Tiêu đề 5. STUDY OF CELL WALL CONSTITUENTS AND CELL INCLUSIONS.pdf ✅

PHARMD GURU Page 1 CELL WALL: External covering of plant cells is known as cell wall. It is a rigid non-living secretion of the cell membrane which is composed of cellulose, pectin, hemicellulose, glycoprotein and lignin. Scientist Robert Hooke was the first to observe cell wall in cork of the oak tree in the year 1665. It is characteristic to only plant cells as it is absent in animal cells. The basic function of cell wall is to protect the constituents present within the cell and also to provide shape to the cell. LAYERS OF THE CELL WALL: Cell wall is further divided into four layers: 1. MIDDLE LAMELLA: Middle lamella is the first layer formed during cell division. It forms the outer wall of the cell and holds the adjacent cells together. It is composed of cellulose, calcium and magnesium crystals. 2. PRIMARY CELL WALL: Primary cell wall is formed by the secretion of cellulose and other polysaccharides after the formation of middle lamella. It is found inner to the middle lamellum. It is made up of a rigid skeleton of cellulose microfibrils embedded in a gel-like matrix which is made up of cellulose, hemicellulose, glycoproteins and pectic compounds. The basic function of primary cell wall is to increase the volume of the cell. Certain STUDY OF CELL WALL CONSTITUENTS AND CELL INCLUSIONS
PHARMD GURU Page 2 areas of the primary cell walls are relatively thinner which are referred as primary pit fields. In higher plants, cytoplasm occurs as bridges (called plasmodesmata) which pass through these pit fields and help to connect the neighboring cells. 3. SECONDARY CELL WALL: Secondary cell wall is located on the inner side of the primary cell wall. It is formed once the cell enlargement stops and the plant cell begins the deposition of more polysaccharides (cellulose, hemicellulose) and other materials such as lignin or suberin. Presence of inelastic materials such as lignin makes it hard and tough. It also bears pits, which are the areas where the cell wall material has not been deposited. 4. TERTIARY CELL WELL: Tertiary cell wall is a very thin layer, formed on the inner side of the secondary cell wall and composed of cellulose, hemicellulose and xylan. Tertiary cell wall is commonly found in the xylem tracheids of gymnosperms. FUNCTIONS: 1. Cell wall provides mechanical protection from physical injury, insects and pathogens. 2. It provides tensile strength to the cell by preventing it from rupturing due to turgor pressure. 3. Provides mechanical support and imparts shape to the cell. CELL INCLUSIONS: Cell-inclusions refer to the non-living components found in the cytoplasm. These include vacuoles, ergastic substances and calcium oxalate crystals. 1) VACUOLES:  Vacuoles are single membrane, fluid-filled, sac-like structures that occupy a large percent of the total volume of a plant cell.  The vacuolar membrane is called tanoplast and fluid within the vacuole is called cell sap.
PHARMD GURU Page 3  The various components of the cell sap are mineral salts, sugars, organic acids, oxygen, carbon dioxide, blue and pink coloured pigments (anthocyanins), toxic wastes and secondary products of metabolism. FUNCTIONS: 1. Tonoplast allows the entry of water, ions and other materials into the vacuole by osmosis. 2. The dissolved anthocyanin pigments in the vacuole are responsible for the colour of flowers, fruits, buds and leaves. 3. Some secondary products and other waste by-products such as calcium oxalate crystals, alkaloids and tannins stored in the vacuoles provide protection from herbivores. 4. Reserve food (like sucrose and mineral salts) stored in the vacuole can be used by the cytoplasm when required. 2) ERGASTIC SUBSTANCES: Ergastic substances of plants include all those non-living cytoplasmic inclusions which are basically products of plant metabolism. There are of 3 types. A) RESERVE FOOD: Reserve food refers to the excess food that is stored in an insoluble form for further use. When the need arises, these insoluble food reserves are subjected to enzymatic hydrolysis.
PHARMD GURU Page 4 CARBOHYDRATES: Both water soluble and water insoluble carbohydrates are utilized as reserve food. Example of water soluble simple sugars (carbohydrates) include glucose, fructose and sucrose. Water insoluble carbohydrates i.e., polysaccharides also function as reserve food. Some examples include starch, inulin, cellulose, mucilage etc. PROTEINS: These are basically nitrogenous compounds which may or may not exhibit water solubility. When acted upon by proteolytic enzymes, they degrade to yield amino acids. They are found as an aleurone layer when they exist in an amorphous form (E.g: Maize). On the contrary, when they exist in a crystalline form, they are found as aleurone grains (E.g: Castor seeds). FATS AND OILS: Both fats and oils are composed of fatty acids and glycerol, but differ in their state. Fats are solid while oils exist as liquids at room temperature. Endosperm of certain seeds like sesame, castor, peanut etc., are rich in fats. B) SECRETORY PRODUCTS: Secretory products are specially secreted by the plants and they serve a specific function. COLOURING SUBSTANCES: These include photosynthetic pigment chlorophyll, anthocyanins, xanthophylls, flavonoids etc. These substances function to impart colour to the plants and leaves which helps to attract insects which in turn help in pollination and seed dispersal. ENZYMES: Enzymes are water-soluble, nitrogen containing compounds that catalyze the hydrolysis of reserve foods such as carbohydrates and proteins. NECTAR: Nectar is a sweet solution secreted by certain specialized glands in numerous flowers. It helps to attract insects which help in pollination.