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Chapter-8, Cell: The Unit of Life, , Cell, The cells are the smallest entity that are capable of independent existence and, carrying out all essential functions of life. Thus, cell forms the basic structural and, functional unit of life., , Robert Hooke (1635-1703) was the first to observe cell in a cork but what he saw, was actually the cell walls enclosing spaces left by dead protoplasts. Anton van, Leeuwenhoek was first to observe, describe and sketch a free living cell. He, observed bacteria, protozoa, spermatozoa, red blood cells, etc. Robert Brown (1831), discovered the presence of nucleus in the cells of orchid root., Leeuwenhoek is called father of microscopy. Electron microscope was invented by, Max Knoll and Ernst Ruska (1931)., , Cell Theory, I n 1838, Matthias Schleiden, a German botanist observed that all plants are, composed of different kinds of cell which form the tissue of the plant. Theodore, Schwann (1838), a British zoologist, reported that animal cells had a thin outer layer,, which is now known as plasma membrane., , Based on Schwann's studies of the plant cells, he concluded the presence of cell wall, is a unique character of the plant cells. He proposed the hypothesis that the bodies, of animals and plants are composed of cells and product of cells., Schleiden and Schwann together formulated cell theory. Rudolf Virchow (1855),, explained that cells divided and new cells are formed from pre-existing cells (Omnis, cellula e cellulo). He modified the cell theory and gave a final shape to it., , Features of Cell Theory, i., I., , All living organisms are composed of cells and products of cells., All cells arise from pre-existing cells. Viruses are exception to cell theory as they do, not have cellular organisation., , An Overview of Cell, The plant cell has a distinct cell wall as its outer boundary and just within it is the cell, membrane. Human cells have an outer membrane as the delimiting structure of the, , cell. Inside each cell is a dense plasma membrane bound structure called nucleus., This nucleus contains the chromosomes which in turn contain the genetic material,, DNA.
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Cells that have membrane bound nuclei called eukaryotic whereas cells that lack a, membrane bound nucleus are prokaryotlc. In both prokaryotic and eukaryotic cell,, a semi-fluid matrix called cytoplasm occupies the volume of the cll. The cytoplasm, , is the main arena of cellular activities in both the plant and animal cells., Besides the nucleus, the eukaryotic cells have other membrane bound distinct, structures called organelles like the endoplasmic reticulum (ER), the Golgi complex,, , lysosomes, mitochondria, microbodies and vacuoles. The prokaryotic cells lack such, membrane bound organelles. Different cell organelles are embedded in cytoplasm., Various chemical reactions occur in it to keep the cell in the Tiving state., , Ribosomes are non-membrane bound organelles found in all cell-both eukaryotic as, well as prokaryotic. Within the cell, ribosomes are found not only in the cytoplasm, , but also within the organelles like chloroplasts and mitochondria and also attached, on rough ER. So, ribosome is also called an organelle within an organelle. Animal, cells contain another non-membrane bound organelle called centriole which helps in, , cell division., Cells differ greatly in size, shape and activities. For example: Mycoplasmas, the, smallest cells, are only 0.3um in length while bacteria could be 3 to 5 um. the largest, isolated single cell is the egg of an ostrich. Human red blood cells are about 7.0 um in, , diameter. Nerve cells are some of the, , longest cells, , Md coe, , dng d, , nT, , und and t e r n a e, , Diagram ing dilleren thspnef tha cel, , Cells also vary greatly in their shape. They may be disc-like, polygonal, columnar,, cuboid, thread-like or even irregular.
