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The cell (meaning “small room”) is the basic structural, functional, and biological unit of all known living organisms. All living organisms are made up of building blocks we call the cell. Many living things consist of vast numbers of cells working in concert with one another. A number of cells in an organism varies.
The cell is surrounded by a cell or plasma membrane. which contains many biomolecules such as proteins and nucleic acids. Cell consists of different organelles like cell nucleus, Mitochondria, Endoplasmic reticulum, Golgi apparatus, Lysosomes, Peroxisomes, centrosome, Vacuoles, Ribosome.
In some organisms, the body is made up of many cells (multicellular) while in others it is made up of a single cell (unicellular). In a human body, it is calculated that there are 1014 cells on an average. Cells, whether living on their own or as part of a multicellular organism, are usually too small to be seen without a light microscope.
Similarly, shape and size of the cells also vary extensively (see Fig). Some multicellular forms (Rotifers) may be smaller in size than many unicellular forms, i.e., Amoeba, Spirostomun, etc. Whereas some single cells like the eggs of birds and reptiles are quite large. A nerve cell may attain a length of several meters. Red blood cells are only 5-8 µ in diameter.