Transport in plants

• Xylem

  Tissue that transports water and dissolved minerals from roots to leaves
• Phloem


  Tissue that transports food substances (sucrose and amino acids) from leaves to other parts of the plant
• Xylem vessels


  • Formed by a series of long cells joined end to end
  • Hollow with no cell contents
  • Cell walls become thickened and impregnated with lignin, making them very strong and impermeable
• Phloem sieve tubes


  • Formed by vertical columns of cells
  • Perforations in end walls allow substances to pass from cell to cell
  • Cell walls not lignified and cell contents do not die, although they lose their nuclei
• Xylem and phloem are present in the midrib of the leaf, as well as in the leaf veins
• Xylem and phloem form vascular bundles that join to form a transport system through the whole plant
• In the stem, the vascular bundles form a cylinder in the cortex
• In the root, the vascular bundle is made up of groups of specialised cells in the centre of the root
• Root hair cells


  • Tiny, tube-like outgrowths from the cells of the root's outer layer
  • Provide a large surface area to take up water from the soil by osmosis and to absorb mineral ions by active transport
• Uptake and transport of water and ions

  1. Water enters root hair cells

  2. Passed on to cells in the root cortex

  3. Enters xylem vessels

  4. Moves up the stem and into the leaves

  5. Xylem passes along the midrib before branching into the leaf mesophyll cells
• Scientists are still unsure of the exact pathway taken by water through the cortex of the root
• Most water travels in or between the cell walls, rather than through the cytoplasm of each cell
• Transpiration


  The loss of water vapour from leaves
• Water evaporating from the leaves


  Causes suction, which pulls water up the stem
• The flow of water up the xylem vessels is called the transpiration stream
• Increasing temperature


  Increases the rate of transpiration
• Increasing wind speed


  Increases the rate of transpiration
• Increasing humidity


  Decreases the rate of transpiration
• Wilting


  Occurs when the rate of transpiration exceeds the rate of water uptake by the roots
• Path taken by a water molecule from the soil until it reaches a mesophyll cell of a leaf to be made into sugar


  1. Water absorbed by roots from soil

  2. Water passes up xylem vessels in vascular bundles

  3. Water evaporates from leaf surface into atmosphere
• Mechanism for moving water through the plant

  1. Water evaporates from spongy mesophyll cells into air spaces

  2. Water vapour diffuses out of leaves through stomata

  3. Evaporation from leaves causes suction which pulls water up stem

  4. Water travels up xylem vessels in vascular bundles (transpiration stream)
• Increasing temperature


  Increases transpiration rate
• Increasing wind speed


  Increases transpiration rate
• Increasing humidity


  Reduces transpiration rate
• Wilting


  Occurs when leaf cells lose water faster than they can absorb it from xylem vessels, causing cells to lose turgor and become flaccid
• Transpiration is an unavoidable consequence of photosynthesis as the pathway that allows carbon dioxide in will also let water vapour out
• Transpiration may help cool the leaf when exposed to strong sunlight by absorbing latent heat as water evaporates
• Some plants close their stomata around midday to reduce transpiration, but this raises the question of how they avoid overheating
• A rapid water flow may be needed to obtain enough mineral salts, which are in very dilute solution in the soil
• Evaporation may also help to cool the leaf when it is exposed to strong sunlight
• In some cases, an increased transpiration may not increase the uptake of minerals
• A leaf exposed to direct sunlight will absorb heat and the increase in temperature may kill the cytoplasm
• Water evaporating from a leaf absorbs its latent heat and cools the leaf down
• There are plants whose stomata close at around midday, reducing transpiration significantly
• In order to photosynthesise, a leaf needs to take in carbon dioxide from the air. The pathway that allows carbon dioxide in will also let water vapour out, whether the plant needs to lose water or not
• Plants need to keep a careful balance between the optimum intake of carbon dioxide and a damaging loss of water
• Stomata


  Openings in the leaf surface that allow gas exchange
• Opening and closing of stomata

  Triggered by cell walls in response to light intensity, carbon dioxide concentration and humidity
• Stomata react to water stress, closing before wilting happens to delay the start of it
• Petroleum jelly prevents evaporation from leaf surfaces