Topic 3 - mass transport in plants

Question 1

how is water transported in plants?

Answer 1

• through the xylem vessels
• long continuous columns that also provides structural support to the stem

Question 2

explain cohesion-tension theory?

Answer 2

• water molecules form hydrogen bonds with each other causing them to stick together
• surface tension of water causes this sticking effect
  = as water is lost from transpiration, more water can be drawn up the stem

Question 3

what are the three components of phloem vessels?

Answer 3

sieve tube elements = form a tube to transport in the dissolved form of sap
companion cells = involved in ATP production for active loading of sucrose into sieve tubes
plasmodesmata = gaps between cell walls where the cytoplasm links, allowing substances to flow

Question 4

name the process where organic materials are transported around the plant?

Answer 4

translocation

Question 5

how does sucrose in the leaf move into the phloem?

Answer 5

sucrose enters companion cells of the phloem vessels by active loading, using atp and a diffusion gradient of hydrogen ions. sucrose diffuses from companion cells into sieve tube elements through plasmodesmata

Question 6

how do phloem vessels transport sucrose around the plant? (mass flow hypothesis)

Answer 6

1. In source / leaf sugars actively transported into phloem;
2. By companion cells;
3. Lowers water potential of sieve cell / tube and water enters by
   osmosis;
4. Increase in pressure causes mass movement (towards sink /
   root);
5. Sugars used / converted in root for respiration for storage.

Question 7

give evidence for the mass flow hypothesis of translocation

Answer 7

• ring of bark removed from a woody stem = bulge of fluid forms above the ring which has a high conc of sugars = evidence there is a downward flow of sugars
• metabolic inhibitor = stops atp production = translocation stops - this is evidence that active transport is involved.

Question 8

give evidence against the mass flow hypothesis of translocation

Answer 8

the structure of sieve tubes seems to hinder mass flow

Question 9

how can ringing experiments be used to investigate transport in plants?

Answer 9

bark + phloem of a tree are removed as a ring leaving behind the xylem
eventually the tissues above the ring swells due to accumulation of sucrose as the tissue below begins to die
therefore sucrose must be transported to the phloem

Question 10

how can tracing experiments be used to investigate transport in plants?

Answer 10

plants are grown in the presence of radioactive co2 which will be incorporated into the plants leaves
using autoradiography we can see the areas exposed to radiation correspond to where the phloem is