B6 Translocation

Question 1

Name the tissue that transports organic substances in plants.

Answer 1

Phloem

Question 2

Name the process used to transport organic material through a plant.

Answer 2

Translocation

Question 3

Name two substances usually transported in the phloem

Answer 3

Sucrose
Amino acids

Question 4

Describe and explain the structure of phloem

Answer 4

Sieve tube elements form a tube of cells with perforations in end walls
Companion cells involved in ATP production for active loading of sucrose/amino acids into phloem
Plasmodesmata allow movement of organic substances between companion cell and phloem

Question 5

Describe how sucrose is transported in phloem

Answer 5

At the source sucrose is actively transported into the phloem
By companion cells
Water potential in phloem decreases
Water enters phloem by osmosis
Produces high hydrostatic pressure
Mass flow /transport towards the sink
At sink sucrose removed /unloaded from phloem

Question 6

Name two methods used to investigate the translocation of materiel in a plant

Answer 6

Ringing experiments
Use of tracers

Question 7

Give evidence for the mass flow hypothesis of translocation

Answer 7

Sap is released when the stem is cut - must be pressure in the phloem
Higher concentration of sucrose in leaves than the roots
Increased sucrose levels in the leaves results in increased sucrose in the phloem

Question 8

Give evidence against the mass flow hypothesis of translocation

Answer 8

Structure of sieve tubes seems to hinder mass flow
NOT all solutes move at the same speed as they would in mas flow
Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first
Organic materials move in opposite directions in phloem tissue

Question 9

How can ringing experiments be used to investigate transport in plants?

Answer 9

Bark and phloem of tree removed in a ring, leaving the xylem intact
Tissues missing the ring will swell due to accumulation of sucrose
Sucrose must be transported in phloem

Question 10

How can tracing experiments be used to investigate transport in plants?

Answer 10

Plants grown in presence of radioactive carbon dioxide which will be incorporated into sucrose
Use autoradiography to ‘see’ if areas exposed to radiation correspond with the position of phloem

Question 11

How is sucrose loaded into the phloem at the source?

Answer 11

Hydrogen ions are actively transported into leaf cells using ATP generated by companion cell.
Hydrogen ions combine with sucrose molecules and move down H+ concentration gradient into companion cell (cotransport)
Sucrose diffuses through plasmodesmata down a concentration gradient into phloem

Question 12

What is a sieve tube plate as shown on the diagram?

Answer 12

Perforations in the cross cell wall of phloem

Allows the flow of sucrose solution between phloem cells

Question 13

Name the type of tissue shown in the diagram

Answer 13

Phloem

Question 14

What type of experiment is shown in the picture?

Answer 14

Ringing experiment

Question 15

Why is the stem swollen in the picture

Answer 15

Phloem has been removed

Sucrose solution can not travel along the phloem tissue

Sucrose solution causes the stem to swell

Question 16

Name the technique shown in the picture

Answer 16

Autoradiograph

Question 17

What treatment would have been given to the leaf labelled A?

Answer 17

Radioactive carbon dioxide given to the leaf

Uses the carbon dioxide to make radioactive glucose in photosynthesis

Question 18

How is the autoradiography picture (shown on the right) produced?

Answer 18

Plant placed between photographic plates

Radioactive carbon causes an image to form on the plate

Movement of sugar around the plant can be visualised