9.2 Transport in the phloem of plants Flashcards
What is translocation?
Plants transport organic compounds from sources to sinks.
The transport of organic solutes in a plant.
What is the phloem?
Sugar and amino acid transport in the plant.
What is the phloem made of?
The Phloem is composed of sieve tubes. Sieve tubes are composed of columns of specialised cells called sieve tube cells. Individual sieve tube cells are separated by perforated walls called sieve plates. Sieve tube cells are closely associated with companion cells.
What are sources in a plant?
- Leaves
- Green stems
- Storage tissues in germinating sedds
- Tap roots or tubers at the start of growth season
What are sinks?
Roots that are growing or absorbing mineral ions using energy from cell respiration.
Parts of the plant that are growing or developing food stores
- Developing fruits
- Developing seeds
- Developing leaves
- Developing tap roots or tubers
What directions do the phloem and the xylem go in?
Xylem one way.
Phloem is both ways.
Which is passive and which is active? Phloem and Xylem?
Xylem is a passive
Phloem, transport happens because of built pressure which is created by energy.
How does transport in the phloem work?
Active transport is used to load organic compounds into phloem sieve tubes at the source. Sucrose is the most prevalent solute in phloem sap. The phloem loading can occur 2 ways.
APPOPLAST ROUTE
- A significant amount of sucrose travels through cell walls from mesophyll cells to the cell walls of companion cells., and sometimes sieve cells, where a sucrose transport protein then actively transports the protein in.
- H+ ions are actively transported out of the companion cell from surrounding tissues using a proton pump powered by ATP. The build up for H+ ions then flows down its concentration back into the cell through a co-transport protein, and sucrose is pulled through as well.
SYMPLAST ROUTE
- Sucrose travels between cells through connections between cells called plasmodesmata.
- Once sucrose reaches the companion cell it is converted to an oligosaccharide to maintain the sucrose concentration.
FLOWS THROUGH THE CELLS NOT THROUGH THE MEMBRANES
So at this point the companion cell is full of sucrose. The build up for sucrose in the companion cell draws water into the companion cell by osmosis. The rigid cell walls combined with the incompressibility of water result in a build up of pressure. Water will therefore flow from this area of high pressure to an area of low pressure. This area of low pressure is the sinks.
At the sink end, sucrose is withdrawn from the phloem and either utilised as an energy source or converted to starch. In either case, the loss of solute causes a reduction in osmotic pressure and the water that carried the solute to the sink is then drawn back in to the transpiration stream in the xylem.
What is the most prevalent solute in the phloem?
Sucrose. Sucrose is not as readily available for plant tissues to metabolise directly in respiration and therefore makes a good transport form of carbohydrate as it will not be metabolised during transport.
What is the appoplast route?
Way of loading the Phloem:
APPOPLAST ROUTE
- A significant amount of sucrose travels through cell walls from mesophyll cells to the cell walls of companion cells., and sometimes sieve cells, where a sucrose transport protein then actively transports the protein in.
- H+ ions are actively transported out of the companion cell from surrounding tissues using a proton pump powered by ATP. The build up for H+ ions then flows down its concentration back into the cell through a co-transport protein, and sucrose is pulled through as well.
SO IT FLOWS THROUGH THE CELL MEMBRANES
What is the symplast route?
A method of filling the phloem.
SYMPLAST ROUTE
- Sucrose travels between cells through connections between cells called plasmodesmata.
- Once sucrose reaches the companion cell it is converted to an oligosaccharide to maintain the sucrose concentration.
FLOWS THROUGH THE CELLS NOT THROUGH THE MEMBRANES
What do the phloem sieve tubes do?
The functions of the phloem include loading of carbohydrates, transport of those carbohydrates, sometimes over long distances; and unloading of the carbohydrates at sinks.
The phloem is composed of sieve tubes. Sieve tubes are composed of columns of specialised cells called sieve tube cells. Unlike the vascular elements of the xylem, sieve tube elements are living, although they do have reduced quantities of cytoplasm and no nucleus.
One reason they are living is because they depend on the membrane to help maintain the sucrose and organic molecule concentration that has been established by active transport. Sieve tube cells are closely associated with companion cells. This is due in part to the fact that the sieve tube cell and its companion share the same parent cell. The companion cells perform many of the genetic and metabolic functions of the sieve tube cell and maintain the viability of the sieve tube cell. The companion cell has lots of mitochondria to facilitate the active transport of sucrose.
The infolding of the plasma membrane cell increases the phloem loading capacity using the apoplastic route. Plasmodesmata connect the cytoplasm of companion cells with the sieve tube cells and have a larger diameter than plasmodesmata found in other parts of the plant to accommodate the movement of oligosaccharides and genetic elements between the two cells.
The rigid cell walls of the sieve tube cell allow for the establishment of the pressure necessary to achieve the flow of phloem in the sieve tube cell.
How does sucrose move in the phloem?
The build up of solutes when the phloem has just been loaded either by the apoplast route (membranes) or the symplast route (cells). because the phloem has just been loaded with sugar, water follows by osmosis, because the phloem walls are so rigid the pressure builds up, and so the solute moves from an area of high pressure to an area of low pressure THE SINK.
What is water potential?
Water potential is a measure of the tendency of water to move from one area to another. The more positive the more likely it is to move, so the less solute it has in it.
How are companion cells adapted?
Lots of mitochondria to facilitate the active transport of sucrose.
The infolding of the plasma membrane cell increases the phloem loading capacity using the apoplastic route. Plasmodesmata connect the cytoplasm of companion cells with the sieve tube cells and have a larger diameter than plasmodesmata found in other parts of the plant to accommodate the movement of oligosaccharides and genetic elements between the two cells.
