and more unicellular organisms has characteristic of all living fo CON nilar to structure like an this activity that sizes have similar der a microscope. el will all look the e of the onion they come from pre-existing cells. Activity 5.2 that we see are the onion bulb. d cells. Not only that we observe . However, there on their own. Every multi-cellular organism a produce cells of their own kind. All cells the multicellular organisms s from a single cell. How? Cells divide We know that there is a divis beings. This means that d the human body perform dif The human body has a hear We can try preparing temporary a stomach to digest food and mounts of leaf peels, tip of roots of division of labour is also se onion or even peels of onions of different cell. In fact, each such cel sizes. specific components within After performing the above activity, let organelles. Each kind us see what the answers to the following performs a special function questions would be: new material in the cell (a) Do all cells look alike in terms of waste material from the shape and size? (b) Do all cells look alike in structure? functions because of thes cell is able to live and (C) Could we find differences among organelles together consti cells from different parts of a plant called the cell. It is intere body? (d) What similarities could we find? are found to have the sa matter what their fur Some organisms can also have cells o organism they are found different kinds. Look at the following picture It depicts some cells from the human body. uestions covered by He observed with the help microscope. with the scovered the rater for the t Brown in nucleus in coined the the fluidदो आज सेल लुक एलाइक इन द टाइम ऑफ शेप एंड साइज
Answers
Robert Hooke (1665) observed the so called cells for the first time in a thin slice of cork under a very primitive microscope invented by him. He coined the term “cell”. Living cells were seen for the first time by Anton Van Leeuwenhoek (1632-1723), with his improved microscope. Much later (1838-39) cell-theory was proposed by two German biologists separately – viz., M.J. Schleiden for plants and Theodor Schwann for animals. According to them, “Cells are the structural and functional units of living organisms.” Later, Rudolph Virchow (1855) extended the cell theory and suggested that all living cells arise from pre-existing cells (Omnis cellula e cellula).
Size of Cell
Usually the cells are microscopic and their size varies between 10 µm and 100 µm. The smallest cells are those of PPLO (Pleuropneumonia like organisms) whose size may vary between 0.1 to 0.4 µm. The largest cell is the egg of ostrich measuring about 15 cm in its outer diameter. The longest animal cell is the nerve cell which may be approximately one metre long, while the longest plant cell is the sclerenchymatous fibre of Boehmeria nevia (about 55 cm long).
(i) The ratio between the volume of the nucleus and that of the cytoplasm.
(ii) The ratio of the cell surface to the cell volume.
(iii) The rate of metabolism.
(iv) The size and the number of chromosomes
Shape of cel
There is a great variation in the shape of cell. Some cells, e.g., Amoeba, slime moulds and WBCs have a constantly changing shape while others (e.g., neurons, muscle cells, RBCs, etc.) have a characteristic shape. The shape is governed by the plasma membrane and the cell wall, if present.