introduction cell walls
plastids cytoskeleton
vacuoles microbodies

Cell walls:

Cell walls are actually quite complex 3D multi-layered structures, built up layer by layer from glucose units linked to form cellulose chains:

parallel chains of cellulose make up the smallest fibers called micelles. Micelles in turn are bundled up to form microfibrils and these in turn are bundled to form the macrofibrils:

Strangely enough, one would assume a plant wall made of glucose units should be easily sought after and digested by herbivores, but the linkage between the glucose molecules requires an enzyme which most animals do not possess..

As cells serve different functions, it is to the plants advantage to control the properties of the cell walls; sometimes cells should be flexible ( give an example) , others times the structural integrity of a plant may depend on a rigid cell...

Plants moderate cell wall flexibility by the addition of molecules such as:

1. to increase flexibility a plant adds more pectin between the microfibrils... think of jelly production where pectin is added to 'hold together the juice. The final texture is 'solild' yet very flexible. Pectin is formed from glucose. Instead of a CH2OH, there is a OH-C=O onto which Ca+ or Mg+ bond to . IF however pectin is acidified, a H will attach to the C=O and it is no longer linked to a Ca.. so when a cell wall needs to 'loosened up'as during growth or expansion, H+ are pumped out, pectin is free from the Ca bridges and the fibers can expand out. When neutralized they come together...

2. if the plant wants a really rigid wall, it adds lignin to the wall... this molecule shows up red in the prepared slides, and you wii notice it wherever there are heavily lignified xylem or sclerenchyma cells for support.

3. if a plant wants the cell to be water insoluble, it will coat or impregnate the walls with suberins (found in cork), cutin (walls of the epidermis) or waxes...

4. Hemicelluose resembles cellulose but is more soluble and less ordered...note in the above diagram that the hemicelluloses are linked to the surface of the cellulose microfibrils by hydrogen bonds. Some of these heimicelluloses are cross-linked in turn to acidic pectins by neutral pectin molecules. The glycoproteins in turn are attached to the pectin. Ca bridges form between the pectins..

5. Glycoproteins opposite to the waxes add more solubility to a cell wall... the question is why would a plant want a cell wall to which water is attracted??

How are these components arranged?

After a primary wall is laid, it is held together by a layer known as the middle lamella..heavily dosed with pectic substances, it binds the two adjacent walls..Secondary walls which may be heavily lignified are laid internal to the primary walls, and dependent on the cells function, additional walls may be laid internal to it.

How many layers depends on

1. IF the cell is active and photosynthesizing, it will lay down just 1 wall, and this wall will contain many plasmodesmata which cross through the pit fields;

The canals are lined with the plasmamembrane through which tubules of ER extrude. This unit called a desmotubule insures that materials can move from one cell to the next..

2. IF the cell dies as to form woody tissue, secondary walls are laid with additional hemicellulose and lignin.

The orientation of the microfibrils is quite important:

In a small cell, the fibers can be laid haphazardly as in a all shape. IF the cell elongates, then the fibrils are laid in rings, and as the cell expands they move from a horizontal position to one of 45 degrees. Some rings are oriented in one direction, others in an opposite so a strong net effect is achieved.

 


Functions of the cell wall:

1. They prevent the vacuoles from over expanding-- form a counter pressure

2. They are a defense against predators...speculate how...

3. Sites of lysomsomal activity

4. In xylem cells , when dead these walls form the structure which allows the rapid transport of fluids and in living cells the secretion of substances.