Transport in plants

• multicellular plants need transport systems for -

  • metabolic demands - many underground areas of the plant need glucose, mineral ions, hormones and oxygen transported to them and waste materials removed.
  • Size - many plants are very tall and need substances to be transported to both the root tips and the topmost leaves.
  • Surface area : volume ratio - cannot rely on diffusion alone to supply their cells with everything they need.
• Transport systems in dicotyledonous plants - Contains a vascular system featuring the xylem and phloem held together in vascular bundles in leaves, stems and roots.

  The structures and functions of the xylem -

  • non-living
  • transport of water and mineral ions, and support
  • long hollow structures
  • spirals of lignin helps reinforce the lumen so it does not collapse.
  • Xylem parenchyma store food and contain tannin deposits which is a bitter chemical to prevent attack from herbivores.
• The structure and functions of the phloem -

  • Living
  • transports food in the form of organic solutes around the plant from the leaves.
  • supplies cells with sugars and amino acids for cellular respiration
  • sieve tube elements connected together to form the phloem tube.
  • areas between cells, sieve plates are perforated which let phloem contents flow through.
  • companion cells - linked to sieve tube elements by many plasmodesmata.
• Water transport -

  • turgor pressure (hydrostatic pressure) as a result of osmosis provides a hydrostatic skeleton to support the stems and leaves.
  • turgor also drives cell expansion - enables plant roots to force their way out of tarmac and concrete.
  • Loss of water by evaporation helps to keep plants cool
  • mineral ions and products of photosynthesis are transported in aqueous solutions.
  • water is a raw material for photosynthesis
• Movement of water into the root -

  • root hairs cells are microscopic so can penetrate easily between soil particles.
  • each hair has a large SA:V ratio and there are thousands on each growing root tip
  • Thin surface layer (cell wall and cell surface membrane) where diffusion and osmosis can take place easily.
  • concentration of solutes in cytoplasm of root hair cells maintains a water potential gradient between soil water and the cell. Soil water with higher water potential than cytoplasm so water moves in to the root hair cell by osmosis.
• The symplast pathway - the symplast is the continuous cytoplasm of living plant cells that is connected through plasmodesmata. the root hair cell has a higher water potential than the next cell along, this is a result of the water moving in from the soil which makes the cytoplasm more dilute. Water moves from the root hair cell into the next cell along by osmosis and this continues until the xylem is reached.
• The apoplast pathway - the apoplast is the cell walls and the intercellular spaces. Water moves into the xylem from the intercellular spaces and this in turn, pulls more water through the apoplast behind them due to cohesion. The pull from the water moving into the xylem and up the plant along with cohesive forces between the molecules creates tension meaning that there is a continuous flow through the open structure of the cellulose wall, which offers little or no resistance
• Movement of water into the xylem - water moves along the symplast and apoplast pathways until it reaches the endodermis. The casparian strip is a band of waxy material called suberin that forms an waterproof layer. Water in apoplast pathway can no longer move forwards and is forced into the cytoplasm, joining the water in the symplast pathway. In order to reach the cytoplasm, water must pass through the selectively permeable membrane, excluding potentially toxic solutes in the soil from reaching the tissues.
• Endodermal cells move mineral ions into the xylem by active transport. As a result, water potential of the xylem cells is much lower than the water potential in the endodermal cells and this increases the rate of water moving into the xylem by osmosis down a water potential gradient from endodermis through symplast pathway.
• Root pressure -

  • Some poisons (cyanide) affect the mitochondria and prevent production of ATP. If cyanide is added to root cells so no energy supply, root pressure disappears.
  • Root pressure increases with a rise in temperature and falls with a fall in temp, suggesting chemical reactions are involved.
  • If levels of oxygen or respiratory substrates fall, root pressure falls.
  • Xylem sap may exude from the cut end of stems at certain times. In natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions, overnight when transpiration is low (guttation).
• Transpiration - When stomata are open to allow an exchange of carbon dioxide and oxygen between the air inside the leaf and the external air, water vapour also moves out by diffusion and is lost. This loss is called transpiration. Transpiration is an inevitable consequence of gaseous exchange.
• Transpiration stream -

  • water molecules evaporate from surface of mesophyll cells into air spaces in the leaf and move out of stomata into air by diffusion down a concentration gradient.
  • Loss of water by evaporation from mesophyll cell lowers the water potential of the cell, water moves into cell from adjacent cell by osmosis (apoplast and symplast).
  • Repeated in leaf to xylem and water moves into cells by osmosis.
  • Water molecules form hydrogen bonds with carbohydrates in xylem (adhesion). Also forming bonds with other water molecules (cohesion). Water moves up the xylem against gravity.
• Cohesion- tension theory -

  • changes in diameter of trees. When transpiration is high, tree shrinks in diameter. When transpiration is low, diameter increases.
  • Xylem vessel is broken - air is drawn in to xylem rather than water leaking out.
• Stomata - rate of transpiration controlled by opening and closing of the stomata. Turgor-driven process. When turgor is low, asymmetric configuration of guard cell walls closes the pore. When the environmental conditions are favourable, guard cells pump in solutes by active transport, increasing their turgor. Hormonal signals trigger turgor loss from the guard cells, which close the stomatal pore and conserve water.
• Factors affecting transpiration -

  • light will open the stomata for gaseous exchange and most will close in the dark.
  • Relative humidity - amount of water vapour in the air. High humidity will lower rate of transpiration.
  • Temperature - increase in temp means increase in rate of evaporation. Increase in temp increases the concentration of water vapour that the air can hold before it becomes saturated.
  • air movement - wind will increase transpiration.
  • soil-water availability - amount of water available in soil can affect transpiration rate. Transpiration reduced if the soil is dry.
• Translocation -

  Main sources of assimilates in plants -

  • green leaves and stems
  • tubers and tap roots - storage organs
  • food stores in seeds when they germinate

  Main sinks in a plant -

  • roots that are growing/ actively absorbing mineral ions
  • meristems that are actively dividing
  • any parts of the plant that are laying down food stores - developing seeds, fruits, storage organs.
• Phloem loading - the movement of sucrose from the leaves to the rest of the plant. Apoplast route - sucrose from the source travels through the cell walls and cell spaces to companion cells+sieve elements down concentration gradient maintained by removal of sucrose into phloem. In companion cells, sucrose is moved into cytoplasm across cell membrane in active process. Hydrogen ions are actively pumped out companion cells into surrounding tissue by ATP. H+ return to companion cell via transport protein. Build up of sucrose = water in by osmosis.
• Phloem unloading -

  Main mechanism of unloading - diffusion of sucrose from phloem to surrounding cells. Rapidly moves through the cells by diffusion or is converted into other substances that the plant needs and so that a concentration gradient of sucrose can be maintained between the contents of the phloem and surrounding cells. Loss of solutes leads to rise in water potential of phloem, water moves out into surrounding cells by osmosis. Some water that carried the solute to the sink is drawn into the transpiration stream in the xylem.
• Xerophytes - adaptations to conserve water - live in dry areas.

  • Thick, waxy cuticle - minimise water loss
  • sunken stomata - microclimate of still, humid air
  • reduced number of stomata - reduces water loss by transpiration
  • reduced leaves - water loss greatly reduced by area
  • hairy leaves - microclimate of still, moist air
  • curled leaves - confines stomata in microclimate of still, humid air
  • succulents - store water in parenchyma tissue in stems and roots.
  • Leaf loss - preserve water by losing leaves when water is not available
  • root adaptations - getting lots of water from soil
• Hydrophytes - need to cope with living in water or wet conditions.

  • very thin/ no waxy cuticle - loss by transpiration not an issue
  • many always open stomata - maximising gaseous exchange
  • reduced structure - water supports leaves and flowers so no structure
  • wide, flat leaves - capture as much light as possible
  • small roots - water diffuses straight into leaf tissue
  • large surface area of stems and roots under water - maximises area for photosynthesis
  • air sacs - enable leaves and flowers to float to the surface
  • aerenchyma - large air spaces - buoyant leaves and stems