Introduction
Cells are the basic structural and functional units of all living organisms. Plant cells contain various constituents that perform important physiological and biochemical functions. Understanding these structures helps in studying medicinal plants, evaluating crude drugs and identifying plant tissues in pharmacognosy.
Main Cell Constituents
Cell constituents include the cell membrane, cytoplasm, nucleus and several cell organelles such as plastids, mitochondria, Golgi bodies, endoplasmic reticulum, ribosomes and vacuoles. Plant cells also contain specific inclusions like starch grains, oils, tannins and calcium oxalate crystals.
1. Plasma Membrane
The plasma membrane surrounds the cytoplasm and controls the movement of substances in and out of the cell. It is a semi-permeable membrane made of a phospholipid bilayer with embedded proteins. It helps in transport processes, cell communication and maintaining internal balance.
2. Cytoplasm
Cytoplasm is the fluid matrix inside the cell where various metabolic reactions occur. It contains water, minerals, enzymes, proteins and organelles. It supports and suspends all cell structures and serves as the site for chemical activities like glycolysis and protein synthesis.
3. Nucleus
The nucleus is the control center of the cell. It contains DNA and regulates growth, cell division and metabolic activities.
Main Components
- Nuclear membrane: Double-layered membrane with pores.
- Nucleoplasm: Fluid medium inside the nucleus.
- Nucleolus: Site of ribosomal RNA formation.
- Chromatin: Genetic material containing DNA.
4. Mitochondria
Mitochondria are the “powerhouses” of the cell. They generate ATP through respiration. They have a double membrane, with inner folds called cristae. Mitochondria contain their own DNA and enzymes for energy production.
5. Plastids
Plastids are large organelles found in plant cells. They store food and perform photosynthesis.
- Chloroplasts: Contain chlorophyll and carry out photosynthesis.
- Chromoplasts: Store pigments like carotenoids in flowers and fruits.
- Leucoplasts: Colourless plastids that store starch (amyloplasts), oils (elaioplasts) and proteins (aleuroplasts).
6. Endoplasmic Reticulum (ER)
ER is a network of membranes connected to the nucleus. It helps in synthesis and transport of materials.
- Rough ER: Has ribosomes and helps in protein synthesis.
- Smooth ER: Involved in lipid synthesis and detoxification.
7. Golgi Bodies
Golgi bodies modify, package and transport proteins and lipids. They are important in secretion, formation of cell wall materials and synthesis of polysaccharides.
8. Ribosomes
Ribosomes are small structures responsible for protein synthesis. They may be free in cytoplasm or attached to rough ER.
9. Vacuoles
Vacuoles are large fluid-filled sacs in plant cells. They maintain turgor pressure and store water, pigments, salts, sugars, secondary metabolites and waste products.
10. Centrosome (in some lower plants)
Centrosomes help in cell division by forming spindle fibres. They are more prominent in animal cells but may appear in some lower plant types.
11. Cell Inclusions
Inclusions are non-living substances stored in the cell. They play important roles in storage, protection and identification of crude drugs.
A) Carbohydrate Inclusions
- Starch grains: Found in potato, ginger and cereals; vary in shape and hilum pattern.
- Inulin: Stored in chicory and dandelion.
- Cellulose and mucilage: Present in plant cell walls and some seeds.
B) Proteins
- Aleurone grains: Found in castor, maize and other seeds.
- Contain protein matrix and sometimes crystals.
C) Fats and Oils
- Stored in seeds like castor, sesame, sunflower and almond.
- Appear as oil droplets under the microscope.
D) Tannins
Tannins are polyphenolic compounds found in bark, leaves and fruits. They have astringent action and appear as brownish masses.
E) Alkaloids
Present as crystalline or amorphous forms in specialized cells. Examples include morphine (opium), quinine (cinchona) and caffeine (tea).
F) Calcium Oxalate Crystals
Common diagnostic inclusions useful in pharmacognosy. They appear in various forms:
- Prisms
- Rosettes
- Needles (raphides)
- Sandy crystals
12. Secretory Structures
Certain plant cells secrete oils, resins, gums and latex.
- Oil glands: Found in citrus, eucalyptus and clove.
- Resin ducts: Present in pine and ginger.
- Latex tubes: Found in poppy, rubber and neem.
Detailed Notes:
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