cellwall

PLANT EXTRACELLULAR MATRIX - CELL WALL

I. Initial material deposited during cell plate formation

A. Initially rich in pectins (middle lamella), amount of cellulose increases over time

B. Subsequent material deposited through plasmalemma to space between middle lamella and plasmalemma

II. Primary function is support

III. Many other diverse secondary functions dependent of pattern and type of materials

A. Determines cell shape

B. Cell to cell communication

C. Protection from mechanical damage and disease organisms

D. Recognition and binding of external molecules

E. Control of water pathways (lignin)

IV. Extracellular matrix of all eukaryotes share two common structural elements

A. Fibers

1. Long, semicrystalline elements
2. Provide resistance to stretching and other tensile forces

B. Network

1. More or less elastic, interlocked assembly of branched molecules
2. Hold fibers in place
3. Provide resistance to compressive forces via trapping and retarding flow of water molecules

V. Physical properties of extracellular matrices determined by

A. Kinds of fiber and network molecules present

B. Degree of crosslinking of fibers via network molecules

C. Amount of trapped water

D. Types of additional material within the matrix
1. Lignin in plant cell walls

DETAILS OF PLANT EXTRACELLULAR MATRIX COMPONENTS

I. Fiber

A. Cellulose = long straight chains of B1,4-linked glucose

1. 20 - 50% dry weight
2. 15% volume

B. Form Micofibrils with quasi-crystalline structure (10-85 nm diameter)
via hydrogen bonding between cellulose molecules

1. Primary Cell Wall
2. Secondary Cell Wall

C. Callose = long chains of B1,3-linked glucose

1. Sieve tube elements and wounded plant cells

II. Network

A. Hemicellulose = branched chains of glucose + other sugars

1. 20% dry weight
2. Form sheathing layers around microfibrils via hydrogen bonds
3. May be neutral or acidic inl pH range of cell wall

B. Pectins = branched chains of polysaccharides with galacturonic acid or galacturonic acid + rhamnose backbones

1. 25 - 30% dry weight
2. Form stable gels of various viscousity via water trapping
3. Highly acidic in pH range of cell walls = electrostatic bonds with other molecules (Interactions with Ca+2 might play a linkage role?)

III. Other network components

A. Various enzymes

1. Transferases involved in moving sugar groups from precusers to network components
2. Hydrolytic enzymes that remove sugar groups and create binding sites in network components
3. Peroxidases, phosphatases, proteases, oxidases, and reductases
a. Some of these hydrolyze molecules of cell walls of invading fungi and bacteris

B. Structural Glycoproteins rich in hydroxyproline or glycine

1. Extensins confer rigidity to cell wall via ionic attractions with negatively charged network components and covalent bonds with other extensins and pectins

C. Lectins

1. Soluble glycoproteins with multiple binding sites to cross link carbohydrate groups of other network components
2. Can bind with chitins of invading fungi
3. Part of recognition mechanism between root hairs and Rhizobium bacteria in legume roots

D. Silica important component of cell walls of grasses and Equisitum

E. Lignin

1. Dense, insoluble substance formed by complex alcohols covalently linked into branched network
2. Lignification makes cell walls hydrophobic which is important in function of endodermis of monocot roots and vascular cells

F. Suberin, Cutin, and Waxes

1. Insoluble substances formed by complex fatty acids covalently linked polymers
2. These substances make cell walls hydrophobic which is important in function of endodermis of dicot roots, epidermal cells, and periderm (bark) cells

IV. Structure of cell wall only partially understood through enzymatic digestion studies

A. Cellulose microfibrils surrounded by hydrogen-bonded hemicellulose sheath

B. These units cross-linked via hydrogen-bonded pectins

C. Cross-linkage may be reinforced via extensins and other glycoproteins

D. This network is open enough to admit water, ions, and small molecules but has great resistance to stretching and compression

E. Proportions of cellulose, hemicellulose, and pectins varies between

   1. Layers within the cell wall
   2. Different types of cells
   3. Different species of plants

F. Size and orientation of cellulose microfibrils varies between

1. First formed primary cell wall

a. Some cells have thin primary cell walls
b. Some cells have thick primary cell walls

2. Latter formed secondary cell wall

a. Composed of three layers (S1,S2,S3)
b. Most cells with secondary cell walls undergo apotosis and have no cytoplasm in lumen at maturity