Transport In Phloem

Question 1

What substances are transported in phloem by translocation?

Answer 1

1. Sucrose
2. Amino acids
3. Fatty acids and glycerol

Question 2

Describe the structure of phloem.

Answer 2

• many sieve tubes made up of living cells
• no organelles inside the sieve tubes for an unrestricted flow
• each sieve tube has a companion cell beside it
• companions cells provide ATP for active transport

Question 3

What is a source in a plant?

Answer 3

Where the sugars are made

E.g. leaves which photosynthesis

Question 4

What is a sink in a plant?

Answer 4

Part of the plants which need sugar

E.g. roots, fruits, stem, growing leaves

Question 5

Describe the Mass Flow Hypothesis in terms of the transport of sucrose through the phloem.

Answer 5

1. Leaf photosynthesis to produce glucose which is quickly converted into sucrose
2. Companion cells produce ATP through aerobic respiration in the mitochondria which is used to actively transport the sucrose from the companion cells into the phloem
3. A higher concentration of sucrose in the phloem reduces water potential so water moves into the phloem from the xylem by osmosis down a water potential gradient.
4. A higher concentration of water now in the phloem increases hydrostatic pressure which causes the sucrose to move down a hydrostatic pressure gradient (translocation)
5. Sucrose is actively transported into the companion cells in the roots from the phloem.
6. Concentration of sucrose lowers in the phloem so the water potential increases higher than the xylem and water moves back into the xylem from the phloem by osmosis down a water potential gradient

Question 6

How can ringing experiments be used as evidence for mass flow?

Answer 6

• remove a complete ring of phloem from a part of a plant
• over a few weeks, a swelling of photosynthetic products will form above the ring as translocation has been prevented through the phloem down the plant

Question 7

How can radioactive tracers be used as evidence for mass flow?

Answer 7

• use of radioactive isotope 14CO2 can be reacted in photosynthesis and its movement is able to be traced using auto radiography
• evidence if the traced carbon is found in the leaves and roots

Question 8

How can aphid mouthparts be used as evidence for mass flow?

Answer 8

• can be used to investigate pressure by piercing phloem with aphids and removing their bodies
• sap flows out quicker near the leaves showing there is a pressure gradient involved in mass flown

Question 9

How can respiratory inhibitors be used as evidence for mass flow?

Answer 9

• stops production of ATP through aerobic respiration in the mitochondria
• mass flow stops proving that ATP is required

Question 10

What evidence is there against mass flow?

Answer 10

1. Sugar travels to many different sinks, not just the one with the highest water potential
2. Sieve plates would create a barrier to mass flow. A lot of pressure would be needed for solutes to get through at a reasonable rate.