Plant Cell Walls - 13
311 Syllabus

Cellulose Synthesis

Particle Rosettes of Cellulose Synthase are present within the hydrophobic Middle of the Plasmalemma.

Microtubules present in the Cortical Cytoplasm (just beneath the Plasmalemma) provide the Orientation of Cellulose Microfibrils.

If  Microtubules are disrupted by drugs, Cellulose deposition occurs but the pattern is random.

When Microtubles reorganize, cellulose deposition becomes organized parallel to the Microtubules.

The Rosettes may contain an enzyme (Sucrose Synthase) which provides the glucose units that are made into Cellulose.

This cartoon tries to illustrate how Cellulose Fibrils and Matrix Components are synthesized at the Plasmalemma. The cream colored Boogie Board represents the lower half of the Plasmalemma. The Rosettes synthesize new Cellulose Molecules which become oriented parallel to the Microtubules just beneath them in the Plasmalemma.  The Cellulose Microfibrils lay on the outside of the Plasmalemma, NOT in the Middle.

Matrix components arrive in Golgi Vesicles which fuse with the Plasmalemma and release their contents into the Wall area.

Matrix Polymers are Synthesized by the Golgi

Golgi Vesicles arrive at the Plasmalemma and fuse with it.

Their contents are release into theWall.

Matrix Polysaccharides are less organized vs Cellulose

Heicelluloses

Bind to Cellulose Microfibrils &

Link Microfibrils

Xyloglucan (Glucose & Xylose)

Cellulose Backbone with Side Branches

Pectins are Gel Forming Matrix Components

Composition = Acidic Sugars & Neutral Sugars (Galacturonic Acid &   Rhamnose)
to form molecules like Rhamnogalacturonan

They have Long Backbones with Side Chains.

Their Carboxyl Groups have a  Neg. Charge  and are linked by Ca++

They also have some stable Covalent Linkages.

They may be linked to the Structural Proteins in the Matrix.

Structural Proteins

There are Several Types with Different Major Amino Acids

They have highly Repetitive Structures

They are initially Soluble but more Insoluble proteins are produced as the wall matures or upon wounding.

Wounding -> Increased concentration of Proteins.

Characteristic Proteins found in different types of Tissues

Hydroxyproline Rich Proteins (HRGPs)

Protect from Wounding & Desiccation

Provide Wall Strengthening

Provide Nucleating sites for Lignin

Similar to those in certain Chlorophyta

Arabinogalactan Proteins

Water Soluble

Cell Signaling & Differentiation