The cell is the smallest structural and functional unit of body.the human body consist about 100 trillion cells. Different types of cells of the body possess features, which distinguish one type from and are specially adapted to form one or few perticular functions. e.g. the red blood cells transport oxygen from lungs to tissue ,muscle cell is specilized for the function of contraction ,the intestinal mucosal cells specilized for absorption of foodstuffs and so on. However mammalian cells have an overall common structure and certain basic characteristics , which are described here.
A typical consists. Of 3 basic components :
• Cell membrane
Fig : Structure of typical cell showing Various oragnelles
• Cell Membrane
Cell membrane or the plasma membrane is the protective semipermeable sheath, enveloping the cell body. It separates the contents of cell from the external environment and controls exchange of materials between the fluid outside the cell ( extracellular fluid ) and the fluid inside the cell ( intracellular fluid ). A detailed knowledge of its structure is essential for the understanding of cell Functions.
Electron microscopy has shown that cell membrane/or plasma membrane has trilayer structure having total thickness of 7-010 nm .The three layers consist of two electron dense layers separated by an electron lucent layer. Biochemically, the cell membrane is composed of complex mixture of lipid 40% , protein 55% and carbohydrates 5% .
• Functions of Cell Membrane
The cell membrane performs the following functions:
• It protects the oragnelles in cytoplasm.
• The selective permeability of the cell membrane helps in acting as barrier to harmful substances.
• It helps in absorbing nutrients.
• It helps in maintaining the shape and size of cells.
Cytoplasm is an aqueous substance (custom) containing a variety of cell oragnelles and other structures.The structures dispersed in the cytoplasm can be broadly divided into 3 groups .- oragnelles, inclusion bodies and cytoskeleton.
The oragnelles are the permanent components of the cells, which are bounded by limiting membrane and contain enzyme; yenar they participate in the cellular metabolic activity. These include the mitochondria ,the endoplasmic reticulum, ribosomes , Golgi apparatus, peroxisomes, centrosome and centriole.
• Cytoplasmic inclusions
The cytoplasmic inclusions are the temporary components of certain cells . A few examples of cytoplasmic inclusions are :
- Protein as secretory granules
- Melanin pigment
The cytoskeleton is a complex network of fibres that maintains the structure of cell and allows it to change shape and move .It primarily consists of microtubules, intermediate filament and microfilaments along with proteins which anchor and tie them together.
Nucleus is present in all the eukaryotic cells .The nucleus consists of an outer nuclear membrane enclosing nucleoplasm and nucleoli.
It controls all the cellular activities including reproduction of the cell.
- Nuclear Membrane
The nuclear membrane brand is double – layered porous structure having 40-70 mm wide space called perinuclear cistern,which is continuous with the lumen of endoplasmic reticulum.The outer layer of the nuclear membrane is continuous with endoplasmic reticulum.
The exchange of materials between nucleoplasm and cytoplasm occurs through nuclear membrane.
The Nucleoplasm or the nuclear matrix is gel – like ground substance containing large quality of genetic material in the form of deoxyribonucleic acid ( DNA ). When cell is not dividing ,the Nucleoplasm appears as dark-staining thread like material is called as chromatin. During Cell division, the chromatin material is converted into road – shaped structures,, the chromosomes. There are 46 chromosomes (23 pairs) in all dividing cells of the body except the gamate ( sec cells ) which contain only 23 chromosomes ( haploid number ) .
The nucleus may contain one or more rounded bodies called nucleoli. The nucleoli areore common in growing cells or in cells actively synthesizing proteins.
The nucleoli are the site of synthesis of ribosomal RNA .