4.4 - Translocation of Sucrose Flashcards
Explain the mass-flow hypothesis.
Companion cells use ATP. To move hydrogen ions into the tissue surrounding the phloem, companion cells use ATP (energy).
Hydrogen ions move back into companion cells. This creates a diffusion gradient and causes the hydrogen ions to diffuse back into the companion cells, which is a form of facilitated diffusion involving co-transporter proteins.
Hydrogen ions help assimilates enter the cell. Hydrogen ions moving back into the companion cells pass through co-transporter proteins, where assimilates are dropped into the cell as each ion passes, causing the concentration to increase.
Assimilates diffuse down the concentration gradient. This results in the assimilate diffusing out of the companion cells, going down the concentration gradient, where they the enter sieve tube elements through links, known as plasmodesmata.
Hydrostatic pressure decreases. Once they enter the sieve tube elements, the water potential inside the tube decreases, causing water to enter via osmosis, increasing the hydrostatic pressure in the sieve tube.
Assimilates removed. As water moves down the sieve tube from a high pressure to low pressure area, diffusion or active transport removes the assimilates into the surrounding cells.
The sieve tube’s water potential increases. This increases the sieve tube’s water potential, meaning water leaves by osmosis and produces pressure in the phloem at the sink.
What are the strengths of the mass-flow hypothesis?
- you can use radioactive isotopes to mark glucose and use autoradiography to trace the sucrose through the phloem
- if steam is used to kill a ring of bark on a tree then it stops movement in the phloem
- aphids feed off phloem tubes and exude sap at the other end of the body, this is evidence of high hydrostatic pressure in the phloem
What are the weaknesses of the mass-flow hypothesis?
- does not explain why there can be bidirectional movement of the sieve tube
- doesn’t explain why there’s movement at different speeds in the sieve tube
- does not explain the presence of sieve plates
- does not explain why sieve tube cells and companion cells need to be alive for movement in the phloem to occur
