Is phloem tissue alive or dead ? >ALIVE
In flowering plants, the phloem is made up from sieve tube elements and companion cells
Use the mouseover to test whether you can identify the different parts of this simplified phloem diagram :
Use the mouseover to define the jobs and characteristics of:
The sieve elements > These are the actual cells that make up a sieve tube. They transport photosynthetic product around the plant from source to sink. They contain very little cytoplasm, it forming a thin layer around the outside of the cell. They have no nucleus or ribosomes. Where two sieve element met there is a sieve plate.
The companion cells > These maintain the sieve tube elements. They have a normal plant cell structure although compared to most plant cells they have more mitochondria and ribosomes. Numerous plasmodesmata link the companion cells with the sieve elemnets.
So, you think you really know phloem structure....
Mouseover to add the labels:
TRANSLOCATION- how are substances transported by the phloem
Translocation is the term used to describe the transport of manufactured
substances (sometimes called assimilates) within the phloem.
The movement of material in the phloem is by mass flow. This is an
active process and therefore requires energy. Sucrose is actively loaded into
the sieve elements, usually within the photosynthesizing leaf. This causes
the water potential to go down, and water moves into the sieve tube down a
water potential gradient by osmsosis.
Elsewhere along the sieve tube, at a sink (a growing region, eg a shoot),
sucrose is being actively removed and water follows by osmosis. These water
movements cause a pressure difference, a hydrostatic pressure, along
the phloem tube and water and assimilate flows from the high pressure areas
to the low pressure areas. This process is summarised in the diagram below:
