What is a cell?

All living things—lions, butterflies, grass, elm trees, dogs, people, and everything else alive—are built of cells. There are millions of cells in the bodies of most plants and animals. Some plants and ani­mals, however, are made up of just one cell.

Not all cells are the same shape or size. Some, such as red blood cells, are disk-shaped while others, such as muscle cells, are long and narrow. Nerve cells have very irregular shapes. Some plant cells are brick-shaped while others are round. There are many other shapes of both plant and animal cells, too. Most cells are far, far too small to be seen without a microscope. Some are just barely large enough to be seen with the naked eye. And some, because of the food stored in them, are rather large. The yolk of a newly formed hen's egg, for instance, is a cell.

Different kinds of cells do different kinds of work. Certain animal cells carry messages. Others carry oxygen. Some fight diseases. Still others make the animal move. Some plant cells take in water. Some manu­facture food, and so on.