translocation

Question 1

define translocation

Answer 1

transportation of assimilates through the phloem, from sources to sinks, requiring energy

Question 2

define source

Answer 2

part of a plant which loads assimilates into phloem sieve tubes
- leaves

Question 3

define sink

Answer 3

part of a plant that removes assimilates from phloem sieve tubes
- meristems

Question 4

what part of a plant can be both a source and a sink?

Answer 4

roots:
- sucrose can be stored in the roots (sink).
- during growth, sucrose is transported from roots to leaves to provide energy for growth. (source)

Question 5

define mass flow

Answer 5

materials move together in same direction

Question 6

explain the process of active loading in the phloem

Answer 6

• in companion cell, ATP (provided by mitochondria) is used to actively transport H+ ions out the cell, into surrounding tissue cells
• this creates a concentration gradient as there are now more H+ ions in surrounding tissue than in companion cell
• a H+ ion binds to a co-transporter protein in companion cell membrane, and re enters the cell
• a sucrose molecule binds to the cotransport protein at the same time, and is moved into the cell, against its concentration gradient, via facilitated diffusion
• sucrose molecules move into the sieve tube element by diffusion

Question 7

explain the stages of translocation, how assimilates are transported from the sources to the sinks of a plant through the phloem.

Answer 7

1) active transport loads assimilates into sieve elements at the source
2) water potential is lowered as sucrose concentration increases, water moves by osmosis from the xylem into the sieve tube
3) this creates high turgor pressure inside sieve elements at the source end of the phloem
4) this causes mass flow of water and assimilates through phloem from source to sink
5) there is now a high sucrose concentration in sieve element, and it diffuses into the sink
6) as sucrose leaves, water potential at the sink increases, and water moves out the sieve elements back into the xylem through osmosis, lowering turgor pressure
7) sucrose is converted into other substances like starch for storage or glucose and fructose for respiration
8) pressure gradient is maintained from source to sink through this process and therefore mass flow can continuously occur