The human body is made from a single cell called zygote, which is derived from the fusion of ovum (female reproductive cell) and spermatozoon (male reproductive cell). From this single cell, all specialized cells develops that forms the tissues from which different organs and system of the body develops. This is how the human body is formed. In other words our body is made up of masses of cells. Hence, the cell is designated as the unit of living matter and fundamental building block of life. For a more explicit definition, “the cell is the structural and functional unit of the body that is capable of carrying on the processes of life independently” as:
1. Assimilation of nourishment
2. Growth and repair
3. Reproduction and
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4. Excretion of waste products.
Structure
All cells are made up of a jelly-like, opaque, colorless substance called protoplasm which is bounded by a delicate membrance and contains various microscopic and submicroscopic structures. Some cells even have the power of movement. Each cell has two principal units: 1. Cytoplasm and2. Nucleus.
1. Cytoplasm:
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The cytoplasm is the protoplasm inside the cell that is surrounded by the cell membrance but outside the nucleus. The cytoplasm contains molecules of protein known as ribonucleic acids (RNA) which act as a messenger carrying information out from the nucleus to the cytoplasm. Several granular structures called mitochondria are also present in the cytoplasm. Mitochondria are referred as the power station of the cell, as they are involved in oxidative reactions which results in the controlled release of energy from the nutrient materials and the formation of adenosine triphosphate (ATP), the main energy-yielding producing substance in the cell, within the cytoplasm there are clear spaces called vacuoles which contains waste materials or secretions formed by the cell.
2. The Nucleus
The Nucleus is a round dense body occupying the centre of the cell. Its shape size position and no vary. It is surrounded by a nuclear membrance and the protoplasm within the nucleus is called nucleoplasm. If the nucleus is removed the cell dies.
The characteristic compound of the nucleus is deoxyribonucleic acid (DNA) which contains the genetically inherited information required for the maintenance of the cell. The nucleoplasm stores the information necessary for the cell to grow and to divide into two daughter cells. This information is stored in the genes, which are strung to form chromosomes. It is these chromosomes which determine the specific characteristics of the cell and it is through these that the hereditary traits pass from one generation to another. Chromosomes are normally visible under the microscope when the cell is about to divide.