A Collection of Cells
The Smallest UnitCells are sometimes thought of as being the smallest unit of a living organism. The adult human body is made up of about 10 trillion cells. Even the largest cell produced by the body, a fertilized egg, is too small to be seen by the naked eye.
Cells come in many different sizes and shapes and perform a myriad of functions. Many cells are attached to other cells to form tissues, such as skin tissue or muscle tissue. Other cells move around the body freely, like white blood cells. Some cells have as their primary function the production of certain substances. Different types of glandular cells, for instance, secrete hormones, enzymes, and mucous.
Other cells don’t produce substances but have specific functions. Heart muscle cells, for example, pulsate continuously for the duration of their existence, and nerve cells conduct electrical impulses throughout the body. Cells multiply at different rates, too. Hair cells, blood cells, and cells in the intestinal lining reproduce quickly, while nerve cells seldom if ever divide.