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?

Answer 6

• sucrose moves into tube elements, water potential inside the phloem is reduced
• this causes water to enter from the xylem by osmosis, increasing the hydrostatic pressure
• water moves along the sieve tubes towards areas of lower hydrostatic pressure
• sucrose diffuses into surrounding cells where it is needed

Question 7

give evidence of the mass flow hypothesis of translocation

Answer 7

sap is released when stem is cut which means there must be pressure in the phloem
there is a higher concentration of sucrose in the leaves than the roots
increasing sucrose levels in the leaves means there is increased sucrose in the phloem

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