3B - Transport in plants - Phloem

Question 1

What are solutes?

Answer 1

Dissolved substances.

Question 2

What does the phloem transport?

Answer 2

Solutes - mainly sugars like sucrose but also hormones.

Question 3

What is the structure of phloem?

Answer 3

• Made of living cells but have few organelles.
• Have ends that form sieve tube structures, through which cytoplasm can pass.
• Companion cells to keep sieve tube element alive (they respire on the element’s behalf).

Question 4

What is the concentration of solutes like at different ends of the phloem?

Answer 4

High concentration at the source, e.g. the leaf.

Low concentration at the sinks, e.g. the roots.

Question 5

What is the pressure like at different ends of the phloem?

Answer 5

High pressure at the source, low pressure at the sinks.

Question 6

What is the name for the movement of solutes?

Answer 6

Translocation.

Question 7

What are solutes sometimes called?

Answer 7

Assimilates.

Question 8

Is translocation active or passive?

Answer 8

Active - it is energy-requiring.

Question 9

In what direction does translocation move solutes?

Answer 9

From source to sink - usually with gravity.

Question 10

What is the source (of a solute)?

Answer 10

Where the solute is made.

Question 11

What is the sink (of a solute)?

Answer 11

Where it is used up.

Question 12

What helps to maintain a concentration gradient from the source to the sink?

Answer 12

Enzymes.

Question 13

How do enzymes maintain a concentration gradient from the source to the sink?

Answer 13

By changing the solutes at the sink (e.g. by breaking them down or making them into something else).

Question 14

How and where are solutes moved through a plant?

Answer 14

Actively transported by co-transport from the source cells to the companion cells to the sieve tubes.

Question 15

Explain the mass flow hypothesis (how the xylem and phloem interact)

Answer 15

• Solutes actively transported from the companion cells into the sieve tubes of the phloem at the source (leaves).
• This lowers the water potential inside the sieve tubes so water enters them by osmosis from the xylem and companion cells.
• This created high pressure inside the sieve tubes at the source end of the phloem.
• Solutes are removed at the sink end from the phloem to be used up.
• 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.
• The result is a pressure gradient from the source end to the sink end.
• This gradient pushes solutes along the sieve tubes towards the sink.
• When they reach the sink the solutes will be used (e.g. in respiration) or stored (e.g. as starch).

Question 16

What supporting evidence is there for mass flow and phloem?

Answer 16

Ringing experiments - If a ring of bark (which includes the phloem but not the xylem) is 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. This is evidence that there’s a downward flow of sugars.

Tracer experiments - A radioactive tracer such as 14C (by for example exposing the plant to radioactive CO2 in a lab) can be used to track the movement of organic substances in a plant.

Aphids - Used to investigate pressure in the phloem. They pierce the phloem, then their bodies are removed leaving the mouthparts behind, which allows the sap to flow out. The sap flows out quicker nearer the leaves than further down the stem. This is evidence that there’s a pressure gradient.

Metabolic inhibitors - These stop ATP production. If one is put into the phloem, then translocation stops. This is evidence that active transport is involved.

Question 17

What evidence is there against the theory of mass flow in phloem?

Answer 17

Sugars travel to many different sinks, not just the one with the highest water potential, as the model would suggest.

The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 18

How can the translocation of solutes be demonstrated experimentally?

Answer 18

In an experiment using radioactive tracers.

• Supply a part of a plant (often a leaf) with an organic substance that has a radioactive label (for example, carbon dioxide containing the radioactive isotope 14C. This can be supplied to a single leaf by being pumped into a container which completely surrounds the leaf).
• The radioactive carbon will then be incorporated into organic substances produced by the leaf (e.g. sugars produced by photosynthesis), which will be moved around the plant by translocation.
• The movement of the substances can be tracked using autoradiography. To see where the radioactive tracer has spread to in a plant, the plant is killed (e.g. by freezing it using liquid nitrogen) and then the whole plant (or sections of it) is placed on photographic film (the radioactive substance is present wherever the film turns black.
• The results demonstrate the translocation of substances from source to sink over time (for example, autoradiographs of plants killed at different times show an overall movement of solutes, e.g. the products of photosynthesis, from the leaves towards the roots).