Introduction
Plant cells have an external rigid covering called the cell wall. It protects the cell, gives shape and supports the plant. Inside the cell, non-living structures like vacuoles, reserve foods and crystals are known as cell inclusions. Understanding both cell wall and cell inclusions helps in identification, evaluation and purity testing of crude drugs in pharmacognosy.
Cell Wall
The cell wall is a non-living, rigid structure present outside the cell membrane. It is made of cellulose, hemicellulose, pectin, lignin and glycoproteins. Robert Hooke first observed cell walls in cork cells in 1665. Cell walls occur only in plant cells, not in animal cells.
Layers of Cell Wall
1. Middle Lamella
The middle lamella is the first layer formed during cell division. It lies between two adjacent cells and acts like a cementing layer. It is rich in pectin along with calcium and magnesium salts. Its main function is to bind neighbouring cells together.
2. Primary Cell Wall
This layer forms after the middle lamella. It contains cellulose microfibrils embedded in a matrix of pectin, hemicellulose and glycoproteins. It is elastic and allows cell enlargement. Thin areas in the wall are called pit fields. Plasmodesmata pass through these pits to connect cytoplasm of neighbouring cells.
3. Secondary Cell Wall
The secondary wall develops after the cell stops growing. It has more cellulose, hemicellulose and hard substances like lignin and suberin. This layer is thick, rigid and provides strength. It also contains pits where wall deposition is absent.
4. Tertiary Cell Wall
This is a thin inner layer present in some cells, especially tracheids of gymnosperms. It contains cellulose, hemicellulose and xylan.
Functions of Cell Wall
- Gives mechanical protection against injury and pathogens.
- Prevents bursting of the cell by controlling turgor pressure.
- Provides support and maintains proper cell shape.
Cell Inclusions
Cell inclusions are non-living components present in the cytoplasm. These include vacuoles, ergastic substances and calcium oxalate crystals.
1. Vacuoles
Vacuoles are large, fluid-filled sacs surrounded by a membrane called the tonoplast. The fluid inside is known as cell sap. The sap contains mineral salts, sugars, pigments (like anthocyanins), organic acids, gases and waste products.
Functions
- Tonoplast regulates entry of water and ions into the vacuole.
- Pigments in vacuoles give colour to leaves, flowers and fruits.
- Vacuoles store waste products, secondary metabolites and protective compounds.
- Store food reserves like sucrose and salts.
2. Ergastic Substances
These are non-living products of metabolism stored inside cells. They include reserve foods, secretory products and excretory products.
A) Reserve Food Substances
- Carbohydrates: Simple sugars (glucose, fructose, sucrose) and polysaccharides (starch, inulin, cellulose, mucilage) act as reserve food.
- Proteins: Found as aleurone layer (amorphous) or aleurone grains (crystalline) e.g., maize and castor seeds.
- Fats and Oils: Present in endosperm of seeds like castor, sesame and peanut.
B) Secretory Products
- Colouring substances: Chlorophyll, anthocyanins, xanthophylls and flavonoids help in photosynthesis and attract insects.
- Enzymes: Nitrogenous compounds that catalyse metabolic reactions.
- Nectar: Sweet fluid secreted by flowers to attract pollinators.
C) Excretory Products
- Alkaloids: Nitrogenous compounds stored for plant protection. Examples: morphine, quinine, caffeine.
- Glycosides: Compounds yielding sugar and non-sugar parts on hydrolysis. Examples: sennosides, digitoxin.
- Tannins: Non-nitrogenous compounds with astringent action. Found in arjuna, myrobalan.
- Resins: Solid or semi-solid exudates insoluble in water.
- Gums and mucilages: Water-soluble polysaccharides stored or produced after injury.
- Volatile oils: Aromatic oils used as carminatives and antiseptics. Examples: ginger, eucalyptus, cinnamon.
3. Calcium Oxalate Crystals
These are solid, water-insoluble crystals found in many plant cells. Their shape helps identify crude drugs and detect adulteration.
Types of Calcium Oxalate Crystals
- Prisms: Regular prism shapes (e.g., senna, rauwolfia).
- Single acicular crystals: Long needle-like crystals (e.g., gentian, squill).
- Rhombic crystals: Diamond-shaped crystals (e.g., kurchi).
- Rosettes: Star-like clusters arranged in spherical form (e.g., cascara, stramonium).
- Sandy or microsphenoidal crystals: Small tetrahedral crystals found in groups (e.g., cinchona, belladonna).
Detailed Notes:
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