Cell Wall

Characteristics

Layer of structural material found external to the protoplasm. Usually 0.1-10mm thick, composed of polysaccharide with small amounts of glycoproteins. Contrasts with the plasma lemma which is less that 0.01mm thick and composed of phospholipids and glycoproteins

Functions

1. Support: tough flexible sometimes rigid. Acts as pressure vessel to prevent over-expansian of cells when water enters. Governs shape and size of cell and fastens it to its neighbour’s.

2. Protection: forms a barrier to entry of of pathogens or toxic substances. Cell walls differ in their penetrability and cells can rapidly bolster their walls in response to attempted invasion. Against mechanical stress.

3. Recognition: cell wall metabolism helps cell to detect approach of pathogens

4. Signaling

Structure

1. Primary cell wall: thin, flexible layer formed while cell is growing. Carbohydrates + glycoproteins

2. Secondary cell wall: thick layer formed inside primary cell wall after cell is fully grown. Not found in all cell types. Carbohydrates plus glycoproteins but some cells have lignin which strengthen and waterproof the wall

3. Middle lamella: layer found outside cell wall, forms interface between adjacent plant cells and glues them together. Carbohydrates are pectine.

Cellulose

Principal component of cell wall and exist as a system of fibrillation of different sizes. Fibrils composed of microfibrils each about 10nm wide.

Made up of Ɓ-glucose molecule- polymerized to form long, unbranched chains which are held together by hydrogen bonds.

Amorphous (randomly arranged). Crystalline (arranged in a parallel fashion)

Microfibrils

Provide tensile strength of primary wall. They do not bond

Hemicellulose

Random amorphous structure with little strength. Polysaccharide of simpler structure of cellulose. Long branched molecules connect microfibrils to each other at the surface by hydrogen bonds. Cross link microfibrils into a complex network.

Pectin

Complex set of polysaccharide. Main component is galacturonic acid. Allows primary cell wall extension and plant growth

Proteins

Modify cell wall components and customize them to confer appropriate properties to cell walls. Contribute to signaling

Middle lamella

Cements adjacent cells. Composed of calcium and magnesium pectates which function as the cement. When Ca is added to pectic molecules they become gelatinous and hydrophilic. If Ca is extracted the tissue will fall apart.

Type of cell wall

1. Primary: No definite arrangement of microfibrils (amorphous). Very flexible because of high proportion. Aids in growth and enlargement of the cell and assists in the cell turbidity

2. Secondary: Difinite arrangement (crystalline). Low elasticity reduced flexibility and very high tensile strength. Found in differentiated cells