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In biology, the cell is the basic structure of organisms. All cells are made by other cells.
The environment outside of the cell and the inside of the cell are separated by the cell membrane. Inside some cells, some parts of the cell stay separate from other parts by membranes. These separate parts are called organelles (like small organs). They each do different things in the cell. Two examples are the nucleus (where DNA is), and mitochondria (where usable energy is created).[1][2]
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There are 2 basic kinds of cells: prokaryotic cells and eukaryotic cells. Prokaryotes, bacteria and archaea, are simple cells with no internal organelles. Eukaryotes are complex cells with many compartments within the cell. Eukaryotes store their genetic information (DNA) in a compartment called the nucleus. In a prokaryotic cell the DNA is not separated from the rest of the cell. In general, all living things (organisms) that are made up of multiple cells are made up of eukaryotic cells.
The only kinds of prokaryotic organisms that survived to the present are bacteria and archaea.[3] Prokaryotic organisms evolved before eukaryotic organisms, so at one point the world consisted of nothing but prokaryotic organisms.
Unicellular organisms are made up of one cell. Examples of unicellular organisms are:
Unicellular organisms live without other cells to help them. Many of these organisms need to:
All organisms must:
Some may:
Multicellular organisms are made from many cells. They are complex organisms. This can be a small number of cells, or millions of cells. All plants and animals are multicellular organisms. The cells of a multicellular organism are not all the same. They have different shapes and sizes, and do different work in the organism. The cells are specialized. This means they do only some kinds of work. By themselves, they cannot do everything that the organism needs to live. They need other cells to do other work. They live together, but cannot live alone.
Cells were discovered by Robert Hooke. He used a microscope to look at organisms in 1665. He named cells after the Latin word cella, meaning room. He did this because he thought cells looked like small rooms. The idea of cell was then deeper explored by a Czech J.E. Purkyňe in 1837. Finally, three German biologists - Schleiden, Schwann and Virchov - figured out three rules about all cells:
It was in 1838 and 1839. These ideas still are the basic ideas of cell theory.[4]
The body cells of metazoans divide by simple mitotic cell division. Sexual reproduction is ancestral in eukaryotes, and in metazoa it is carried out by specialised sex cells in a process called meiosis.
Prokaryotic cells reproduce using binary fission, where the cell simply splits in half. For both mitosis and binary fission the cell must replicate all of its genetic information (DNA) so that each new cell will have a copy.
