substance exchange

Mass Transport in Plants

  • Central Idea: How plants transport water and nutrients throughout their structures.

Main Branches

  1. Xylem
    • Transports water and minerals from roots to leaves.
    • Made up of dead cells.
    • Includes:
      • Tracheids
      • Vessel Elements
  2. Phloem
    • Transports sugars and other organic compounds from leaves to other parts of the plant.
    • Made up of living cells.
    • Includes:
      • Sieve Tube Elements
      • Companion Cells

Xylem

  • Tracheids
    • Long, thin cells with tapered ends.
    • Water moves from cell to cell through pits.
  • Vessel Elements
    • Shorter, wider cells with perforated ends.
    • Water moves through perforations to adjacent cells.

Phloem

  • Sieve Tube Elements
    • Long, thin cells with perforated ends.
    • Sugars and other organic compounds move through perforations to adjacent cells.
  • Companion Cells
    • Provide metabolic support to sieve tube elements.
    • Connected to sieve tube elements by plasmodesmata.

Transpiration and Translocation

Transpiration

  • Definition: The loss of water vapour from the leaves of a plant
  • Factors affecting transpiration:
    • Temperature
    • Humidity
    • Wind speed
    • Light intensity

Mechanism of transpiration:

  • Water vapour diffuses out of the stomata
  • Water molecules form hydrogen bonds with each other, creating a continuous column of water in the xylem vessels
  • Water is pulled up the xylem vessels by transpiration pull

Adaptations of plants to reduce water loss:

  • Stomatal closure
  • Reduced leaf size
  • Thick cuticle
  • Hairs on leaves
  • Rolled leaves

Translocation

  • Definition: The movement of organic substances (e.g. sugars) from one part of the plant to another
  • Mechanism of translocation:
    • Sugars are produced in the source (e.g. leaves) by photosynthesis
    • Sugars are loaded into the phloem sieve tubes
    • Water moves into the phloem by osmosis, creating a high pressure at the source end and a low pressure at the sink end
    • Sugars move from high pressure to low pressure by mass flow
  • Factors affecting translocation:
    • Temperature
    • Light intensity
    • Availability of water and minerals
  • Adaptations of plants to increase translocation:
    • Increased number of phloem sieve tubes
    • Increased phloem loading rate
    • Increased number of companion cells
    • Increased surface area of leaves

Plants are constantly exposed to harsh weather conditions and they lose a lot of water from their leaves.

Describe how plants replace the water that they have lost.

Plants lose water through a process called transpiration. This creates tension in the xylem formed by hydrogen bonds between water molecules and the sides of the xylem vessel elements. Water is then pulled upwards by this tension, however, cohesive properties are also present. Cohesion is formed by hydrogen bonding between water molecules and this allows water molecules to be drawn upwards to replace the water that has been lost in the leaves.

  • The hydrostatic pressure gradient set up by water entering the source and leaving towards the sink does not explain why sap can move up and down the plant
  • Sieve plates contain large pores. How can an increase in hydrostatic pressure be created when pores are present
  • Phloem cells are living. However, it is not fully established why living cells are important