4. Translocation Flashcards
What is Translocation
The movement of assimilates (dissolved substances sa. sugars (sucrose), amino acids, hormones) from where they are made (source) to where they are required (sink).
What type of process in Translocation & where does it happen
- Its an active process - meaning it requires ATP energy (to create a pressure difference)
- Occurs in the phloem vessels
Translocation in terms of concentration
Translocation moves substances from “sources’ to ‘sinks’.
The source of a substance is where its made (so its at a high concentration there).
The sink is the area where its used up (so its at a lower concentration there)
Eg of ‘sources’ & ‘sinks’
The source for sucrose is usually the leaves (where its made), & the sinks are the other parts of the plant, especially the food storage organs & the meristems (areas of growth) in the roots, stems & leaves
Eg of when some parts of a plant are both a sink AND a source
Sucrose can be stored in the roots. During the growing season, sucrose is transported from the roots to the leaves to provide the leaves with energy for growth. In this case, the roots are the source & the leaves are a sink
How do enzymes affect Translocation
Enzymes maintain a concentration gradient from the source to the sink by changing the dissolved substances at the sink (eg. by breaking them down or making them into smth else).
This makes sure theres always a lower concentration at the sink than at the source
Eg of how enzymes increase Translocation
- In potatoes, sucrose is converted to starch in the sink areas, so theres always a lower concentration of sucrose at the sink than inside the phloem.
- This makes sure a constant supply of new sucrose reaches the sink from the phloem
- In other sinks, enzymes sa invertase break down sucrose into glucose (& fructose) for use by the plant - again this makes sure theres a lower concentration of sucrose at the sink
What is the mass flow hypothesis
Scientists still arent certain exactly how the dissolved substances (solutes) are transported from source to sink by translocation. The best supported theory is the mass flow hypothesis
Explain the mass flow hypothesis
1.
- Active transport is used to actively load the solutes (eg. sucrose from photosynthesis) into the sieve tubes of the phloem at the source (eg. the leaves).
- This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem & companion cells
- This creates a high pressure inside the sieve tubes at the source end of the phloem
2. Unloading of sucrose at the sink
- 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 pressure inside the sieve tubes
3.
- The result is a pressure gradient from the
source end to the sink end.
- This gradient pushes solutes along the sieve tubes to where they’re needed
see pg94 for dia of mass flow hypothesis
What is Active loading
- Active loading is used to move substances into the companion cells from surrounding tissues, & from the companion cells into the sieve tubes, against a concentration gradient.
- In other words, substances enter the phloem at the source by active loading
- Loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP & co-transporter proteins
How is sucrose moved to where it needs to go
- The concentration of sucrose is usually higher in the companion cells than the surrounding tissue cells, & higher in the sieve tube cells than the companion cells
- So sucrose is moved to where it needs to go using active transport & co-transporter proteins
Method of how sucrose is moved to where it needs to go using active transport & co-transporter proteins
- In the companion cells, ATP is used to actively transport hydrogen ions (H+) out of the cell & into surrounding tissue cells
- This sets up a concentration gradient - there are more H+ ions in the surrounding tissue than in the companion cell
- An H+ ion binds to a co-transporter protein in the companion cell membrane & re-enters the cell (down the concentration gradient)
- A sucrose molecule binds to the co-transport protein at the same time. The movement of the H+ ion is used to move the sucrose molecule into the cell, against its concentration gradient
- Sucrose molecules are then transported out of the companion cells & into the sieve tubes by the same process
How is the energy needed for active transport of the H+ ions produced
ATP is one of the products of respiration. The breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions
Describe the movement of Translocation
Movement is bidirectional (from source to sink)
What is the liquid being transported in Translocation called
‘phloem sap’
3 stage process of Translocation
3 stages to the movement of assimilates (sucrose) from source to sink
- Active loading at the source into the phloem sieve tube
- Mass flow of sucrose through the sieve tube elements (involves water from xylem)
- Unloading of sucrose at the sink
Adaptations for Active loading
- Companion cells have infoldings in their cell surface membrane to increase the available SA for the active transport of solutes & many mitochondria to provide the energy for the proton pump
- This mechanism permits some plants to build up the sucrose in the phloem to up to 3x the concentration of that in the mesophyll
