Multicellular organisms need transport systems to exchange substances with their environment since not all cells are in direct contact. Transport systems allow organisms to take in needed molecules and remove waste. In plants, xylem transports water and minerals up from the roots while phloem transports organic compounds like sucrose made by leaves throughout the plant. Movement occurs through cell walls and plasmodesmata aided by pressure gradients from transpiration and active transport of ions.

1. Transport

2. 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.

3. Surface area to volume ratio

4. Transpiration
• What do you remember?

5. • It is a consequence of gas exchange

6. Investigating factors affecting
transpiration rate
• Presentations

7. Dicotyledonous plants
• Distribution of xylem and phloem
Root

8. •Stem

9. Leaves

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

11. • Cellulose cell wall
strengthened with
lignin

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

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

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

15. Movement of water from soil to xylem

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

17. Apoplastic Pathway
• Moves through the cell wall

18. 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.

19. 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.

20. Role of ions in plants
• Nitrates
• Magnesium

21. Xerophytic Plants
• video

22. 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.

24. 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.

25. 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.