Why do multicellular organisms need transport systems?
• Too big so not all of their cells have contact with the external environment.
• All organisms need to exchange substances with their environment - take in needed molecules and get rid of waste.

Surface area to volume ratio

Transpiration
• What do you remember?

• It is a consequence of gas exchange

Investigating factors affecting transpiration rate
• Presentations

Dicotyledonous plants
• Distribution of xylem and phloem

Xylem
• Transports water
• Supports plant
Two types:
Vessels – no end walls
Tracheids – pits in end walls

• Cellulose cell wall strengthened with lignin

Phloem
• Transporting organic substances
• Sieve tube elements join end to end
• Sieve plates between
• Nucleus and cytoplasm pushed to side

• Kept alive by companion cells
• Lots of mitochondria
• Connected to sieve tube by plasmodesmata

Movement of water
• From cell to cell – down a water potential gradient.
• From cell to environment - first by osmosis then diffusion

Movement of water from soil to xylem

Symplastic pathway
• Through the cytoplasm and the vacuole
• Entering adjacent cells through plasmodesmata or cell walls

Apoplastic Pathway
• Moves through the cell wall

Casparian strip
• In the stele the cells have a thick, waterproof band of suberin in cell wall
• Apoplastic pathway stopped.
• This helps with root pressure.

Roots to leaves
• Mass flow due to cohesion and adhesion
• Hydrostatic pressure (transpiration decreases pressure at top)
• Root pressure (increases pressure at bottom)
– active secretion of solutes in to the xylem.

• Nitrates
Role of ions in plants
• Magnesium

• video
Xerophytic Plants

Translocation
• Food is transported from source to sink
• Mainly sucrose
• Actively pumped into the sieve tubes at the source
• Water follows by osmosis, creating high pressure.
• Sugar is then removed by active transport, and water again by osmosis, lowering the pressure in the sieve tube at the sink.

Sugar Loading and Unloading
• There are two known mechanisms
• In one, it then enters the phloem by attaching to sucrose transporter proteins embedded in the plasma membranes of the sieve elements and companions cells.
• In the second mechanism, sucrose enters through plasmodesmata
• they unload from the phloem either through plasmodesmata or by crossing from one cell to another across the cell walls.

Loading
• Hydrogen ions are pumped out of the companion cell in to the source cell
• Hydrogen ions flow back by diffusion through a carrier protein that also carries sucrose.
• High concentration of sucrose in companion cell causes sucrose to flow into phloem through plasmodesmata.