plants

• Structure of the root

  • Epidermis
  • Exodermis
  • Cortex (parenchyma)
  • Phloem
  • Xylem
  • Endodermis
• Structure of a stem

  • Epidermis
  • Collenchyma
  • Parenchyma cortex
  • Vascular bundle
  • Pith
  • Interfascicular cambium
  • Xylem (vessels, tracheids, fibres, parenchyma)
• Xylem


  Dead cells that transport water and minerals up the plant and provide mechanical strength and support as they are strengthened by waterproof lignin
• Transpiration


  The loss of water as water vapour, by evaporation and diffusion out of the open stomata, from the leaves of plants. It leads to the transpiration stream.
• Transpiration stream


  Water moves into the root and enters the xylem (root pressure). Cohesive forces between water molecules and adhesive forces between water molecules and the hydrophilic lining of the xylem create a transpiration pull as the water leaving the xylem into the leaf cells pulls on molecules below.
• Factors increasing transpiration


  • Lower humidity
  • Higher temperature
• Hydrophyte - water plants


  • Little/no waxy cuticle as no need to conserve water
  • Stomata on upper surface as lower surface submerged
  • Poorly developed xylem as no need to transport water
  • Large air spaces (aerenchyma) provide buoyancy and act as reservoirs of gas
• Mesophyte - live with adequate water


  • Close stomata at night to decrease water loss
  • Shed leaves in unfavourable conditions, e.g. winter
  • Underground organs and dormant seeds survive winter
• Xerophyte - water is scarce


  • Thick waxy cuticle reducing water loss by evaporation from epidermal tissue
  • Sunken stomata increasing humidity in an air chamber above the stomata, reducing diffusion gradient and therefore water loss
  • Rolled leaves - reduces area of leaf exposed directly to air
  • Stiff interlocking hairs trap water vapour inside rolled leaf, reducing water potential gradient and therefore water loss
• Translocation


  The phloem transports the products of photosynthesis from the source (the leaf) to the sink (area of use or storage). This is bidirectional through the phloem.
• Experimental evidence for translocation

  • Ringing experiments (removal of phloem) show accumulation of sucrose products on leaf side of the ring but none on root side
  • Using aphids to sample sap from the phloem
  • Radioactive labelling of carbon dioxide which will become incorporated into sucrose can be used to determine the rate of transport in the phloem
  • Sources and sinks can be determined by autoradiography using radioactively labelled carbon dioxide
• Theory of mass flow

  Sucrose made at source lowers water potential. Water enters cells and sucrose is forced into phloem (loading). This increases hydrostatic pressure and therefore mass flow occurs along the phloem to the root where sucrose is stored as starch, water potential is less negative and water moves into the xylem.
• Pathways for movement of water across the root cortex

  • Apoplast (from cell wall to cell wall)
  • Symplast (from cytoplasm to cytoplasm through plasmodesmata)
  • Vacuolar (from vacuole to vacuole)
• Endodermis


  Impregnated with areas of suberin called the casparian strip. This blocks the apoplast pathway, forcing water into the symplast pathway. Minerals are selected to move into the symplast by active transport. This sets up a water potential gradient with lower water potential in the xylem, so water moves in by osmosis resulting in a force called root pressure.
• Adaptations for transport in plants

  • Vessels
  • Tracheids
  • Phloem (sieve tubes carry sucrose and amino acids, sieve elements end in sieve plates containing pores through which cytoplasmic filaments extend linking cells, no other organelles are in the sieve elements, companion cells contain many mitochondria for ATP and the organelles for protein synthesis)
• Pathways for movement of water across the root cortex

  • Apoplast
  • Symplast
  • Vacuolar
• Root hair cell


  • Has a large surface area for the absorption of water
• Endodermis


  Impregnated with areas of suberin called the casparian strip
• Casparian strip


  Blocks the apoplast pathway, forcing water into the symplast pathway
• Minerals movement into the symplast


  Selected by active transport
• Minerals selected into the symplast

  Sets up a water potential gradient with lower water potential in the xylem
• Lower water potential in the xylem


  Water moves in by osmosis resulting in a force called root pressure
• Adaptations for transport in plants

  • Apoplast
  • Symplast
  • Vacuolar
• Apoplast


  Movement of water from cell wall to cell wall
• Symplast


  Movement of water from cytoplasm to cytoplasm through plasmodesmata
• Vacuolar


  Movement of water from vacuole to vacuole
• As temperature increases


  The kinetic energy of molecules, including water vapour, increases
• Kinetic energy of molecules increasing

  Molecules will diffuse and evaporate faster and osmosis will also take place at a higher rate
• Humidity


  If the air contains more water vapour, this decreases the water potential gradient and fewer water molecules can diffuse out of the leaves
• As wind speed increases


  Water vapour on the surface of the leaf is blown away
• Water potential gradient between the inside and outside of the leaf increasing

  There is a higher rate of diffusion of water molecules
• If the light intensity is higher

  The rate of photosynthesis in the guard cells of stomata increases and the stoma are more likely to open
• More stomata opening


  The higher the rate of diffusion of water molecules from the leaf
• Light intensity


  Affects the overall rate of photosynthesis, which uses water as a reactant
• Most water that a plant absorbs is lost during transpiration but some water is used in photosynthesis