Translocation

Question 1

What are the three components of phloem vessels?

Answer 1

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

Question 2

How does sucrose in the leaf move into the phloem

Answer 2

Sucrose enters the companion cells of the phloem vessels by active loading, which uses ATP and a diffusion gradient of hydrogen ions. Sucrose then diffuses from companion cells into the sieve tube elements through the plasmodesmata

Question 3

How do phloem vessels transport sucrose around the plant?

Answer 3

As sucrose moves into the sieve tube elements, the water potential inside the phloem is reduced. This causes water to enter via osmosis from the xylem and increases hydrostatic pressure. Water moves along the sieve tube towards areas of lower hydrostatic pressure. Sucrose diffuses into surrounding cells where it is needed

Question 4

Give evidence to support mass flow hypothesis of translocation

Answer 4

• Sap is released when a stem is cut so there must be pressure in the phloem
• There is a higher sucrose concentration in the leaves than the roots
• Increasing sucrose levels in the leaves results in increased sucroses in the phloem

Question 5

Give evidence against the Mass flow hypothesis of translocation

Answer 5

• the structure of sieve tubes seems to hinder mass flow
• not all solutes move at the same speed as they would in mass flow
• sucrose is relieved at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first

Question 6

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

Answer 6

The bark and phloem of a tree are removed in a ring, leaving behind the xylem. Eventually the tissues above the missing ring swells due to accumulation of sucrose as the tissue below begins to die. Therefore sucrose must be transported in the phloem

Question 7

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

Answer 7

Plants are grown in the presence of radioactive CO2, which will be incorporated into the plants sugars. Using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is