Transport in plants- Phloem

Question 1

What do phloem transport?

Answer 1

-Phloem transport solutes

Question 2

The importance of sieve tube elements in phloem?

Answer 2

Form the tube for transporting solutes- have no nucleus and few organelles

Question 3

Importance of companion cells?

Answer 3

There is a companion cell for each sieve tube element- carry out living functions for sieve cells (provide energy needed for active transport of solutes)

Question 4

Describe translocation

Answer 4

-movement of solutes (sugars and amino acids) to where theyre needed in a plant
-Energy requiring process
-Moves solutes from sources to sinks- source is where its made (at high conc) and sink is where its used up (at low conc)
-Enzymes maintain a conc gradient from the source to the sink by changing the solutes at the sink

Question 5

Explain the mass flow hypothesis-

Answer 5

1)
-Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the the source (leaves)
-This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells
-Creates a high pressure inside the sieve tubes at the source end of the phloem
2)
-At the sink end, solutes are removed to be used up
-Increasing the water potential inside the sieve tubes, so water leaves the sieve tubes by osmosis- lowering the pressure
3)As a result, there is a pressure gradient from the source end to the sink end
-Gradient pushes the solutes along the sieve tubes to towards the sink
-When they reach the sink, the solutes will be used (in respiration) or stored.

Question 6

How to investigate translocation?

Answer 6

Tracers and ringing experiment

Question 7

Describe the tracers

Answer 7

-Involves radioactively labelling carbon 14C.
-Plants are provided with radioactively labelled carbon, over time this is absorbed into the plant and used in photosynthesis to create sugars which all contain radioactively labelled carbon- the products of photosynthesis will then be moved around the plant by translocation.
-Movement of substances can be tracked by using autoradiography- slices from stems are cut and placed on x-ray film that turns black when exposed to radioactive material.
-When the stem are placed on the x-ray film the section that contains sugar will turn black
-Results demonstrate translocation of substances from source to sink over time.

Question 8

Describe the ringing experiment

Answer 8

-Ring of bark and phloem are peeled and removed off a tree trunk.
-Trunk swells above the removed section, concluding that there is a higher concentration of fluid above the removed section than below.
-Evidence that theres a downward flow of sugars and proves that phloem transports sugars.