Lecture 10- Phloem

Question 1

What name is given to the phloem transport cells?

Answer 1

Sieve tube elements

Question 2

How are phloem transport cells different to that of the xylem?

Answer 2

They are living

They transport carbohydrates

Question 3

How do sieve tube elements join?

Answer 3

Enlarged plasmodemata pores called sieve plates

Question 4

How do sieve tube elements become hollow?

Answer 4

Membrane that encloses central vacuole (tonoplast) dissapears, nucleus and other components break down

Question 5

What is the name given to movement of carbohydrates and other solutes in the phloem?

Answer 5

Translocation- from sources to sinks

Question 6

What does it mean for a tree to be girdled?

Answer 6

A ring of bark containing phloem was removed

Question 7

What happens after a tree is girdled?

Answer 7

Organic solutes collect in the phloem above the girdle- swelling occurs
Bark, then roots, the whole tree dies because sugar isn’t translocated downwards

Question 8

What are the three characteristics of translocation?

Answer 8

• Stops if phloem is killed
• Proceeds in both directions simultaneously
• Inhibited by compounds that inhibit respiration and limit ATP supply

Question 9

How do aphids feed?

Answer 9

Drilling into sieve tubes and inserting their stylet

Pressure in sieve tube forces sap through stylet into aphid

Question 10

How do plant physiologists sample phloem?

Answer 10

Cut body of aphids away from stylet- phloem sap continues to flow and can be collected/analysed

Question 11

How do plant physiologists find out how long translocation takes to occur?

Answer 11

Using radioactive tracers

Question 12

What did all of these experiments and more lead to the theory of?

Answer 12

Pressure flow model

Question 13

What are the two steps of translocation?

Answer 13

• Loading

- Unloading

Question 14

What do the two steps of translocation require?

Answer 14

Energy

Question 15

What is loading?

Answer 15

Transport of solutes from sources into sieve tubes

Question 16

What is unloading?

Answer 16

Removal of solutes at sinks

Question 17

What happens during loading?

Answer 17

• Solute concentration is sieve tubes is greater than surrounding cells
• Water enters by osmosis- greater pressure potential
• Water and solutes push towards sink

Question 18

What happens at the sink?

Answer 18

Unloading
-Solutes are unloaded by active transport
-This maintains the pressure gradient
Water moves back into the xylem

Question 19

What two conditions must be met for the pressure flow model to be valid?

Answer 19

• Sieve plates must be unobstructed so bulk flow is possible

- There must be an effective method for loading/unloading solutes

Question 20

Why did early electron microscopes show that the sieve plates were blocked by fibrous proteins?

Answer 20

These proteins were a response to damage when phloem was prepared for study

Question 21

What does secondary active transport do in sieve tubes?

Answer 21

Loading of sucrose into companion cells and sieve tubes by sucrose-proton symport

Question 22

Explain how sucrose is carried from the apoplast to the symplast.

Answer 22

Sucrose-proton symplast (coupled), the apoplast has a high concentration of protons supplied by primary active transport (the proton pump)

Question 23

How are solutes transported in sink regions?

Answer 23

Solutes are transported actively out of the sieve tube elements into surrounding tissues

Question 24

How do many substances move through the symplast?

Answer 24

Via the plasmodesmata

Question 25

Where are plasmodesmata more abundant?

Answer 25

Sink tissues- to allow passage of large molecules

Question 26

How do plants and viruses allow more large molecules to pass?

Answer 26

Change the permeability of the plasmodesmata by producing movement proteins

Question 27

Why are biologists interested in these movement proteins?

Answer 27

To be able to modify plasmodesmata

-For example: divert more photosynthetic output to seeds to increase crop yield