9.4 Translocation Flashcards
How do plants transport organic compounds in the phloem
From sources to sinks by translocation
What are assimilates
The products of photosynthesis that are transported in an energy requiring process
What is the main assimilate in plants
Sucrose
What are the sources of assimilates
Green leaves abs stems
Storage organs such as tuber and tap roots
Food stores in seeds
What are sinks in plants
Growing roots
Actively dividing meristems
Any part of the plants laying down good stores
What are the two main waits assimilates load into the phloem
Apoplast route
What happens to sucrose at the start of the apoplast route in phloem loading (translocation)
Sucrose from the source travels through the cell walls and intercellular spaces to the companion cells and sieve elements
What happens to sucrose once moved into companion cells
It is moved into the cytoplasm in an active process
Hydrogen ions are actively pumped out of the companion cell into the surrounding tissue using ATP
The hydrogen ions are returned to the companion cells down a concentration gradient via a co-transport protein - sucrose is co-transported
What happens when sucrose is co-transported with the hydrogen ion back into the companion cells
Sucrose concentration increases which causes water to move into the companion cells which leads to a build up of turgor pressure
Water carrying assimilates then move into the tubes of the sieve tube elements abs reduces the companion cell pressure
What happens to water carrying the assimilates as it moves out of the companion cells and into the sieve tube elements
It moves by mass flow into areas of low pressure - the sinks
What is phloem unloading
Diffusion of sucrose from phloem to surrounding cells
When phloem loses solutes this leads to a rise in its water potential so water also moves out of the surrounding cells
