Learn about the history and evolution of the cell theory , which states that all living things are composed of one or more cells and that cells only arise from pre-existing cells. Explore the three parts of the classical cell theory and the three additions of the modern cell theory . The Cell Theory is one of the basic principles of biology. It states that all living things are composed of cells and cells are the basic units of life. These findings led to the formation of the modern cell theory , which has three main additions: first, that DNA is passed between cells during cell division; second, that the cells of all organisms within a similar species are mostly the same, both structurally and chemically; and finally, that energy flow occurs within cells. Every cell in the human body is a different type of cell . According to cell theory , each of these cells originated from the zygote, a single cell produced upon the fertilisation of an egg by a sperm. Then, this cell multiplies, divides, and starts differentiating into the various types of cells that make up the human body.
