Class 8 science Introduction Both living and non-living things are composed of molecules made from chemical elements such as Carbon, Hydrogen, Oxygen, and Nitrogen. The organization of these molecules into cells is one feature that distinguishes living things from all other matter. The cell is the smallest unit of matter that can carry on all the processes of life. All the living organisms carry out certain basic functions. Anton Van Leeuwenhock Robert Hooke These functions are performed by the structural unit of an organ, called as cell. Any function performed by the organisms is the outcome of the activity of cell.
Discovery of Cell Before the 17th century, no one knew that cells existed, since they are too small to be seen with the naked eye. The invention of the microscope enabled Robert Hooke, (1665) and Anton Van Leuwenhoek (1675) to see and draw the first ‘cells’, a word coined by Hooke to describe the cells in a thin slice of cork, which reminded him of the rooms where monks lived. In 1665, Robert Hooke, an English scientist while observing a dried section of cork (bark of tree) with a crude microscope, observed small chambers in it and named them as cells. Cork is a dead plant structure. He observed that cork had a several tiny compartments in it. He called these tiny compartments as cells. However the cell discovered by Hooke, were dead and only cellulose walls were visible. Anton Van Leeuwenhoek (1974) was the first to observe living cells like bacteria, red blood cells and protozoa. Robert Brown (1931) reported the presence of nucleus in root cells of orchids.
Cell The study of the cell is termed as cytology. • •
In 1938, Schleiden and Schwann discovered that all animals and plants are made up of cells. They put forward the ‘Cell Theory’. Later it was modified by Rudolf Virchow, who said that cells arise from pre existing cells.
Cell Theory consists of three principles: • All living things are made of cells. • Cells are basic units of structure and function in living things. • All cells arise from pre-existing cells by cell division. Organisms showing variety in shape, size and number: • There are millions of living organisms of varying shapes and sizes. Similarly, in an organism itself there are cells present of varying shape and size, depending upon the function performed by them. • The single celled organisms are called unicellular organisms. A single celled organism is able to perform all the necessary functions that multicellular organisms perform. For example – Bacteria, Amoeba, Paramecium, yeast etc.