Transport in plants - phloem

Question 1

What does the phloem transport?

Answer 1

Organic solutes

Question 2

What are solutes?

Answer 2

Dissolved substances

Question 3

What are the main solutes that the phloem transports?

Answer 3

Sugars

Question 4

How is the phloem arranged?

Answer 4

In long tubes

Question 5

What three parts make up the phloem?

Answer 5

Sieve tube elements
Sieve plates
Companion cells

Question 6

What are sieve tube elements?

Answer 6

living cells which form the tube for transport

Question 7

What does every sieve tube element have paired with it?

Answer 7

Companion cell

Question 8

What is the purpose of a companion cell?

Answer 8

To carry out all functions for the sieve tube elements

Question 9

Why do sieve tube elements need companion cells?

Answer 9

They have no nucleus

Question 10

What do companion cells provide?

Answer 10

Energy for transporting the solutes

Question 11

What is the purpose of sieve plates?

Answer 11

Not known

Question 12

What is translocation?

Answer 12

The movement of solutes to where its needed

Question 13

During translocation, where do the solutes move from? Which way along the concentration gradient?

Answer 13

Source to sink

High to Low

Question 14

How are enzymes used in translocation?

Answer 14

They maintain a concentration gradient from source to sink by changing solutes at the sink so that theres always a lower concentration gradient at the sink

Question 15

What is the best supporting theory for translocation?

Answer 15

Mass flow hypothesis

Question 16

What happens at the source according to the mass flow hypothesis?

Answer 16

1) Active transport actively loads solutes from companion cells into the sieve tubes of the phloem
2) This lowers the water potential inside the sieve tubes
3) Water enters via osmosis from xylem and companion cells
4) Creating a high pressure inside the sieve tubes at the source end of the phloem

Question 17

What happens at the sink end according to the mass flow hypothesis?

Answer 17

1) Solutes are removed from the phloem increasing water potential inside the sieve tubes
2) Water leaves the tubes by osmosis
3) Lowers the pressure inside the sieve tubes

Question 18

The higher the concentration of sucrose at the source…

Answer 18

The higher the rate of translocation

Question 19

What does translocation create? What does this cause?

Answer 19

Pressure gradient from the source end to the sink

Causing the solutes to be pushed along the sieve tubes towards the sink

Question 20

What do companion cells contain lots of?

Answer 20

Mitochondria

Question 21

Why is the ATP released by companion cells needed?

Answer 21

To actively load the solutes into the phloem at the source

Question 22

How is bark used as supporting evidence for the mass flow hypothesis?

Answer 22

The fluid from the bulge, created by the removal of a ring of bark, has a higher concentration of sugars than below the ring
The sugars can’t move past the area where the bark has been removed

Question 23

What is evidence that there is a pressure gradient during translocation?

Answer 23

When pierced, the sap flows out quicker nearer the leaves than further down the stem

Question 24

What is used as evidence that there is active transport involved in translocation?

Answer 24

If a metabolic inhibitor, which blocks ATP production, is put in then translocation stops

Question 25

What is evidence against the mass flow hypothesis?

Answer 25

The sugar travels to many different sinks, not just the one with the highest water potential like suggested

Question 26

How are sieve plates used as an argument against mass flow hypothesis?

Answer 26

They create a barrier so a lot of pressure would be needed