Transport

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

Root

•Stem

Leaves

Xylem• Transports water• Supports plant

Two types:Vessels – no end wallsTracheids – 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.

Role of ions in plants

• Nitrates

• Magnesium

Xerophytic Plants

• video

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.