Plants: Exchange and Transport

Xylem and Phloem
1. Xylem
  1. transport water and minerals from the roots up to the leaves.
  2. Xylem vessel
    1. dead cells, thick walls and lignin (waterproofs the walls of the cell)
    2. Lignin allows the stem to be flexible due to the patters in the cell wall, e.g. spiral or reticulate.
    3. some places lignification is not complete which leaves pores called pits. this allows water to leave one vessel and enter another, or into living parts of a plant.
  3. Adaptations
    1. dead skin cells aligned end to end to form column
    2. narrow tubes, capillary action is effective (tubes don't burst)
    3. pits in lignified walls to allow water movement to adjacent vessels
    4. lignin deposited in walls in spiral, annular or reticulate patterns to allow xylem to stretch
    5. no nucleus, cell contents, end wall or cytoplasm; good flow of water
2. Phloem
  1. transport sugars from one part of plant to another
  2. 2 types of cell.
    1. Sieve Tubes
      1. not true cells as they do not have a nucleus and very little cytoplasm
      2. sucrose is transported (cells lined end-end) , sucrose dissolved in water to form sap.
      3. cross walls at intervals called sieve plates, perforated by pores to allow sap to flow
      4. thin walls and 5-6 sided
    2. Companion Cells
      1. in between sieve tubes with large nucleus and dense cytoplasm
      2. numerous mitochondria to produce ATP. They carry out metabolic processes needed by sieve tube elements.
        includes using ATP as a source of energy to load sucrose into sieve tubes
      3. cytoplasm of cell and sieve tube elements are linked through plasmodesmata (gaps in cell walls allowing communication and flow of substances)
Water
1. Water Potential
  1. measure of tendency of water molecules to diffuse from one place to another
  2. water moves from high water potential to low potential
  3. pure water has a water potential of zero.
    1. cells have a negative water potential
      1. because they contain dissolved salts and sugars
    2. lower water potential: more negative; higher water potential: less negative
  4. turgidity and plasmolysis
    1. when water leaves a cell the cytoplasm and vacuole shrink. The cytoplasm loses contact with the cell wall, this is known as plasmolysis
    2. when water enters a cell the water pressure exerts pressure on the cell wall. The cell wall becomes full with water, this is known as being turgid.
  5. pathways water molcules can take
    1. apoplast pathway
      1. cellulose cell walls have many water filled spaces between cellulose molecules, water does not pass through any plasma membranes which means dissolved mineral ions and salts can be carried with the water.
    2. symplast pathway
      1. water enters cytoplasm through plasma membrane, (passes through plasmodesmata from one cell to the next), water can move through the continuous cytoplasm from cell to cell.
        Plasmodesmata: gaps in the cell wall that contain a thin strand of cytoplasm
    3. vacuolar pathway
      1. water enters cytoplasm, and passes through the vacuole back out into the cytoplasm and a vacuole of an adjacent cell
Transpiration
1. the loss of water from the aerial parts of a plant due to evaporation
2. water vapour diffuses down a water potential gradient
3. 3 processes are involved in transpirartion
  1. 1. osmosis from the xylem to mesophyll cells
  2. 2. evaporation from the surface of the mesophyll cells into the intercellular spaces
    1. spongy mesophyll cells have large air spaces between them which help water vapour diffuse
  3. 3. diffusion of water vapour from the intercellular spaces out through the stomata
4. water that is lost needs to be replaced, water is useful in a number ways
  1. water keeps the cells turgid
  2. required for photosynthesis
  3. flow of water carries essential nutrients and minerals
5. potometer
  1. estimates the rate of water loss
  2. cut the shoot underwater to prevent any air bubbles getting in
  3. what affects the rate of transpiration?
    1. wind speed
    2. surface area
    3. temperature
    4. humidity
    5. size and position of stomata
    6. light
    7. waxy cuticle
6. xerophytes
  1. plant that is adapted to reduce water loss so that it can survive in very dry conditions
  2. structural and behavioural adaptations to reduce water loss
    1. smaller needle like leaves: reduces total surface area
    2. densely pack spongy mesophyll: reduces surface area exposed to the air
    3. pits containing stomata: trap air which can become saturated with water vapour (water vapour potential has been reduced)
    4. rolled up leaves so lower epidermis is not exposed: traps air so it can become saturated (water vapour potential has been reduced)
    5. hair on the surface: traps air so it becomes saturated (water vapour potential has been reduced)
Translocation
1. transport of assimilates throughout the plant, in the phloem tissue
2. sugars are transported in the phloem in the form of sucrose
3. source and sink
  1. source
    1. releases sucrose into the phloem
    2. water potential reduces at the sieve tube element as sucrose enters
    3. water molecules move into the sieve tube element by osmosis
      1. this increases the hydrostatic pressure in the sieve tube (at the source)
  2. sink
    1. removes sucrose from the phloem
    2. sucrose molecules move by diffusion or active transport from the sieve tube into surrounding cells
      1. this increases the water potential in the sieve tube
    3. water molecules move into the surrounding cells by osmosis
      1. this decreases the hydrostatic pressure in the sink (at the phloem)
4. sucrose can be converted to starch from storage or used in a metabolic process; e.g. respiration
  1. reduces sucrose concentration in cells
5. mass flow
  1. a flow of water in the phloem is produced by the water moving down the hydrostatic pressure gradient
  2. the flow of water carries sucrose and other assimilates along the phloem
  3. it can flow in any direction and at the same time

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	Created by tasnia.a.98 almost 10 years ago