The cell is the basic functional unit of life. It was discovered by Robert Hooke and the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing and is often called the building blocks of life. Some organisms, like most bacteria, are unicellular (consist of a single cell). Other organisms, like humans, are multicellular. Humans have about 100 trillion cells or 1014, a typical cell size is 10 m and a typical cell mass is 1 nanogram. The largest cells are about 135 microns in the anterior horn of the spinal cord, while cerebellar granule cells, the smallest, may be about 4 meters and the longest cell can achieve from the heel to the lower brain stem (cells Pseudounipolar). The largest known cells are not fertilized ostrich egg, which weighs 3.3 pounds.
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