Phloem

Question 1

What are solutes?

Answer 1

Dissolves substances

Question 2

What does phloem transport?

Answer 2

Solutes (mainly sugars like sucrose) around plants

Question 3

Why are sieve tube elements important for phloem tissue?

Answer 3

They are living cells that form a tube for transporting solutes

they have no nucleus and few organelles

Question 4

Why is it important that companion cells are there in phloem tissue?

Answer 4

Carry out living functions for sieve cells e.g providing energy needed for active transport of solutes

Question 5

What is translocation?

Answer 5

Movement of solutes (e.g sugars like sucorse and amino acids) to where they are needed in the plant solutes are sometimes called assimilates

energy requiring process in phloem

Question 6

What is the source of solute?

Answer 6

Where it is made (so higher concentration)

Question 7

What is sink?

Answer 7

Area where it is used up (lower concentration there)

Question 8

What is the source for sucrose?

Answer 8

Usually the leaves (where it is made)

Question 9

What are the sinks ?

Answer 9

Other parts of the plant , especially food storage organs and meristems (areas of growth) in roots , stems and leaves

Question 10

Why are enzymes important?

Answer 10

Maintain concentration gradient from the source to the sink by changing the solutes at the sink (e.g breaking them down/making them into something else)

This always make sure that there is a lower concentration at sink than source

Question 11

Examples of enzymes involved

Answer 11

• Potatoes , sucrose converted into starch in sink areas so lower conc of sucrose at sink than inside phloem
• Make sure it is a constant supply of new sucrose reaches sink from phloem

Question 12

What is mass flow hypothesis?

Answer 12

Scientists aren’t exactly sure how solutes are transported from source to sink

best supported theory is mass flow hypothesis

Question 13

What is the first step of mass flow hypothesis?

Answer 13

1. Active transport is used to actively load the solutes (e.g sucrose from photosynthesis) from companion cells into sieve tubes of phloem at the source (e.g leaves)
2. Lowers the water potential inside the sieve tubes so water enters tube by osmosis from the xylem and companion cells
3. Creates a high pressure inside the sieve tubes at the source end of the phloem

Question 14

What is the second step of mass flow hypothesis?

Answer 14

1. At sink end , solutes are removed from phloem to be used up
2. Increases water potential inside the sieve tubes , water also leaves tubes by osmosis
3. Lowers pressure inside the sieve tubes

Question 15

What is the third step of mass flow hypothesis?

Answer 15

1. Result is a pressure gradient from source end to the sink end
2. Gradient pushes solutes along the sieve tubes towards the sink
3. When they reach the sink , solutes will be used up (e.g respiration) or stored (e.g as starch)

Question 16

What do experiments show?

Answer 16

Some sucorse is transported also through the cell walls of the phloem

Question 17

Supporting evidence

Ring of bark

Answer 17

• If a ring of bark (phloem) is removed from a woody stem , bulge form around ring. Fluid from bulge has high concentration of sugars than fluid from below the ring - downward flow of sugar

Question 18

Supporting evidence

radioactive tracer

Answer 18

• A radioactive tracer such as radioactive carbon (¹⁴C) can be used to track the movement of organic substances in a plant

Question 19

Supporting evidence

Pressure in phloem investigated by aphids

Answer 19

• The sap flows out quicker nearer the leaves than further down the stem
• Evidence that there is a pressure gradient

Question 20

Supporting evidence

Metabolic inhibitor (stops ATP production)

Answer 20

• This is put on phloem
• Translocation stops
• Evidence active transport is involved

Question 21

Objections

Answer 21

• Sugar travels to many different sinks not just to do with highest water potential as model suggest
• 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

Question 22

How can translocation of solutes be demonstrated experimentally?

(explain radioactive tracers)

Answer 22

• Radioactive carbon incorporated into organic substanes produced by leaf e.g sugar by photosynthesis which will move around by translocation
• Use autordiography to reveal where the tracer spread ,
• Plant killed and whole plant/sections placed on photograph film (film turns black)
• Results demonstrate translocation from source to sink over time , show overall movement of solutes

Question 23

What happens if there is a higher concentration of sucrose at source?

Answer 23

Higher rate of translocation

Question 24

A scientist is investigating where the products of photosynthesis are translocated to in a plant. To do this several upper leaves of the plant were exposed to a radioactive tracer in form of radioactively-labelled CO2.

Plant was then left for 24 hours before autoradiograph of whole plant was taken

1) explain why leaves of the plant can act as a source in translocation
2) The autoradiograph showed radioactivity in roots and fruits , explain why seen in fruits

Answer 24

(1) Leaves act as source because they are part of plant where solutes/products of photosynthesis are made
(2) Radioactive solutes/products of photosynthesis have been translocated to fruits as acting as a sink