Lesson 69 - Translocation in the phloem

Question 1

What is translocation?

Answer 1

• Transport of organic molecules from one part of the plant to another
• Takes place in the phloem
• Transports organic molecules (sugars) from the site of production (photosynthesis), known as the source, to the place the organic molecules (sugars) will be used or stored for future use, known as the sink

Question 2

What is the structure and function of the phloem?

Answer 2

• Formed from cells arranged into a tube, contains: sieve tube elements (sieve plates with pores in between) and companion cells
• Sieve tube elements: Living cells that form the tube for transporting solutes. They have no nucleus/few organelles, for efficient transport of substances through the phloem.
• Companion cells: (1 for each sieve tube element) They carry out living functions for sieve cells, e.g. providing the energy needed for the active transport of solutes.

Question 3

What are the 3 stages of mass flow of sucrose in the plant?

Answer 3

1. Transfer of sucrose from photosynthesising tissue/cell at source (leaf) into sieve tube - loading
2. Mass flow of sucrose through sieve tube down a pressure gradient
3. Transfer of sucrose from the sieve tube into sink cells (root) for use or storage - unloading

Question 4

Which direction is the waterflow in the phloem?

Answer 4

Down the plant - from an area of high hydrostatic pressure to an area of low hydrostatic pressure

Question 5

What are the 2 experiments that provide evidence for translocation?

Answer 5

Ringing experiments
Tracer experiments

Question 6

Describe Ringing experiments

Answer 6

• A ring of protective bark layer & phloem removed around whole circumference of woody stem
• After time, region immediately above the ring starts to swell
• Samples taken from swollen region are rich in sugars/other organic substance
• Non-photosynthetic tissues below the ring die, while those above continue to grow

Question 7

Describe the Tracer experiments:

Answer 7

• Radioactive isotopes can be used to trace the movement of substances in plants (eg. C14 can be used to radioactively label CO2)
• If a plant is grown in an atmosphere of CO2 (14) it will incorporate C14 into the sugars it produces during photosynthesis
• The radioactive isotope can then be traced using autoradiography - which detects radioactive substances using x-ray photography
• Thin cross-sections of the plant stem can be taken and placed on a piece of x-ray film
• The film appears black where it has been exposed to the radiation produced by C14 in the sugars
• The blackened regions correspond to where the phloem is located

Question 8

How is a lower conc. of sucrose at the sink maintained?

Answer 8

Enzymes change the solutes at the sink, by breaking them down or making them into something else.
E.g. sucrose is converted into starch in potatoes.

Question 9

What are assimilates?

Answer 9

Substances that become incorporated into the plant tissue.