Cells PDF

We can try preparing temporary mounts of peels of onions of different sizes. What do we observe? Do we see similar structures or different structures? What are these structures? More to know These structures look similar to each other. Together they form a big structure like an onion bulb! We find f...

We can try preparing temporary mounts of peels of onions of different sizes. What do we observe? Do we see similar structures or different structures? What are these structures? More to know These structures look similar to each other. Together they form a big structure like an onion bulb! We find from this activity that onion bulbs of different sizes have similar small structures visible under a microscope. The cells of the onion peel will all look the same, regardless of the size of the onion they came from. These small structures that we see are the basic building units of the onion bulb. These structures are called cells. Not only onions, but all organisms that we observe around are made up of cells. However, there are also single cells that live on their own. Cells wer e first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive micr oscope. Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time. It was Robert Brown in 1831 who discovered the nucleus in the cell. Purkinje in 1839 coined the ter m ‘protoplasm’ for the fluid substance of the cell. The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells. With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles. The invention of magnifying lenses led to the discovery of the microscopic world. It is now known that a single cell may constitute a whole organism as in Amoeba, Chlamydomonas, Paramoecium and bacteria. These organisms are called unicellular organisms (uni = single). On the other hand, many cells group together in a single body and assume different functions in it to form various body parts in multicellular organisms (multi = many) such as some fungi, plants and animals. Can we find out names of some more unicellular organisms? Every multi-cellular organism has come from a single cell. How? Cells divide to produce cells of their own kind. All cells thus come from pre-existing cells. Activity ______________ 5.2 We can try preparing temporary mounts of leaf peels, tip of roots of onion or even peels of onions of different sizes. After performing the above activity, let us see what the answers to the following questions would be: (a) Do all cells look alike in terms of shape and size? (b) Do all cells look alike in structure? (c) Could we find differences among cells from different parts of a plant body? (d) What similarities could we find? Some organisms can also have cells of different kinds. Look at the following picture. It depicts some cells from the human body. Blood cells Smooth muscle cell Bone cell Ovum Nerve Cell Fat cell Sperm Fig. 5.3: Various cells from the human body SCIENCE 50 Rationalised 2023-24