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: 2: Cells and Tissues
A. CELLS
Unit of Life
The human body and its parts are made up of (i) trillions of microscopic structures called cells, (ii) inter-cellular material (matrix) that the cells produce, and (iii) body fluids. Of these three ingredients, only the cells have the characteristics which distinguish life from non-life viz., growth, metabolism, response to stimuli and reproduction. Cells are thus the smallest living units of the body.
Since cells are the units from which the body is built, we shall commence our study of the body systems with them. The cell is often called the basic element of life. In fact, it is life itself. Can you imagine to compress the functions of a big city viz, transport system, power stations, communication set-up, factories and waste disposal system, all in a tiny sphere of about one-hundredth of a milimeter in diameter? This is exactly what a cell is. It is hard to believe that a structure too small to see with the naked eye should be as complex as a city.
Size and Shape
Cells are microscopic units with great diversity of size and shape. There are 60 billion (60,000,000,000,000) cells in a human body. Though they are of different sizes, nearly all human cells need a high magnification microscope to be seen and a super microscope to peep inside its body. The smallest cells (certain brain cells) are about 1/200 mm and the largest ones (ova) are about 1/4 mm in diameter. They come in a variety of shapes. Some cells are nearly spherical, others are
tiny cubes and still others are cylindrical in shape.
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