transport in plants

• multicellular plants need transport systems for:

  • metabolic demands
  • size
  • SA:V ratio
• Turgor pressure (hydrostatic pressure)= pressure exerted by water molecules on the cell wall
• The loss of water by evaporation helps keep plants cool.
• Root hair cells:

  • specialised epidermal cells
  • microscopic size so can penetrate easily between soil particles
  • each hair has a large SA:V ratio
• Root hair cell:

  • each hair has a thing surface layer through which diffusion and osmosis can take place quickly
  • concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
• Symplast pathway:

  Water moves through the symplast-the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata by osmosis.
• Symplast pathway:

  The root hair cell has a higher water potential than the next cell along as a result of water diffusing in from the soil, which has made the cytoplasm more dilute. So water moves from the root hair cell into the next door cell by osmosis. This process continues from cell to cell across the root until the xylem is reached.
• The symplast pathway:

  As water leaves the root hair cell by osmosis, the water potential of the cytoplasm falls again, and this maintains a steep water potential gradient to ensure that as much water as possible continues to move into the cell from the soil.
• The apoplast pathway:

  Water moves through the apoplast- the cell walls and the intercellular spaces.
• The apoplast pathway:

  Water fills the spaces between the loose, open network of fibres in the cellulose cell wall. As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules.
• The apoplast pathway:

  The pull from water moving into the xylem and up the plant along with the cohesive forces between the water molecules creates a tension that means there is a continuous flow of water through the open structure of the cellulose cell wall, which offers little or no resistance.
• Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis-the layer of cells surrounding the vascular tissue (xylem and phloem) of the roots.
• The Casparian strip is a band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer.
• At the Casparian strip, water in the apoplast pathway can go no further and it is forced into the cytoplasm of the cell, joining the water in the symplast pathway. To do this, water must pass through the selectively permeable cell surface membranes, this excludes any potentially-toxic solutes in the soil water from reaching living tissues.
• The water potential of the xylem cells is much lower than the water potential of the endodermal cells. This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway.
• Once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem itself and move up the plant.
• The active pumping of minerals into the xylem to produce movement of water by osmosis results in root pressure and it is independent of any effects of transpiration. Root pressure gives water a push up the xylem but root pressure is usually not the major factor in water movement up from the roots to the leaves.
• Factors affecting transpiration:

  • Light
  • Humidity
  • Temperature
  • Air movement
  • Soil-water availability
• Transpiration= when the 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
• Transpiration is an inevitable consequence of gaseous exchange.
• At least some stomata need to be open at all times because during the day a plant needs to take in carbon dioxide and at night it needs to take in oxygen for cellular respiration as no oxygen is being produced from photosynthesis.
• Lignin is essential to the walls of a xylem vessel as:

  • it provides strength and support to keep the vessel open
  • it creates a continuous column
  • it prevent the collapse of the vessel