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.
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.
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.
23.
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.
Editor's Notes
Nitrates provide the nitrogen plants need to construct such vital molecules as amino acids and nucleic acids (DNA and RNA). Also some of the vitamins. Lack of nitrates means stunted growth, low yields, yellow leaves. Magnesium is the ion that's vital to chlorophyl function and photosynthesis. Without it leaves turn yellowish and/or red orange in colour, shrivel and die.
