Translocation 3.3.7

Question 1

What is translocation?

Answer 1

The transport of organic assimilates within the phloem is called translocation.

Question 2

What are assimilates?

Answer 2

Sugars and amino acids, made by the plant

Question 3

What is a source?

Answer 3

A part of the plant that loads assimilates into the phloem sieve tubes

Question 4

What is a sink?

Answer 4

A part of the plant that removes assimilates from the phloem sieve tubes

Question 5

What are phloem sieve tubes made of?

Answer 5

Live cells

Question 6

What is the lumen?

Answer 6

The part of the sieve tube that is left as a liquid filled space - formed during development

Question 7

What are sieve plates?

Answer 7

Perforated end walls in the sieve tubes

Question 8

What are sieve tube elements?

Answer 8

The section of phloem sieve tube between the sieve plates

Question 9

Where does the lumen go?

Answer 9

It continues through the sieve plates at each end of the sieve tube elements

Question 10

Where are companion cells found?

Answer 10

Alongside sieve tubes

Question 11

What is the function of companion cells?

Answer 11

Perform metabolic functions that maintain the cell

Question 12

Why is translocation required?

Answer 12

• The leaves produce large amounts of glucose which is converted to sucrose for transport, then when it reaches the cells, it is converted back to glucose for respiration, or to starch for storage.

Question 13

Where is active loading used in the phloem?

Answer 13

Sucrose is loaded into the sieve tube by an active process - it requires energy from ATP in the companion cells

Question 14

State the process of active loading and phloem loading, from source to sink

Answer 14

• Hydrogen ions are pumped out of the companion cell into a source cell (such as a leaf) this creates a concentration gradient.
• H ions diffuse back into the companion cells co-transporting sucrose
• As the sucrose concentration increases in the companion cells it diffused through the plasmodesmata into the sieve tube.
• The movement of sucrose across the phloem is by mass flow - which is caused by a difference in hydrostatic pressure between the two ends of the tube (pressure gradient)
  At the source = high pressure & low water potential
  At the sink = low pressure & high water potential (leads to osmosis out of the sieve tube which reduces hydrostatic pressure)
• Concentration gradient is always maintained as sucrose is constantly in demand

Question 15

Name 2 sources of assimilates

Answer 15

• Green leaves and stems
• Food stores in seeds when they germinate

Question 16

How does sucrose move along the phloem?

Answer 16

By mass flow -from a low water potential (at the source) to a high water potential (at the sink).
And as water leaves the phloem at the sink (due to high water potential) hydrostatic pressure reduces and sap therefore moves from source to sink.