translocation

Question 1

What is translocation?

Answer 1

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

Question 2

What is a source? Give an example

Answer 2

A part of the plant which loads assimilates into the phloem sieve tubes
eg the leaves

Question 3

What is a sink? Give an example

Answer 3

A part of the plant that removes assimilates from the phloem sieve tubes
eg meristems= areas of growth in roots, stems and leaves

Question 4

What is a part of a plant which can be both a source and a sink? Explain how

Answer 4

The roots= sucrose can be stored in the roots (sink). During the growing season sucrose is then transported from the roots to the leaves to provide the leaves with energy for growth. (source)

Question 5

What is the mass flow hypothesis?

Answer 5

• Where are material moves together in the same one direction.
• This is how translocation is believed to happen

Question 6

Explain how active loading happens in the phloem

Answer 6

• in the companion cell, ATP (provided from mitochondria in companion cell) is used to actively transport hydrogen ions out of the cell and into surrounding tissue cells
• this creates a concentration gradient as there is now more hydrogen ions in the surrounding tissue then in the companion cell
• a hydrogen ion binds to a co-transporter protein in the companion cell membrane, and re enters the cell
• a sucrose molecule binds to the co-transporter protein at the same time, and is moved into the cell, against its concentration gradient, via facilitated diffusion
• sucrose molecules then 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 actively loads the assimilates into the sieve elements of the phloem at the source
2) water potential is lowered as sucrose concentration increases, causing water to move by osmosis from the xylem into the sieve tube
3) this creates a high turgor pressure inside the sieve elements at the source end of the phloem
4) this causes the mass flow of water and assimilates through the phloem from the source to the sink
5) there is now a high sucrose concentration in the sieve element, and it diffuses into the sink
6) as the sucrose leaves, water potential at the sink increases, and water moves out of the sieve elements back into the xylem through osmosis, lowering the turgor pressure
7) sucrose is then converted into other substances like starch for storage or glucose and fructose for respiration
8) The pressure gradient is maintained from the source to the sink through this process and therefore mass flow can continuously occur