Transport In Plants - Translocation Flashcards
Define translocation
The movement of assimilates up and down the plant in the stem
What is glucose converted to
Glucose is converted to sucrose for transport
Where are substances moved to and from
Moved from source to sink
What are sources
Assimilates loaded into the phloem
Give examples of sources
- photosynthesising green leaves and stems
- storage organs at the start of the growth season
- seeds when they germinate
What are sinks
Assimilates removed from the phloem
Give examples of sinks
- roots (when growing)
- buds/any actively dividing meristems
- storage organs
- flowers/fruit developing
What happens at the source
- sucrose is loaded into sieve tube elements involving an active process
- this reduces the water potential of the sap
- therefore water follows the sucrose into the sieve element moving down a water potential gradient by osmosis
- this increases hydrostatic pressure
What happens at the sink
- sucrose diffuses out of the sieve tube elements
- this increases the water potential of the sap, so water follows the sucrose out of the sieve elements moving down a water potential gradient by osmosis
- this decreases the hydrostatic pressure in the phloem at the sink
Define mass flow
Water moving in at the source and out at the sink creates a hydrostatic pressure difference in the phloem sieve tube. This forces the sap to move from source to sink by mass flow
How is sucrose moved across the leaf
By the apoplast and symplast pathways. It then needs to be actively loaded into the phloem
Describe the symplast route
In some plants sucrose moves from photosynthesising mesophyll cells to companion cells through plasmodesmata
Describe the apoplast route briefly
Some plants have few, if any plasmodesmata connecting to the companion cells and so sucrose passes across the cell walls. This involves the movement of hydrogen ions
Describe the movement of hydrogen ions in the apoplast route
- hydrogen ions are pumped out of the companion cells using ATP, this leads to a higher concentration of H+ ions outside the cell than inside
- the H+ ions can move back into the companion cells down the concentration gradient through a special co-transporter protein
- they carry with them sucrose molecules, moving them against their concentration gradient
Describe what happens once the sucrose molecules are in the companion cells
They then diffuse from the companion cells into the sieve tube element through the plasmodesmata that connect them
Describe the unloading of sucrose at the sink
- unloading occurs in any tissues that need sucrose
- it is likely that the sucrose moves into the tissues by facilitated diffusion
- once in the tissues the sucrose is converted into something else by enzymes - this maintains a concentration gradient from the phloem to the tissue
How do we know the phloem is used
- radioactively labelled carbon given to plant and tracked
- tree ringing ( vascular tissue removed and bark swells above due to accumulated solutes, water enters by osmosis)
- aphid feeding (used to collect sap)
How do we know it needs metabolic energy
- companion cells have many mitochondria
- use of metabolic poison stops translocation
- high rate of flow of sugars
How do we know it uses this mechanism
- phloem sap has a relatively high pH (hydrogen ions transported out of the neighbouring companion cell)
- higher concentration of sucrose in source than in sink
- difference in electric potential across plasma membrane of companion cells (more negative inside than outside)
What evidence is there against this mechanism
- not all the solutes in the sap flow at the same rate
- sucrose is moved to all parts of the plant at the same rate
- the role of sieve plates is unclear
explain why the tree swells above the cut ring of bark
translocation past ring is prevented, sucrose accumulates, lowering the water potential, causing water to move into area by osmosis
