Living Cells

           A cell is the building block from which all living things are made. A human body is made number of cells. Similar cells combine to form tissues and different tissues joint together to make the various organs of body. The organs work together to make different systems of the body such as nervous system, digestive system, circulatory system, blood system etc.

            The cells in theses systems vary in shapes and sizes and have their own specific functions to perform. A cell is made up of membrane, cytoplasm, lysosmes, nucleus, endoplasmic reticulum, mitochondria and Golgi complex.

            A cell is surrounded by membrane which forms its outer wall. It is a very thin layer and allows food and oxygen to pass into the cell and substances made by the cell or waste products to pass out. Membrane separates cells from one another and gives shape to each cell.

            Cytoplasm is a watery, jelly like material which surrounds the nucleus of the cell. It is living, colorless, translucent and granulated. All the parts of the cell float in it.

            Liposomes are circular tiny packets of digestive chemicals which can destroy harmful substances or worn out parts of the cell. They also play an important part in cell division.

            Nucleus is a compact body. It is the control centre of a cell. It contains the chromosomes and DNA of a cell. DNA keeps the cell alive doing its own special job.

            Endoplasmic reticulum is the folded, membrane like structure in the cytoplasm. Their main function is to transport substances throughout the cell.

            Mitochondria are the cell’s powerhouses. The energy needed for cellular activities is released here from food and oxygen.

            The Golgi complex is the flat arrangement of pockets where some substances are stored and others are prepared for used outside the cell.

Different types of cells

                        Human body has different types of cells. Some of the smallest cells are in the brain, measuring only 0.005mm. The largest are the ovum or egg cells with a diameter of 0.2ml. Cells differ in shapes and sizes. Some of them are round/circle and some are flat. Muscle cells are long and cylindrical in shape up to 60mm in length.

Tissues and Organs

            Despite being independent units, the cells often work together. The cells performing similar function form a tissue. Every tissue has its special function to perform.

            They are structured differently according to the task they perform. Muscles, bones and nerves are three different types of tissues.

Life of Cells

            Living cells grow and many of them reproduce themselves so that a body can grow larger and replace the damaged or dead cells. The life span of some cells is only a few days while some others can live for weeks, months or years. Bone cells last for about 15-20 years, while white blood cells only for four months. Skin cells do not live more than 3 weeks. Nerve cells do not have any reproductive property; therefore, they end with the life itself. 

Internal Structure of an Ordinary Cell
Nucleus

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