Cells are some of the smallest organisms in existence. All living things are made up of cells, yet they are invisible to the naked eye. Cells are the fundamental structural and functional units of life. Cells are made up of many different parts that allow them to function properly. All cells are separated from their surroundings by a cell membrane. The cell membrane regulates what goes in and out of the cell and also helps in the protection and support of the cell. A cell membrane is similar to the walls that surround your home. In plants the cell membrane is surrounded by a cell wall. The cell wall is located on the outside of the cell membrane, and its purpose is to help the membrane protect and support the cell. Because the cell wall is very porous, water, oxygen, carbon dioxide, and other substances can pass through it easily. A cell wall is similar to a fence around your house, because it helps protect other things from entering the cell. The nucleus, first described by Robert Brown, is present in many cells. Whether or not a cell has a nucleus has been used by scientists to divide cells into two general categories. The two categories are eukaryotic cells, which are cells with a nucleus, and prokaryotic cells, which are cells without a nucleus. It turned out that the nucleus is the information center of the cell and contains DNA. It also directs all activities that occur in a living cell. It is like a miniature of the heart and brain of a human body. Most nuclei contain a small area called the nuc...