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CELL WALL.pptx

The document discusses the structure and function of the plant cell wall, highlighting its role as a protective layer composed of multiple layers including the middle lamella, primary, secondary, and tertiary cell walls. It outlines the composition of the cell wall, primarily cellulose, along with other substances like lignin and pectin, and describes various thickening patterns and the function of plasmodesmata in connecting neighboring cells. The cell wall provides shape, mechanical support, and assists in water absorption in root cells.

Education◦
R. NITHYA M. Sc, M. Phil, (Ph. D) ASSISTANT PROFESSOR IN BIOTECHNOLOGY SRI ADI CHUNCHANAGIRI WOMEN’S COLLEGE, CUMBUM THENI DT, TAMIL NADU, INDIA
CELL WALL • Cell Wall is the outer covering of plant Cells. • It is a rigid protective layer around the plasma membrane. • It is present only in plant Cells and Absent from the animal Cells. • It determines the shape of plant Cells. • The cell wall is formed of four Layers, Namely • middle lamella, • primary cell wall, • secondary cell wall and • tertiary cell wall.
Middle lamella is the intercellular matrix located between the adjacent cells. It is the outer layer. It is formed of pectin, lignin and some proteins.
Primary Cell wall is the true cell wall which develops in the still growing Cells. It lies inner to middle Lamella.
Secondary Cell wall found in the mature or non-growing Cells. It is thick and rigid. It is present inner to the primary cell wall and close to plasma membrane. It is composed of cellulose fibrils and an inter-fibrillar matrix. Tertiary cell Wall is thin layer located beneath the secondary cell wall. It is present in some old cells.
Composition The cell wall is mainly formed of cellulose. Other substances present in the cell wall includes hemicellulose, pectin, lignin, fatty acids, minerals, etc.....
STRUCTURE When the cell wall is examined under electron microscope, it shows two components, Namely the matrix and the Fibrils.
MATRIX • The matrix is the ground substance in which the fibrils are embedded. • The Matrix is composed of non- cellulosic materials such as pectin, lignin, fatty acids and minerals. • Cellulose Fibrils remain embedded in the matrix. • The Fibrils are made of cellulose. • Cellulose is a polysaccharide. It is closely related to starch. • It is made up of several glucose units linked together by glycosidic bonds.
• The primary cell wall contains cellulose microfibrils which are loosely arranged and run in all directions.
But, in the secondary cell wall there are densely packed microfibrils which are arranged in parallel orientation.
• About 100 cellulose polymers are arranged in a longitudinal bundle to from an elementary microfibril. • The elementary fibrils are ribbon-like. • About 20 such fibrils are bound together to from a microfibril. • It has about 2000 cellulose molecules. • Many microfibrils join together to from a bundle called macrofibril. • The macrofibril is 0.4 micron wide and has about 500,000 cellulose molecules.
Thickenings in cell wall • When the cells mature, lignin deposits on the cell wall. • Hence the cell wall becomes thickened. • The thickening does not occur uniformly through out the entire surface. • Hence a portion of the wall remains thick and the other regions remain thin. The thickenings occur in the following patterns: • Annular thickenings • Spiral thickenings • Scalariform thickenings • Reticulate thickenings • Pitted thickenings
Annular thickenings : The thickenings occur in the form of rings.
Spiral thickenings : The thickenings occur in the form of spiral band.
Scalariform thickenings : The thickenings in the form of a ladder.
Reticulate thickenings : Here the thickenings occur in the form of a network.
Pitted thickenings : In this type of Thickening, the inner surface of the cell wall is more or less uniformly thickened leaving here and there unthickened areas called pits.
Pits • Pits are depressions formed in the cell walls due to cell wall Thickenings. • The pits are always arranged in pairs lying against each other on the opposite sides of the wall. • They are called pit pairs • A pit occurring singly is called a blind pit. • It has no complementary pit on the opposite side.
Plasmodesmata The Protoplast ( protoplasm) of plant cell remains connected with that of the adjacent cells by delicate threads of cytoplasm. These threads of cytoplasm are called plasmodesmata. The plasmodesmata (singular ) passes through the pore or pit in the cell wall. They maintain the continuity of the Cytoplasm. They conduct stimuli and transport materials between cells. They also function as channels for transporting protoplasmic substances from one cell to other cells.
Function of the cell wall The cell wall has the following functions : • It gives the cells a definite shape. • It gives mechanical support and rigidity to the cell. • In root cells, it helps to absorb water from the soil by imbibition. • It protects the protoplast from turgor pressure developed in it due to intake to more water by osmosis.
CELL WALL.pptx