The cell is the unit of life. Although all human cells are microscopic, they vary in size. Some neurons and muscle cells are relatively large Squamous epithelial cells and osteocytes are smaller and occupy less space.
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Cells are the smallest units that are able to carry out the essential functions of life. Human cells vary in size; as cells mature, they may become specialized and lose some capacities. Not all cells can move, and not all can divide mitotically. However, all living cells can carry out the processes of metabolism, are responsive to stimuli, guard their boundaries, and maintain internal homeostasis by virtue of having a selectively permeable plasma membrane.