transport in plants

Cards (92)

  • The Need for Transport Systems in Plants
    All living organisms have the need to exchange substances with their surrounding environment
  • Plants
    • Need to take carbon dioxide and nutrients in
    • Waste products generated need to be released
  • Exchange site
    The location within an organism where exchange of substances occurs
  • Exchange site in plants
    • Roots (water and minerals)
  • Substances are said to not have entered or left an organism until it crosses the cell surface membrane
  • Small organisms like single-celled Chlamydomonas
    • Able to exchange substances directly with the environment
    • Have a large surface area: volume ratio
    • Have a short diffusion distance
  • Larger organisms
    • Require specialised mass transport systems
    • Increasing transport distances
    • Decreasing surface area: volume ratio
    • Increasing levels of activity
  • Mass transport in plants
    1. Xylem transports water and mineral ions
    2. Phloem transports sucrose and other nutrients
  • Plants have no specialised transport system for oxygen and carbon dioxide
  • Plants do not need a specialised transport system for oxygen and carbon dioxide because they have adaptations that give a high surface area: volume ratio for gas absorption and diffusion, leaves and stems have chloroplasts that produce oxygen and use up carbon dioxide, and plant tissues have a low metabolic rate
  • Xylem
    • Vascular tissue that carries dissolved minerals and water up the plant
    • Provides structural support
    • Stores food
  • Phloem
    • Transports organic compounds, particularly sucrose, from the source (e.g. leaf) to the sink (e.g. roots)
    • Transport can occur up and down the plant
  • Cell types in xylem tissue
    • Tracheids
    • Vessel elements
    • Xylem parenchyma
    • Sclerenchyma cells
  • Xylem vessel elements
    • Thickened cell walls
    • No end plates when mature
  • Location of xylem and phloem in roots, stems and leaves
    • In roots, xylem is in the centre and phloem is on the edges
    • In stems, xylem is on the inside and phloem is on the outside
    • In leaves, xylem is on the upper side and phloem is on the lower side of the vascular bundles
  • Phloem sieve tube elements
    • Line up end to end to form a continuous tube
    • Have companion cells that control their metabolism and load/unload sugars
  • Sieve tube element
    • Line up end to end to form a continuous tube
  • Structure and function of phloem sieve tube elements
    1. Sieve tube elements
    2. Companion cells
  • Companion cells
    • Associated with each sieve tube element
    • Control the metabolism of their associated sieve tube member
    • Play a role in loading and unloading of sugars into the phloem
  • Mature xylem tissue is dead, so there is no evidence of organelles, and they have lignified cell walls, whereas sieve tube elements have no lignin, do have sieve plates, and their companion cells contain nuclei and dense cytoplasm
  • Dicotyledonous (dicots) plants

    • Seeds contain two cotyledons (seed leaves)
    • Network of veins
    • Leaves have broad blades (leaf surface) and petioles (stalks)
    • Tap root with lateral branches
    • Herbaceous dicots have a relatively short life cycle (one growing season) and non-woody tissue
  • Transport systems in plants
    • Vascular system with xylem (transports water and mineral ions from the roots to the rest of the plant) and phloem (transports substances from the source (eg. leaf) to the sink (eg.root))
    • Vascular bundles contain xylem and phloem arranged together
  • Tissue plan diagrams of a dicotyledonous leaf, stem and root
    Diagrams showing the arrangement of tissues
  • Calculating eyepiece graticule unit and actual width of plant stem
    1. Step 1: Calculate the number of divisions
    2. Step 2: Calculate the value of each division
    3. Step 3: Calculate the actual width
  • When drawing tissue plan diagrams, follow instructions carefully, use a sharp pencil, clear continuous lines, and include the magnification
  • Transpiration
    Loss of water vapour from a plant to its environment by evaporation and diffusion
  • Advantages of transpiration
    • Provides a means of cooling the plant via evaporative cooling
    • The transpiration stream is helpful in the uptake of mineral ions
    • The turgor pressure of the cells provides support to leaves and the stem of non-woody plants
  • Transpiration stream
    Movement of water from the roots to the leaves
  • Driving force for movement of water
    Gradient in water potential from the soil (high water potential) to the atmosphere (low water potential)
  • Transpiration only occurs when there is a concentration gradient, with a lower concentration of water molecules in the air outside the leaf
  • Loss of water vapour from leaves
    Results in a lower water potential, creating a concentration gradient between the roots and leaves causing water to move upwards
  • Factors affecting the rate of transpiration
    • Air movement
    • Temperature
    • Light intensity
    • Humidity
  • Air movement
    Affects the concentration gradient of water vapour between the leaf and surrounding air
  • Temperature
    Affects the kinetic energy of water molecules, increasing the rate of transpiration
  • Light intensity
    Affects the opening and closing of stomata, which regulates the rate of transpiration
  • Humidity
    Affects the concentration gradient of water vapour between the leaf and surrounding air
  • The different factors have different effects on the rate of transpiration
  • Uptake of water and minerals in plants
    • Passive process by osmosis and diffusion
    • Active process by active transport
  • Pathways for water and dissolved solutes
    • Apoplastic pathway (through cell walls, dead cells, and xylem)
    • Symplastic pathway (through living cells)
  • Plants must take in a constant supply of water and dissolved minerals to compensate for the continuous loss of water via transpiration in the leaves, and so that they can photosynthesise and produce proteins