Mass Transportin Plants

Question 1

Describe how transpiration works

Answer 1

Water diffuses out of the stomata of the leaf which lowers the water potential
Osmosis occurs from neighbouring tissue into the leaf cells.
The neighbouring cells have lower water potentials
Xylem tissue bring water to these tissues up in a column
The cohesive forces of water allow it to move as one column
Hence water forms an unbroken column from root to stomata

Question 2

What is the evidence for transpiration?

Answer 2

If xylem tissue broken no water leaks out, hence its not being pushed
Tree trunk diameter changes - midday xylem is narrower as its under greater tension so is pulled inwards

Question 3

Which plant tissue is responsible for which type of movement?

Answer 3

Xylem - transpiration

Phloem is tranlocation

Question 4

The phloem moves organic material from _____ to_______

Answer 4

Sinks to sources

Question 5

Describe the mechanism of translocation

Answer 5

1) H+ actively transported out of companion cells
2) diffuses back in with sucrose through co transport proteins
3) high [sucrose] in comapnion cell so it diffuses into phloem
4) high [sucrose] in phloem reduces the water potential meaning water flows into phloem from xylem creating a high hydrostatic force
5) Mean while at sinks sucrose is being rapidly used and so a gradient occurs from the phloem into the respiring cells, the sucrose lowers water potential and hence water follows it
6) therefore in phloem near respiring tissue hydrostatic pressure is less
7) there is a pressure gradient from source to sink that carries with it the organic material

Question 6

Describe two experiments that help provide evidence for the mass flow hypothesis of translocation

Answer 6

Ringing experiments
- by breaking phloem tissue the plant above swells whislt plant below dies

Tracer experiments

• introduce C14 into the plant
• will photosynthesis to produce sugars with C14
• these sugars can be located in the phloem with radioactive papers etc