3.3.7(Translocation)

What is meant by the term assimilates?

• They are substances that have become part of the plant such as nitrogen

What is meant by the term source?

• a source is the part of the plant loads assimilates into the transport system (phloem sieve tubes)

Give an example of a source?

• The leaves photosynthesise and sucrose made is moved to other parts of the plants

What is meant by the term sink?

• A sink is a part of a plant that removes assimilates by storing them from the phloem sieve tubes

Define translocation?

• The movement of assimilates throughout the plant via the phloem tube

What does sap consist of?

• Sap is a solution of sucrose, amino acids and other assimilates that can flow up or down in the plant

Describe the process of active loading

1. H+ ions are actively transported transported out of the companion cells against a concentration gradient

2. A concentration gradient in created, with a higher H+ ion concentration outside of the cell and a low concentration of H+ ions in the cell

3. H+ ions then diffuse into the companion cells through special cotransport proteins, which allow hydrogen ions back into the cell only if they are accompanied by sucrose, which moves against its concentration gradient

4. As the concentration of sucrose in companion cells increases, it can diffuse through the plasmodesmata into the sieve tube

Why is active loading called secondary active transport?

• Because it results from the active transport of H+ ions out of the cell, and moves sucrose against its concentration gradient

What method of movement does sucrose move along the phloem?

• It moves by mass flow

Explain how sucrose moves along the phloem?

• Sucrose moves along the phloem by mass flow

• Sucrose decreases water potential in the phloem which causes water from the xylem and the companion cells to move down a water potential gradient via osmosis

• This generates a high hydrostatic pressure which causes a pressure gradient to form at the source and sink.

• This causes sucrose and water to move down a pressure gradient to the sink where the sucrose moves out of the phloem sieve tubes

Describe and explain how sap moves in along the phloem

1. Sucrose is actively loaded using ATP across a concentration gradient into the sieve tube elements and reduces the water potential

2. Water follows vie osmosis down a water potential gradient, which increases the hydrostatic pressure in the sieve tube elements

3. sap moves down the sieve tube from a higher hydrostatic pressure at the source, to a lower hydrostatic pressure at the sink

4. Sucrose is removed from the sieve tube elements into the surrounding cells via facilitated diffusion, which increases water potential in the sieve tubes

5. Water moves out of the sieve tube via osmosis down a water potential gradient, which therefore reduces the hydrostatic pressure in the sink side of the sieve tubes