Transport in plants

• Dicotyledonous plants


  Plants with two seed leaves and a branching pattern of veins in the leaf
• Meristem


  A layer of dividing cells, here it is called the pericycle
• Phloem


  Transports dissolved assimilates
• Vascular tissue


  Consists of cells specialised for transporting fluids by mass flow
• Xylem


  Transports water and minerals
• The need for a transport system in plants
• Substances plants need to take from and return to their environment

  • Oxygen
  • Water
  • Nutrients
  • Minerals
  • Sugars
• Plants can absorb water and minerals at the roots, but they cannot absorb sugars from the soil
• Plants need a transport system to move water and minerals from the roots up to the leaves, and sugars from the leaves to the rest of the plant
• Vascular tissues


  • Water and soluble mineral ions travel upwards in xylem tissue
  • Assimilates, such as sugars, travel up or down in phloem tissue
• Unlike in animals, there is no pump in plants, and respiratory gases are not carried by the vascular tissues
• Dicotyledonous plants


  • The vascular tissue is distributed throughout the plant
  • The xylem and phloem are found together in vascular bundles
  • These bundles may also contain other types of tissue that give the bundle some strength and help to support the plant
• Vascular bundle in young root

  • Central core of xylem, often in the shape of an X
  • Phloem found in between the arms of the X-shaped xylem tissue
  • Arrangement provides strength to withstand the pulling forces to which roots are exposed
  • Around the vascular bundle is a special sheath of cells called the endodermis
  • Just inside the endodermis is a layer of meristem cells called the pericycle
• Vascular bundles in stem

  • Found near the outer edge of the stem
  • In non-woody plants the bundles are separate and discrete
  • In woody plants the bundles become a continuous ring in older stems
  • This arrangement provides strength and flexibility to withstand the bending forces to which stems and branches are exposed
  • The xylem is found towards the inside of each vascular bundle and the phloem towards the outside
  • In between the xylem and phloem is a layer of cambium
• Vascular bundles in leaf

  • Form the midrib and veins of a leaf
  • A dicotyledonous leaf has a branching network of veins that get smaller as they spread away from the midrib
  • Within each vein, the xylem is located on top of the phloem
• The best plants to dissect and stain to view vascular tissue are celery and busy lizzies
• Companion cells


  The cells that help to load sucrose into the sieve tubes
• Sieve tube elements


  • Make up the tubes in phloem tissue that carry sap up and down the plant
  • The sieve tube elements are separated by sieve plates
• Xylem vessels


  The tubes which carry water up the plant
• Xylem


  • Consists of vessels to carry water and dissolved mineral ions, fibres to help support the plant, and living parenchyma cells which act as packing tissue
• Xylem vessels


  • As they develop, lignin impregnates the walls of the cells, making them waterproof and killing the cells
  • The end walls and contents of the cells decay, leaving a long column of dead cells with no contents - a tube called the xylem vessel
  • The lignin strengthens the vessel walls and prevents the vessel from collapsing
  • The lignin thickening forms patterns in the cell wall such as spiral, annular or reticulate
  • This prevents the vessel from being too rigid and allows some flexibility of the stem or branch
  • In some places lignification is not complete, leaving gaps in the cell wall called pits or bordered pits
• Adaptations of xylem to its function

  • Made from dead cells aligned end to end to form a continuous column
  • The tubes are narrow, so the water column does not break easily and capillary action can be effective
  • Bordered pits in the lignified walls allow water to move sideways from one vessel to another
  • Lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows, and enables the stem or branch to bend
  • There are no cross-walls, cell contents, nucleus or cytoplasm, and the lignin thickening prevents the walls from collapsing
• Phloem


  • Consists of sieve tubes made up of sieve tube elements, and companion cells
• Sieve tube elements


  • Elongated cells lined up end to end to form sieve tubes
  • They contain no nucleus and very little cytoplasm, leaving space for mass flow of sap to occur
  • At the ends are perforated cross-walls called sieve plates that allow movement of sap from one element to the next
• Companion cells


  • Small cells with a large nucleus and dense cytoplasm, located in between the sieve tubes
  • They have numerous mitochondria to produce the ATP needed for active processes
  • They carry out the metabolic processes needed to load assimilates actively into the sieve tubes
• Plasmodesmata


  Gaps in the cell wall containing cytoplasm that connects two cells
• Pathways taken by water

  • Apoplast pathway - through the spaces in the cell walls and between the cells
  • Symplast pathway - enters the cell cytoplasm through the plasma membrane and passes through plasmodesmata
  • Vacuolar pathway - enters and passes through the vacuoles
• Water potential


  • A measure of the tendency of water molecules to move from one place to another
  • Water always moves from a region of higher water potential to a region of lower water potential
  • The water potential of plant cells is always negative due to the presence of solutes
• Water uptake


  The water potential in a plant cell is more negative (lower) than the water potential of the water, so water molecules will move down the water-potential gradient into the cell
• Water loss


  If a plant cell is placed in a salt solution with a very negative (low) water potential, then it will lose water by osmosis
• Movement of water between cells

  Water molecules can pass from one cell to another, moving from the cell with the less negative (higher) water potential to the cell with the more negative (lower) water potential
• Transpiration


  The loss of water vapour from the aerial parts of a plant, mostly through the stomata in the leaves
• More than 95% of the water taken up by a plant is lost in transpiration
• Factors affecting transpiration


  • Light intensity
  • Temperature
  • Relative humidity
  • Air movement (wind)
  • Water availability
• When describing the effects of an environmental factor, state whether the factor is increasing or decreasing to cause the change in transpiration rate
• Potometer


  • A device that can measure the rate of water uptake as a leafy stem transpires
  • Assuming the cells are turgid, more than 95% of water taken up is lost by transpiration, so this gives a reasonable estimate of transpiration rate
  • Precautions to ensure valid results include setting it up under water, ensuring a healthy shoot, cutting the stem under water, cutting at an angle, and drying the leaves
• Adhesion


  The attraction between water molecules and the walls of the xylem vessel
• Cohesion


  The attraction between water molecules caused by hydrogen bonds
• Transpiration stream


  The movement of water from the soil, through the plant, to the air surrounding the leaves
• Measuring volume and transpiration rate

  1. Calculate volume of a cylinder (length of capillary tube)

  2. Rate = volume/time