Biology F211 Transport in plants.

Transport in plants

Xylem

• Xylem Vessels
• Long collum of dead cells
• lignified to make xylem waterproof, strong and ridgid and flexible
• where lignification is not complete, there are lateral pits for water to travel between the xylem vessels
• Flow of water is not impeded because there are no end walls, no cell contents, no nucleus or cytoplasm.
• Transports water up the xylem due to transpiration

Phloem

• Phloem tissue consists of 2 elements, sieve tube elements and companion cells.
• Transports assimilates up and down the plant via translocation
• sieve plates on sieve tubes
• companion cells have lots of mitochondria to provide ATP for loading assimilates into the seive tubes.
• Sieve tubes and companion cells are linked by plasmodesmata which allow communication and flow of minerals between the cells.

Water transport between cells

• Apoplast pathway - Water moves through the water filled spaces between the cellulose molecules in the cell wall, the water does not pass through any membranes meaning the dissolved minerals can be carried with the water
• Symplast pathway - Enters through the plasma membrane and travels through the cytoplasm and then through the plasmondesmata from one cell to the next cell
• Vacuolar pathway - Similar to symplast, but is not confined to the cytoplasm, also passes through vacuoles aswell.

Water uptake and movement up the stem

• Osmosis only occurs due to the uptake of minerals from the soil. When the minerals are acticely loaded into the root hair cell it reduces the water potentia inside the cell, and because water moves from a high water potential to a low water potential, the water follows the minerals.
• The role of the casparian *****:
• Blocks the apoplast pathway between the cortex and the xylem
• Ensures that water and dissolved minerals have to pass into the cytoplasm of the cell
• Once the disolved minerals are into the cytoplasm they can be actively transported into the xylem
• This lowers the water potential of the xylem so the water in the cortex follow into the xylem
• The casparian ***** is made of suberin (fatty, waxy layer)
• 

How does water move up the stem?

• Root pressure - when minerals are actively moved into the xylem, this forces water into the xylem…