2.4 Cellular Component

Plant Cell Wall

FUNCTION OF CELL WALL

Maintain cell shape.

Provides rigidity and structure

Provide protection to the cell

COMPONENT OF CELL WALL

microfibrils

• A long and thin molecule.• Made of repeating molecules of glucose attached end to end in a

b(1-4) linkage

cellulose

• Made of a bundle of cellulose molecules.

Macrofibril•Microfibrils are cemented together by

a matrix of pectin and hemicellulose. •The microfibrils wind together to form fine threads which may coil around one another like a cable. •This "cable" is called Macrofibril.•Cellulose molecules wound in this manner are as strong as an equivalent thickness of steel.

Lignin

• a very strong ridged molecule that is found in cells which have a strong supporting function.

Suberin Cutin Waxes

• Fatty substances which are commonly found in the cell walls of cells on the outside surface of the plant, and act to water proof the outer surface of the plant to prevent evaporation and dehydration

•The spaces between the fibrils are not entirely filled with matrix.• They generally allow water, air and dissolved materials to pass freely through cell wall.•Hence, cell wall are permeable.•But, the wall does not determine which materials can enter and which cannot, because the function is carried out by plasma membrane that is located below the cell wall .

LAYERS OF CELL WALL

PRIMARY WALL

• The only layer of cell formed as long as the cell continues to grow.

• Elastic and allow for considerable stretching during cell growth.

• This is because the fibrils that form the primary wall are arranged randomly to form a rather loose network.

MIDDLE LAMELLA

• Binds the wall of two cells together • A sticky, jellylike layer of pectin, that

generally present in the form of calcium and magnesium pectates.

• When the a fruit ripens, pectin dissolves & the cells become less tightly bound to one another. It’s these loosely arranged cells that make a ripening fruit become softer.

The Various Layers of Plant Cell Wall

SECONDARY WALL

• Formed by depositing further layers of cellulose fibrils on primary cell wall.

• Deposited by the protoplasm and located between plasma membrane and primary cell wall.

• Much thicker than primary cell wall & composed of a succession of compact layers or lamellae.

• The cellulose microfibrils in each lamella lie parallel to each other , are tightly packed and are generally oriented at angles of 60- 90 degrees to the fibrils of the adjacent lamellae . This arrangement gives additional strength to the cell wall.

• Secondary wall usually contain other materials such as lignin , which make them even stiffer & impermeable to water.

• Once deposition of the secondary cell wall is completed, many cells die , leaving the hard tube formed by their walls to function as a mechanical support, and for internal transport in the plant .

PLASMODESMATA

• Region of living connection between adjacent plant cell.

• Formed when portions of endoplasmic reticulum are trapped across the middle lamella as a new cell wall is laid down between two newly divided plant cells, & these eventually become the cytoplasmic connection between cells.

• Here the wall is not thickened further ,& depressions or thin areas known as pits are in the walls. Pits normally pair up between adjacent cells.