Transport in Plants - Phloem 3B

Question 1

What is the function of phloem tissue

Answer 1

To transport solutes

Question 2

What are solutes

Answer 2

dissolved substances

Question 3

Types of phloem tissue

Answer 3

sieve tube elements and companion cells

Question 4

What do companion cells do

Answer 4

carry out functions for sieve cells

Question 5

sieve tube structure

Answer 5

few organelles and no nucleus

Question 6

companion cell structure

Answer 6

many mitochondria and a nucleus

Question 7

Define translocation

Answer 7

the movement of solutes from the source to the sink

Question 8

How is the concentration gradient contained in translocation

Answer 8

by enzymes, they change the solutes at the sink so there’s always a lower concentration at the sink

Question 9

What does translocation require

Answer 9

energy in the form of ATP

Question 10

Mass flow hypothesis

Answer 10

1. Active transport used to actively load the solutes from the companion cells into the sieve tubes this lowers the water potential inside the sieve tubes so water enters the tube by osmosis from the xylem and companion cells this creates a higher pressure inside the sieve tubes at the source end of the phloem
2. At the sink end, solutes are removed from the phloem, this increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis this lowers the pressure inside the sieve tubes
3. The result is a pressure gradient from the source end to the sink end
   Gradient pushes solutes towards the sink end to be used

Question 11

Mass flow hypothesis supporting evidence

Answer 11

1. If a ring of bark was removed from a woody stem, a bulge forms above the ring, the fluid from the bulge has a higher concentration of sugars than the fluid from below the ring - evidence of downward movement of sugars
2. A radioactive tracer can be used to track the movement of organic substances in a plant
3. Pressure in the phloem can be investigated using aphids, the sap flows out quicker near the leaves - evidence that there’s a pressure gradient
4. If a metabolic inhibitor is placed inside the phloem, translocation stops - evidence that active transport is involved

Question 12

Mass flow hypothesis objections

Answer 12

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

Question 13

How can translocation be demonstrated experimentally

Answer 13

by using radioactive carbon, by using a process called autoradiography, the plant is killed and placed on photographic film, substance is present where film turns black results demonstrate downward movement of solutes (translocation)