: Water- & water-soluble material is moved up from the roots
  • Phloem Tissue: Sugars travel from the source (leaf material) to respiring tissues or roots for storage (sink)
• Vascular Tissue within the Root

  • Endodermis
  • Primary Xylem
  • Primary Phloem
• Vascular Tissue within the Stem

  • Primary Phloem
  • Cambium
  • Primary Xylem
  • Medulla/Pith
  • Cortex
• structure to function of the Xylem

  1. Narrow tubes allow for transportation of water molecules via capillary action

  2. Gaps within the waterproof lignin allows for water to leak out into neighbouring tissues

  3. Rings/spirals of lignin allows for the xylem to stretch as the plant grows/bends

  4. Lack of cell content allows for ease of movement of water

  5. Thick lignin walls prevent the tubes from collapsing
• Transportation of water within cells

  • Apoplast Pathway - travels outside the cells
  • Vacuolar Pathway - travels through the vacuoles of each cell
  • Symplast Pathway - travels through the cytoplasm of each cell
• Photosynthesis

  6CO2 + 6H2O -> C6H12O6 + 6O2
• Movement from the root to the leaves

  Transpiration: Passive process which cools the plant, maintains and regulates osmotic pressure, enables mass flow of minerals
• Transportation across the root

  1. Casparian Strip: Active Transportation of minerals/salts across the membrane, lowering water potential and allowing for osmosis to occur

  2. Root Pressure: due to actively transported minerals forcing osmotic gradient

  3. Capillary Action: Cohesion of water molecules pulls the 'chain' of molecules by tension from above

  4. Transpiration pull: evaporation of water from cell surface reduces hydrostatic pressure
• Water loss by transpiration is unavoidable
• Plants must exchange gases via stomata


  • So, must be open during daylight
• Plants have evolved structural and behavioural adaptation to reduce this water loss


  • A waxy cuticle reduces non-stomata evaporation through the epidermis
  • Stomata are commonly found on the underside of the leaf to reduce direct heating evaporation effects
  • Stomata are closed at night as there is little need for gas exchange
  • Deciduous plants lose leaves in the winter as there is no available ground water due to frozen ground
• Plants adapted to reduce water loss to enable them to live in dry conditions


  • Sunken Stomata/presence of hairs creates a local humidity/decreases effect of air currents
  • Thicker waxy cuticle makes the epidermis more waterproof
  • Reduced number of stomata
  • Xerophytes
• Plants which live in water/high water areas

  • Thin or no waxy cuticle
  • Multiple always open stomata
  • Often on the upper leaf to maximise gas exchange
  • Small root systems
  • Large surface areas of underwater stems/roots
  • Hydrophytes
• The movement of sugars through to the phloem tubes

  1. H+ ions are actively removed from neighbouring companion cells

  2. Resulting in a proton-gradient which allows for the H+ ions to diffuse back through co-transporter proteins bringing sucrose through

  3. Sucrose is then actively loaded into the sieve tube, which reduces water potential, allows for more water to enter the sieve tube through osmosis increasing hydrostatic pressure allowing for sucrose movement
• At the sink

  1. Sucrose is used by the cells, both for storage as starch or directly for respiration

  2. This forms a sucrose gradient between the sieve tubes and the surrounding cells

  3. Allowing for the diffusion of sucrose out of the sieve tubes
• Mass Flow is bidirectional!
• Depends on where the sugars are being produced and where they are being used

  • Summer: Source: Sugars are produced in leaves via photosynthesis, Sink: Starch storage within the roots
  • Winter: Source: Sugars are produced in roots due to break down of Starch, Sink: Maintenance of plant + new leaves
• components of the vascular tissue within the leaf

  components:

  • waxy cuticle
  • upper epidermis
  • palisade mesophyll cell
  • vascular bundle: bundle sheath, phloem, xylem
  • spongy mesophyll cell
  • air space
  • lower epidermis
  • stoma with two guard cells