The Fundamental Unit of Life

Cell is the structural and functional unit of all the living organisms, which is capable of carrying out all life processes. Robert Hooke observed honeycomb like compartments, the microscope and named them ‘cells’. They were dead cells. In 1674, Leeuwenhoek discovered living cells in pond water. The cell theory was presented by two biologists – Schleiden (1838) and Schwann (1839). It was further expanded by Virchow (1855). There are two types of organisms based on the number of cells present in them: • Unicellular organisms: They are single celled. For example: Mycoplasma, Amoeba • Multicellular organisms. They possess two or more cells for example plant and animal cells. Cells are enclosed by a plasma membrane. In plant cells, a cell wall is located outside the plasma membrane. In eukaryotes, the nucleus is separated from the cytoplasm by a nuclear membrane and it directs all the life processes of the cell. The endoplasmic reticulum (ER) is of two types: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Rough endoplasmic reticulum has ribosomes on its surface. Endoplasmic reticulum helps in intracellular transport and serves as a manufacturing surface. Golgi apparatus consists of stacks of membrane-bound vesicles that help in the storage, modification and packaging of substances manufactured in the cell. Plant cells possess plastids, which are of two types namely chromoplasts and leucoplasts. Mitochondria are also called “power house of a cell”. Both plastids and mitochondria are semi-autonomous organelles. Prokaryotic cells lack membrane-bound organelles whereas eukaryotic cells are membrane bound.

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