The Sucrose Loading Mechanism

Question 1

Carbohydrates are generally transported in plants in the form of sucrose because

Answer 1

It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)

It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported

Question 2

Loading of assimilates (eg. sucrose)
The molecules may move by the:

Answer 2

symplastic pathway (through the cytoplasm and plasmodesmata) which is a passive process as the sucrose molecules move by diffusion

apoplastic pathway (through the cell walls) which is an active process

Question 3

If the sucrose molecules are taking the apoplastic pathway then modified companion cells (called ………..) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. This is an ………..process and therefore requires ATP as an energy source

Answer 3

transfer cells

active

Question 4

The large concentration of hydrogen ions in the cell wall of the companion cell results in the

Answer 4

hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell

Question 5

The hydrogen ions move through a ………………protein. While transporting the hydrogen ions this protein also carries….

Answer 5

cotransporter

sucrose molecules into the companion cell against the concentration gradient for sucrose

Question 6

The hydrogen ions move through a cotransporter protein.
While transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose

The sucrose molecules then move INTO VIA ………

Answer 6

into the sieve tubes via the plasmodesmata from the companion cells

Question 7

Companion cells have infoldings in their cell surface membrane to increase

Answer 7

the available surface area for the active transport of solutes and many mitochondria to provide the energy for the proton pump

Question 8

This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the

Answer 8

mesophyll

Question 9

A high concentration of sucrose decreases the water potential in the

which causes

Answer 9

phloem and water enters by osmosis

Question 10

phloem and water enters by osmosis
The entry of water results in a ………….(pressure gradient) which enables………..

Answer 10

high pressure

the mass flow of sugars towards sink tissues

Question 11

At the sink tissues, sugars are

Answer 11

unloaded