Anatomy is the study of structure, and physiology is the study of function. Because the structure of living organisms is complex, anatomy is organized by levels, from the smallest components of cells to the largest organs and their relationships to other organs. Gross anatomy is the study of the body's organs as seen with the naked eye during visual inspection and dissection. Cellular anatomy is the study of cells and their components, which require special instruments such as microscopes and special techniques for observation. Cells Often thought of as the smallest unit of living organisms, a cell is made up of many even smaller parts, each with its own function.
Human cells vary in size, but all are quite small.
The largest, a fertilized egg, is too small to be seen with the naked eye. Human cells have a membrane that holds the contents together. However, this membrane is not just a sac. It has receptors that identify the cell to other cells. The receptors also react to substances produced in the body and to drugs taken into the body, selectively allowing these substances or drugs to enter and leave the cell.