Plant Cell Walls

I. Introduction

• Plant cells are different from animal cells 
  
• Functions of the cell wall: middle lamella; turgor pressure; pathogens

II. The Cell Wall and Cell Division in Plants; Plasmodesmata

1. The "wooden box"

2. The formation of the wall: phragmoplast

3. middle lamella

4. plasmodesmata.

• desmotubule.
  
• transport of material through the plasmodesmata
  

5. cellulose microfibrils form the primary cell wall

• What is a cellulose microfibril?
• Why is the beta polymer a better choice for a rigid microfibril than the alpha polymer seem in starch and glycogen?
  
• the direction of microfibril deposition varies
• isotropic
  
• anisotropic
  
• What orientation would you guess would be favored in cell that are undergoing elongation growth?
  
• Investigation has shown that the orientation of cellulose microfibrils is connected with a similar oriented array of cytoplasmic microtubules.
  
• the "cortical array".
• cellulose synthetase.
  

III. Cell Wall Structure

1. The cell wall has one of the more complicated structures found in cells.

2. The study of wall structure using a model system: sycamore cell suspensions

-digest walls with enzymes to make protoplasts 

-walls contain cellulosic and non-cellulosic components.

• organic analysis - chromatography, x-ray diffraction, use of organic solvents, chelators, etc.
• enzymatic analysis - use specific enzymes to attack wall structure in a known way.
• endopolygalacturonase (enzyme isolated from culture medium of fungus Colletotrichum lindemuthianum)
  
• How do you think this fungus survives in nature?
  

-Cellulosic Components

• cellulose 23%
  

-Non-cellulosic Components

• Pectic polymers 46%
• Expansin (Extensin) 10%
  
• Hemicellulose (or Xyloglucan) 21%
  

-Some simple connections

• there are covalent bonds between all pectic polymers and between pectic polymers and hemicellulose;
  
• This is why pectin can be thought of as the "glue" which holds the cell wall together.
  
• Can you think of a commercial use for pectin?
  
• What about the cellulose microfibrils and hemicellulose (xyloglucan)?
  
• What is the key fact that "links" these molecules together?
  
• there is enough xyloglucan in sycamore to encapsulate all of the cellulose fibers in the wall
  
• The bonding between xyloglucan and cellulose (although strong) is weaker than the bonding between cellulose molecules in a microfibril.
• Role of Expansin?
  

IV. Cell Wall Structure and Growth

• Plant growth during germination
  
• What must happen before a plant cell can grow?
  
• What is the force which promotes the expansion of a plant cell?
  
• Some facts about plant cell growth:
  

1. plant cells will grow if placed in an acid buffer (pH 5);
2. the plant hormone auxin (IAA) will promote growth;
3. auxin promotes acidification and "loosening" of the cell wall;
4. auxin stimulates mRNA & protein synthesis by acting directly on nuclear DNA;

What does this all mean?

• The Acid Growth Theory; The Gene Regulation Theory