3.4.2 Transport Of Organic Substances In Plants

Question 1

Phloem loading description

Answer 1

Carbohydrates enter sieve tubes at source
Source is site of photosynthesis sugar production
H+ move from companion cells to spaces within cell walls by active transport
Requires ATP
H+ diffuses down concentration gradient via carrier proteins into companion cells

Question 2

Mass flow description

Answer 2

Organic compounds loaded into sieve tube elements in the leaves
Causes an increase in hydrostatic pressure caused by osmotic influx.
The increase in hydrostatic pressure pushes the contents of the sieve tube elements down a pressure gradient
Organic compounds are unloaded into cells at the sinks resulting in a decrease in hydrostatic pressure

Question 3

Phloem unloading description

Answer 3

Sucrose moved into sink cells by active transport
Requires ATP
Some sink cells- sucrose actively transported into tonoplast (storage vacuole)
This keeps sucrose conc in ceiling low
So more sucrose enters the cell