Cells are among the smallest organisms that exist. All living things are made up of cells, yet invisible to the naked eye. Cells are the basic 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 environment by a cell membrane. The cell membrane regulates what enters and leaves the cell and also helps protect and support the cell. A cell membrane is similar to the walls that surround your house. In plants, the cell membrane is surrounded by a cell wall. The cell wall lies outside the cell membrane and its role 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 home because it helps prevent other elements from getting inside the cell. In many cells there is a nucleus, first described by Robert Brown. Whether or not a cell has a nucleus, scientists have used this method to divide cells into two general categories. The two categories are eukaryotes, which are cells with a nucleus, and prokaryotes, which are cells without a nucleus. The nucleus was found to be the information center of the cell and contains DNA. It also directs all activities that take place in a living cell. It's like a mini-me of the heart and brain of a human body. Most nuclei contain a small area called the nucleus....