Chapter 9 Transport in Plants

• Metabolic demands


  Glucose and oxygen for oxygen.

  Removing waste products.

  Hormones from the site of production to site of effect.
• Vascular system


  A system of transport vessels in animals or plants.
• Vascular bundle


  The vascular system of herbaceous dicots, made up of xylem or phloem system.
• In low external water potential, water moves out of the cell. Plants can survive like this for short periods as they can shrink the cell membrane away from the cell wall. The cell is said to be plasmolysis.
• Hydrostatic pressure (turgor pressure) forms the hydro skeleton.
• Turgor drives cell expansion (i.e., driving roots through soil).
• Water cools plant by evaporations.
• Water transports mineral ions and products of photosynthesis around the plant.
• Water vapour evaporates out from cell walls and diffuses out through the stomata.
• Low hydrostatic pressure at the tope of the xylem vessels draw water up the stem as a continuous column from high to low water potential pressure.
• Water enters root hair cells by osmosis and diffuses across the cortex to the xylem vessels.
• Root hair cells


  Exchange surface in plants where water is taken into the plant from the soil. They contain higher concentrations of solutes than the soil.
• Root hair cells are the site of active transport of mineral ions into the root hair cell, lowering the internal water potential to increasse water uptake by osmosis.
• Adaptations of root hair cells:

  Large surface area; large numbers of protein pumps; membrane proteins for active transport; thin cell wall; lots of mitochondria.
• Water movement pathways


  • Symplast
  • Vascular
  • Apoplast
• Symplast pathway


  The movement of water and solutes through the cytoplasm of the cells via plasmodesmata via diffusion
• Each cell further away from the roots has a lower water potential so water is drawn up the plant.
• Plasmodesmata


  Small channels between cells which aid intracellular communication.
• Vascular pathway


  Water moves through the vacuoles in addition to the cytoplasm.
• Casparian Strip


  Found in the endodermis. Impermeable layer of suberin - waxy material.
• The function of the casparian strip


  To force water into the symplast pathway. This diversion means that water must pass through a selectively permeable membrane, excluding any toxic solutes in soil water from reaching living tissues.
• Endodermis


  The layer of specialised cells surrounding the xylem.
• Root pressure


  Active pumping of minerals into the xylem results in root pressure.
• Evidence for the role of active transport in root pressure

  • No root pressure forms in the presence of Cyanide, a respiratory poison
  • No root pressure forms in the absence of oxygen or respiratory substrates.
  • Root pressure increases with temperature increase and decreases with temperature drop (chemical reactions are involved).
  • Guttation – xylem sap is forced out of pores in the leaves when transpiration is low.-In cut rotted stems, xylem sap will seep out (only possible if there is an upwards force).
• Function of the Xylem

  Transports dissolved minerals and water around the plant.
• Function of the xylem's lignified cell wall

  Strength to withstand hydrostatic pressure so vessel does not collapse; impermeable to water.
• Function of the xylem having no end plates

  Allows mass flow of water and dissolved solutes.
• Function of the xylem having pits in its walls

  For lateral movement of water; allows continual flow in case of air bubbles forming.
• Function of the small diameter of the xylem

  Helps prevent the water column breaking; assists with capillary action
• The function of the phloem

  Transport organic compounds (assimilates) from the source to the sink. The transport can occur up or down the plant.
• The structure of the phloem

  The phloem is made up of a few different cell types, most of it being made up of sieve tube elements. Closely linked to the sieve tube elements are companion cells, which form with them.

  Sieve tube elements line up end to end, forming a continuous tube.
• Function of sieve plates with sieve pores in the phloem

  Allows for continuous movement of organic components.
• Cellulose cell walls in the phloem 


  Strengthens cell wall to withstand hydrostatic pressures.
• The function of phloem cells having no nucleus, vacuole or ribosomes

  Maximises space for translocation of assimilates.
• Companion cells are linked to the sieve tube elements by plasmodesmata. This is where organelles such as the nucleus are found.
• The function of the nucleus and other organelles present in a phloem cell.

  Metabolic to support sieve tube elements; helps with loading and unloading of assimilates.
• Function of transport proteins in phloem cells

  Move assimilates into and out of the sieve tube elements.
• Function of lots of mitochondria in phloem cells

  Provides ATP for active transport of assimilates into and out of companion cells.
• Function of the plasmodesmata


  The link to sieve tube elements allowing organic compounds to move from companion cells into sieve tube elements.
• Transpiration 


  The loss of water vapour from the stems and leaves of a plant as a result of evaporation from cell surface membranes inside the leaf and diffusion down a concentration gradient out through the stomata.