The cells are structural and functional unit of body. Different types of cells are present having different functions, but their basic structures are common to all. Cytoplasmic organelles differ depending upon their functions. Each cell has cell wall enclosing the cytoplasm with nucleus and organelles.
Cell membrane: It is formed by double-layered phospholipid molecule, having membrane protein in between with selective permeability. Oxygen, carbon dioxide, water, steroids, etc. are permeable; whereas glucose, ions or proteins pass by specific transport mechanism.
The cell membrane encloses protoplasm. It has a nucleus covered by nuclear membrane. The cytoplasm contains a number of organelles and inclusion bodies.
Cellular organelles: For the maintenance of proper homeostasis and specialized metabolic function, different organelles are present in cytoplasm of each cell. Common organelles are mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes and lysosomes, etc.
Mitochondria: It is spread all over the cell as granules or round, oval or elongated rods. It has outer and inner membranes with cristae. The cristae are folds of inner membrane. They provide energy.
Endoplasmic reticulum: It is present as extensive network of sacs and cisternae. It is of two types—rough and smooth. Rough endoplasmic reticulum is seen as many flattened interconnected cisternae studded with ribosomes. They are concerned with protein synthesis. Smooth endoplasmic reticulum is devoid of ribosomes. They are concerned with further processing of proteins synthesized by rough endoplasmic reticulum.
Golgi apparatus: It consists of smooth, curved cisternae present near the nucleus for transport of proteins. It adds sugar to form glycoprotein, lipoprotein and glycolipids.
Ribosomes: These are small, dark and dense granules. They may be free or attached to the endoplasmic reticulum, concerned with protein synthesis.
Lysosomes: These are membrane-bound vesicle or dense bodies of various sizes and shapes, containing, lysosomal enzymes with property of phagocytosis (intracellular digestion). They are rich in glycoprotein and are abundant in phagocytic cells.
Peroxisomes: They are also membrane-bound vesicles smaller than lysosomes containing several types of oxidases and catalase. They are present mostly in liver and kidney to remove toxic materials.
CYTOSKELETON OF THE CELLS
Cytoskeleton gives the structural supporting framework and facilitates cell motility. They are made up of three main structural components—microfilaments, intermediate filaments and microtubules, which are formed by proteins.
Microfilament: They are formed by protein actin, present near the cell wall and maintain the shape of the cells. It also forms the core of microvilli.
Intermediate filament: Different types of thick intermediate filaments are present in different types of cells having different names due to its different types of proteins present. They are cytokeratin in skin, vimentin in mesenchymal cells, desmin in muscles, neurofilament in nervous cells, glial filament in glial cells (astrocytes) and lamin in inner layer of nuclear membrane.
Microtubules: These are the largest filaments formed by α- and β-tubulin originating from centrosome in the cytoplasm, which contains a pair of centriole. They are visible in cilia and flagella. They are present in almost all types of cells except red blood cells.8
Centrosome and centriole: They are present near the nucleus having two centrioles lying perpendicular to one another. Centriole has nine microtubules arranged in group of three. They replicate before mitosis and form the spindles.
Cytoplasmic inclusions: They are lipids, glycogen, pigments and crystals.
Nucleus: Except RBC, all living cells have nucleus, covered by double layer of nuclear membranes containing ribosome on its outer layer. Nucleus has chromatin, nucleoli and DNA. Nuclear membrane has nuclear pores to control the movement of metabolites between nucleus and cytoplasm.
Epithelial cells: They have the followings functional surfaces:
- Basal surface: It rests on basement membrane formed by basal lamina and reticular lamina. At some places, basal surface shows infolding of plasma membrane to increase the surface area for transport of fluid and ions.
- Side surface: Specialized for cell-to-cell attachment. It is lying in contact with side surface of other cells. They are:
- Interlocking projections.
- Junctional complexes: The important junctional complexes are:
- Zonula occludens or tight junction.
- Zonula adherens or loose junction.
- Macula adherens or desmosomes.
- Free surface: It shows many types of specialization. It is exposed to either air or fluid in body cavity of system:
- Microvilli: These are ultramicroscopic finger-like projections from free surface of the cell. They consist of a core of apical cytoplasm covered by plasma membrane. They increase the absorptive surface.
- Cilia: These are hair-like processes. These may be motile or nonmotile (stereocilia). Function of motile cilia is to propel liquid, mucus or foreign bodies. They are present in respiratory tract, uterine tube and uterus, etc. Function of non-motile cilia (stereocilia) is absorptive or to act as sensory receptors. They are present in central canal of spinal cord.