Phloem And Transport Of Assimilates

Question 1

What is the function of phloem tissue?

Answer 1

Transports assimilates up and down the plant = translocation

Question 2

What does phloem tissue consist of?

Answer 2

• sieve tubes elements
• companion cells

Question 3

What are assimilates?

Answer 3

eg Sucrose, amino acid

Question 4

What is the structure and function of sieve tube elements?

Answer 4

• sieve tube elements have little cytoplasm, few organelles but no nucleus
• sieve tubes do not have lignified walls
• they have cross walls at intervals called sieve plates
• the sieve plates connect the elements
• the sieve plates allow the sap to flow easily
• the elements are lined up end to end to form a tube which allows the plant to transport assimilates, mainly sucrose
• plasmodesmata link the cytoplasm of companion cells and sieve tube elements

Question 5

What is the structure and function of companion cells?

Answer 5

• have cytoplasm and many mitochondria which produce ATP for active transport
•they will also have many proteins in the plasma membrane and many ribosomes

Question 6

What is translocation?

Answer 6

The movement of assimilates. It is an energy requiring process (requires ATP) that happens in the phloem. Translocation moves assimilates from source to sink

Question 7

What is the source?

Answer 7

The part of the plant that releases the assimilates eg leaf, storage organs

Question 8

What is the sink?

Answer 8

The part if the plant that receives the assimilates eg roots, fruit, shoot, meristem (growth region)

Question 9

What is active loading? (loading the sucrose into the phloem)

Answer 9

1. The companion cells use ATP to actively transport/ pump hydrogen ions out of their cytoplasm and into the surrounding tissue
2. This sets up a concentration gradient and the hydrogen ions diffuse back into the companion cells with the sucrose. The diffusion occurs through co-transporter proteins. These proteins allow the hydrogen ions to bring the sucrose molecules into the companion cells
3. As the concentration of sucrose molecules builds up in the companion cells they diffuse into the sieve tube elements through numerous plasmodesmata

Question 10

Explain the mass flow hypothesis

Answer 10

1. AT THE SOURCE
   Active transport is used to actively load the sucrose the into the sieve tube element, reducing the water potential. Water enters the sieve tube by osmosis from the surrounding tissue. This increases the hydrostatic pressure in the sieve tube element.
2. AT THE SINK
   The solutes are being used at the sink. Sucrose may be converted into starch or be used for respiration. The sucrose leaves the sieve tube element at the sink via diffusion or active transport. This increases the water potential in the sieve tube element. The pressure gradient means that the phloem sap moves from source to sink. This is called the mass flow hypothesis.

Sucrose can be moved up or down the plant, depending on the hydrostatic pressure gradient

Question 11

What is evidence for mass flow?

Answer 11

1. Use of aphids
2. Ringing a tree
3. Radioactive tracers
4. Metabolic inhibitors

Question 12

How is ringing a tree evidence for mass flow?

Answer 12

Ringing a tree Involves removing a ring of bark ( which includes the phloem but not the xylem). A bulge forms above the ringed area because the sugars can’t pass the ringed area. This decreases the water potential and water moves into the cells. Ringing is evidence that there can be a downward flow of sugars

Question 13

How are metabolic inhibitors evidence for mass flow?

Answer 13

Translocation can be stopped by using a metabolic poison that inhibits the formation of ATP - this is evidence that active transport is involved

Question 14

What is some evidence against mass flow?

Answer 14

1) sugar travels to many different sinks not just one with the lowest hydrostatic pressure as the model suggests
2) the sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate

Question 15

How is the use of aphids evidence against mass flow?

Answer 15

• an aphid feeding on a plant stem can be used to show that the mouthparts which are feeding from the phloem contain sugars
• pressure in the phloem can be investigated using aphids
• the sap flows out quicker nearer the leaves than further down the stem - evidence there is a pressure gradient

Question 16

How are radioactive tracers used for evidence against mass flow?

Answer 16

Travers such as radioactive carbon can be used to track the movement of organic substances in a plant