1cell wall.pptx

Cell wall structure and function

1. Cell Wall By: Dr. Sunita Sangwan Assistant professor, Dept of botany, Govt. college, Bhiwani

2. Introduction . Cell wall is protective layer in function and provides mechanical support to the cell. Cell wall determines the shape of the cells and any cell which has lost its cell wall becomes amorphic or in other words loses definite shape. The cell wall was first observed by Robert Hooke in the year 1665 in cork cells of oak tree. Originally it was thought that the cell wall was a non-living secretion of the protoplasm, but now it is known to be metabolically active and is capable of growth. It can be tough, flexible and rigid which provides cell with both structural support and protection.

3. Types of cell wall On the basis of chemical composition of cell wall there are three types of cell wall: Bacterial Cell Wall : made up of Mucopeptide and Muramic acid. Cell wall of Fungi: made up of Chitin. Plant Cell wall: made up of Cellulose.

4. Plant Cell Wall The plant cell wall is a remarkable structure. It provides the most significant difference between plant cells and other eukaryotic cells. The wall is rigid(up to many micrometers in thickness) and gives plant cells a very defined shape. While most cells have a outer membrane , none is comparable in strength to the plant cell wall. The cell wall is the reason for the difference between plant and animal cell functions. Because the plant has evolved this rigid structure.

5. Components Of Plant Cell Wall The plant cell wall composed of : The Middle Lamella The Primary Cell Wall The Secondary Cell Wall The Tertiary Cell Wall

6. Middle lamella It is the outer most layer of the cell wall connecting the two adjoining cells like the cementing layer. Chemically it is composed of calcium and magnesium pectates . It is thin and amorphous layer. It is an isotropic or optically inactive layer of the cell. In addition to these substances proteins are also present in the middle lamellae. It is the first formed layer of the cell wall during the cell division. The cell plate formed during the cytokinesis itself develops into the middle lamellum .

7. Primary wall It is the layer present just under the inner side of middle lamella. It is thin, elastic and extensible in growing cells. Due the elastic nature of the primary wall, it helps in increasing the cell volume. It is anisotropic layer or optically active layer of the cell. Parenchymatous and meristematic cells possess only primary wall and no secondary wall

8. Chemical composition of Primary wall Chemically primary cell wall is composed of cellulose, hemicellulose and pectin. A loose network of cellulose microfibrils embedded in a gel like matrix. The matrix is mainly composed of water, hemicellulose, pectin and glycoprotein. Pectin is the filling material of the matrix. Hemicellulose binds the microfibrils with the matrix and the glycoproteins control the orientation of the microfibrils.

9. Plasmodesmata Certain thinner areas present in the primary cell wall are called as primary pit fields. In multicellular plants, cytoplasmic connections pass through these pit fields from one cell to other. These cytoplasmic bridges found between two cells are called plasmodesmata. Plasmodesmata help in intercellular transportation of cellular material from cell to cell. Diameter is 40-50 nm.

10. Plasmodesmata They form a protoplasmic band called Symplast. It consists of a canal, lined by plasma membrane. It has a simple or branched tubule known as desmotubule . Desmotubule is an extension of endoplasmic reticulum. Plasmodesmata serve as a passage for many substances and they also have a role in the relay of stimuli.

11. Secondary Cell Wall It is extremely rigid and provides strength. It is not found in all cell types. It consists of three layers known as S1(outer),S2(middle) and S3(inner). It is composed of cellulose, hemicellulose and lignin.

12. Chemical composition of Secondary Wall Chemically secondary wall is made up of a framework of cellulose microfibrils and interfibrillar spaces are mostly filled with lignin, suberin , pectin and cutin . Secondary walls are hard, tough and inelastic in nature due to the presence of wall materials like lignin. It grows in thickness, layer by layer with the deposition of materials over the existing structures. The formation of secondary wall is not uniform in all the cells and this result in differentiation of various types of cells like parenchyma, collenchyma and tracheids. The areas in the secondary wall where wall material is not deposited are called pits.

13. Pits and their types Pits are the areas on the cell wall where secondary wall is not deposited. The pits of adjacent cells are opposite to each other. Each pit consists of a pit chamber and a pit membrane. The pit membrane consists of middle lamella and primary wall. Pit membrane has many minute pores and thus it is permeable. The Pits help in the translocation of substances between two adjacent cells. Generally pits develop in pairs. In other words, when a pit is present on the secondary wall of one cell, a similar or dissimilar pit will be present on the opposite side of the adjacent cell. Such type of pits form morphological and functional units called pit pairs. Pits are of two types namely: simple pits and bordered pits.

14. Simple pits In simple pits, usually the pit cavities are uniformly wide on all sides. The pit cavity is not enclosed by secondary cell wall.

15. Bordered pits  In bordered pit, the secondary wall bends like an arch over the pit, this gives a border like appearance and hence the name. The portion of the middle lamellum crossing the pits become thickened and is known as torus.

16. Tertiary Cell Wall Tertiary cell wall is deposited in few cells. It is considered to be dry residue of protoplast . Besides cellulose and hemi-cellulose, xylan is also present.

17. Composition Of Cell Wall The cell wall is mainly composed of carbohydrate materials. The major components of cell wall are cellulose, pectins , hemicelluloses, proteins and phenolics .

18. Cellulose It provides shape and strength to the cell wall. It composes 20-30 % of the dry weight of primary wall and accounts 40-90% of the dry weight of secondary wall.

19. Pectins They are group of polysaccharides, which are rich in galacturonic acid, rhamnose , arabinose and galactose. Pectins are present in high concentration in the middle lamella where they presumably serve the function of cementing adjacent cells together.

20. Hemicelluloses These are matrix polysaccharides built up of a variety of different sugars. They differ in different species and in different cell types. Xylan : It typically makes up roughly 5% of primary cell wall and 20% of secondary cell wall in dicots . This hemi cellulosic polysaccharide is linked with xylose and arabinose .

21. Proteins Different varieties of protein are present in the cell wall, most of which are linked with carbohydrate forming glycoprotein. The cell wall glycoprotein extensin contains an unusual amino acid hydroxyproline (about 40%), which is generally absent from the protoplast. Extensins are present in the primary cell walls of dicots making up one to ten percent of the wall.

22. Primary Cell Wall vs Secondary Cell Wall It is formed in the growing cells It lies on the inside of the middle lamella It is present in all the plant cells Primary cell wall is elastic and flexible in nature It consists of single layer of all material Chemically it is composed of Cellulose 5%, Hemicellulose 50%, Proteins 5%, Lipids 5-10% It is formed in the mature non-growing cells It lies on the inside of the primary cell wall It is present only in certain types of plant cells Secondary cell wall is rigid and non-flexible in nature It is formed of three or more layers Chemically it is composed of, Cellulose 50-94%, Hemicellulose 25%, Proteins in very small amounts, No lipids Primary cell wall Secondary cell wall

23. Primary Cell Wall vs Secondary Cell Wall It is about 1-3µ in thickness Cellulose microfibrils are very loosely arranged Pits are formed in primary cell wall No additional material like lignin or suberin is found Primary cell wall grows in thickness by the process of intussusception It is about 5-10µ in thickness Cellulose microfibrils ae compactly arranged Pits are formed in secondary cell wall Additional material like lignin and suberin is often found impregnated Secondary cell wall increases in thickness by apposition Primary cell wall Secondary cell wall

24. Functions Of Cell Wall Cell wall provides tensile strength and plasticity which are important for keeping cells from rupturing by turgor pressure. Thick cell walls give definite shape and mechanical support to the cell. Cutinization in the cell wall protects the cell from water loss. It protects the internal protoplasm against injury Physiological and biochemical activities in the cell wall promote cell-to-cell communication

25. Functions Of Cell Wall Cell wall also helps in growth and development. It prevents the bursting of plant cells due to endosmosis. The walls of xylem vessels, tracheids and sieve tubes are specialized for long distance transport. In many cases, the cell wall takes part in offense and defense against insects and pathogens.

26. Thanks