Where are the phloem tissues found?
throughout plants, including the stems, roots and leaves
What are phloem composed of?
What are sieve tubues composed of?
columns of specialized cells called sieve tube cells; are living; no nucleus (closely associated with companion cells)
What are individual sieve tube cells separated by?
Perforated walls called sieve plates with pores
What do phloem do?
transport organic compound through the plant down the plant from source to sink
What is translocation?
the transport of organic solutes in a plant from source to sink
Why is the function of the phloem (transport organic compounds) so important in a plant?
Links parts of plant that need supply of sugars and other solutes like amino acids to parts that have a surplus
What is a source in terms of translocation?
areas where sugars and amino acids are loaded into the phoem/provide
What is a sink in terms of translocation?
where sugar and amino acids are unloaded and used/needed
In what direction does the phloem flow?
Flows in either direction (no valves/pumps) as sinks can turn into sources sometimes
What is a similarity between blood vessels and phloem?
both systems a fluid flows inside tubes because of pressure gradients. For both energy is needed to generate meaning both are active processes
What is an example of a source?
photosynthetic tissues: mature green leaves, green stems; storage organs that are unloading: storage tissues in germinating seeds, tap roots or tubers at the start of the growth season
What is a part of a plant that is a sink?
roots that grow or absorb mineral ions using energy from cell respiration; parts of the plant that grow or develop food stores (fruits, seeds, leaves)
What is the most prevalent solute in phloem sap?
Why is sucrose a good transport form of carbohydrates?
Sucrose is not as readily available for plant tissues to metabolize directly in respiration, so it will not be metabolised during transport
What is phloem loading?
This is the mechanism by which plants bring sugars into the phloem. this differs for different plants
What is the apoplast route?
When a significant amount of sucrose travels through CELL WALLS from mesophyll cells to the CELL WALLS of companion/sieve cells where a sucrose transport protein then actively transports the sugar in
How is the concentration gradient of sucrose established between phloem and cell walls companion cells ?
active transport where H+ ions are actively transported out of the companion cell from surrounding tissues using ATP as an energy source; ATP is broken down into ADP +Pi that is used to carry sucrose into the cell; Because H+ concentration outside of the sieve tube is higher, it goes down concentration gradient into the sieve tube
What is the symplast route?
much of sucrose travels between cells through connections (cytoplasm) between cells called plasmodesmata;
What happens to sucrose after it reaches the companion cell?
is converted to an ogliosaccharide to maintain the sucrose concentrations gradient
How is water drawn into the companion cell?
sucrose/carbohydrates is high concentrated inside the companion cell and draws water in by osmosis.
What results in a build -up of pressure inside a phloem?
the rigid cell walls combined with the incompressibility of water causes pressure
What happens to the water inside the phloem once sucrose is withdrawn from it and used?
Loss of solute causes less osmotic pressure and the water that carried the solute to sink is then drawn to the xylem
What can the sucrose be used for in a plant?
energy source for processes (like growth) or conversion into starch
Why do sieve cells have to be living?
They depend on the membrane to help maintain the sucrose and organic molecule concentration that has been established by active transport
Why are sieve cells closely associated with companion cells?
they share the same parent cell
What does a companion cell do?
perform many of the genetic and metabolic functions of the sieve tube cell and maintain the viability of the sieve tube cell
How can the companion cell increase the phloem loading capacity?
The infolding of the plasma membrane increases capacity using the apoplastic route
What are plasmodesmata?
they are connections between the cytoplasm of companion cells and sieve tube cells
What do plasmodesmata do?
accommodates the movement of oligosaccharides and genetic elements between the companion and sieve tube cells