Biology Notes Class 9th

Topics: Cell, Bacteria, Multicellular organism Pages: 2 (573 words) Published: January 24, 2013
Cell is the basic structural unit of life.
Cork is a substance which comes from the bark of a tree.
In 1665, Robert Hooke made the observation of a cell through a self designed microscope. Cell is a Latin word which means ‘a little room’.
Cells were first discovered by Robert Hooke in 1665.
Leeuwenhoek (1674), with the improved microscope, discovered the free living cell in pond water for first time. Robert Brown in 1831 discovered the nucleus in the cell. Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell. The cell theory that all 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 electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles. The cells that are made up of a single cell are called unicellular organisms. Amoeba, Chlamydomonas, Paramoecium and bacteria are examples of unicellular organisms.{ uni = single} The cells that are made up of more than thousands of cells are called multicellular organisms. Animal and plants are examples of multicellular organisms. { multi = many} Plasma membrane is the outer most covering of the cell that separates the contents of the cell from its external environment. It allows the entry or exit of some materials in and out of the cell. It also prevents the entry of unwanted materials. Plasma membrane is also known as selectively permeable membrane. Substances like CO2 or O2 can move across the cell membrane by a process of diffusion. The spontaneous movement of a substance from higher concentration to lower concentration is the same that happens with a cell. Water also obeys the laws of diffusion. The movement of...
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