The variation in cellular structure is due to the specialization of cells.
Cells from an organism may differ in structure depending on the functions they perform. The modification of cellular structure to suit the functions they perform is termed specialization.
For example, the cells of the mouth have more lysosomes than most other cells found in the body of humans because digestion starts from the mouth which requires a lot of digestive enzymes housed in the lysosome.
The red blood cells in humans lack a nucleus unlike other cells of the body because they are specialized to transport oxygen in the body.
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