Roots and Translocation: Structure of Roots, Process of Translocation and Evidence for it
Edited by Jamie (ScienceAid Editor), Taylor (ScienceAid Editor)
Below is a diagram of a cross section of a primary root.
- 1Epidermis.The outside layer of the root that holds everything in place.Advertisement
- 2Parenchyma.Beneath the epidermis are the parenchyma [pah-ren-ki-ma]. They are packing cells and make up the bulk of the root. Generally they are responsible for storing starch and also respiration.
- 3Endodermis.The endodermis is a layer around the vascular bundle. The cells here contain a casparian strip which helps to control water movement into the xylem.
- 4Xylem & Phloem.These are arranged rather strangely. The xylem transports water, and the phloem transports sugar. We will go into more detail below.
This is the process of transporting sugar and takes place in the phloem. How we think it travels through them is called the mass-flow hypothesis.
The source is where food is produced. This would be the leaves. They produce glucose which is converted to sucrose which enters the phloem. This makes the water potential more negative, making water from the surrounding xylem enter.
All of this extra material increases the pressure and forces the solution down and through the sieve plate. Then it gets to the sink where the sucrose is moved by active transport into the parenchyma; where it is made into insoluble starch so the water returns to the xylem.
Therefore the solutes move from source to sink, but it is important to note that it can go in both directions. For example, at night when there is no light for the leaves to produce sugar by photosynthesis, energy has to come from storage.
Evidence for this theory comes from two sources.
- 1Radioactive Tracers.These show the direction of flow of sucrose, This method uses radioactive carbon dioxide where the C is 14C. Carbon dioxide - (CO2 = 1 carbon atom and 2 oxygen atoms). This radioactive carbon dioxide is put in a bag over a leaf and sealed. The carbon dioxide gets converted into glucose and an x-ray can be taken that will show where this radioactive material has moved to.
- 2Ringing Experiments.Advertisement
- If you have problems with any of these steps, ask a question for more help, or post in the comments section below.
Categories : Plants
Recent edits by: Jamie (ScienceAid Editor)