Module 3 : Section 3 - Translocation

Question 1

Define translocation

Answer 1

The movement of dissolved substances

Question 2

What are dissolved substances sometimes called

Answer 2

Assimilates

Question 3

What is translocation

Answer 3

• An energy-requiring process that happens in the phloem
• moves substances from ‘source’ to ‘sinks’

Question 4

Why is sugars transported as sucros

Answer 4

Because it is insoluble and metabolically inactive - so it doesn’t get used up during transport

Question 5

Give an example of translocation

Answer 5

• The source of sucrose is usually leaves (where it is made)
• The sinks are the other parts of the plant, especially food storage organs and meristems in the roots and stems and leaves

Question 6

Some parts of a plant can be

Answer 6

A sink and a source

Question 7

Give an example of parts of plants that are sinks and sources

Answer 7

• sucrose can be stored in the roots
• during growing season sucrose is transported from the roots to the leaves to provide the leaves with energy growth
• in this case the roots are the source and the leaves are a sink

Question 8

What maintains a concentration gradient from the source to the sink

Answer 8

• Enzymes. They do this by changing the dissolved substances at the sink
• e.g. breaking them down or making them into something else
• this make sure there’s always a lower concentration at the sink than at the source

Question 9

Give an example of enzymes maintaining a concentration gradient from the

Answer 9

• In potatoes, sucrose is converted to starch in the sink areas, so there’s always a lower concentration of sucrose at the sink than inside the phloem
• this make sure a constant supply of new sucrose reaches the sink from the phloem
• in other sinks, enzymes such as invertase break down sucrose into glucose for use by the plant, makes sure there’s a lower concentration of sucrose at the sink

Question 10

What best explains phloem transport

Answer 10

Mass flow hypothesis

Question 11

What is the first stage of the mass flow hypothesis

Answer 11

• active transport is used actively to load the solute into sieve tubes of the phloem at the source
• this lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells
• this creates high pressure inside the sieve tubes at the source end of the phloem

Question 12

What is second stage of the Mass flow hypothesis

Answer 12

• At the sink end, solutes are removed 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 pressures inside the sieve tubes

Question 13

What is the third stage of the mass flow hypothesis

Answer 13

• the result is a pressure gradient from the source en to the sink end
• this gradient pushes solutes along the sieve tubes to where they’re needed

Question 14

How do substances enter the phloem at the source

Answer 14

Substances enter the phloem at the source by active loading

Question 15

What is active loading

Answer 15

• active loading is used to move substances into companion cells from surrounding tissues and from the companion cells into the sieve tubes against a concentration gradient

Question 16

The concentration of sucrose is higher in companion cells than surrounding tissue cells and higher in the sieve tubes than in companion cell

So sucrose is moved to where it needs to go using active transport and Co-transporter proteins

How does it work

Answer 16

• in the companion cell, ATP is used to actively transport hydrogen ions out of the cell and into surroundings tissue cells
• this sets up a concentration gradient - there are more hydrogen ions in the surrounding tissue than in the companion cell
• a H⁺ ion binds to a co-transporter protein in the companion cell membrane and re-enters the cell (against concentration gradient)
• a sucrose molecule binds to the co-transporter protein at the same time. The movement of the H⁺ ion is used to move the sucrose molecule into the cell, against concentration gradient
• sucrose molecules are then transported out of the companion cells and into sieve tubes by the same process

Question 17

What supplies the initial energy needed for the active transport of the H⁺ ions

Answer 17

The breakdown of ATP supposed the initial energy needed for the active transport of the H⁺ ions

Question 18

What is the ‘source’ of a substance

Answer 18

the source of a substance is where it’s made (higher concentration)

Question 19

What is the sink

Answer 19

the sink is the areas where the assimilates are used up (lower concentration)