1. Not all cells have a nucleus. Cells are of two types: eukaryotic (those with a defined nucleus) and prokaryotic (those with no defined nucleus).
2. Tissues are groups of similar cells that have a common function. An organ is a structure that is composed of at least two or more tissue types and performs a specific set of functions for the body. Many organs working together to accomplish a common purpose is called an organ system.
3. Tissue is a group of cells that have similar structure and that function together as a unit. Primary types of body tissues include epithelial, connective, muscular, and nervous tissues.
4. Cells differ greatly in size, shape and activities. For example, mycoplasmas, the smallest cells, are only 0.3 µm in length, while bacteria could be 3 to 5 µm. The largest isolated single cell is the egg of an ostrich. Among multicellular organisms, human red blood cells are about 7.0 µm in diameter. Nerve cells are some of the longest cells. Cells also vary greatly in their shape. They may be disc-like, polygonal, columnar, cuboid, thread-like, or even irregular. The shape of the cell may vary with the function it performs.