In biology, the cell is the basic structure of organisms. All cells are made by other cells. The outside of the cell is a 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. Some of these are ribosomes, nucleus (where DNA is), and mitochondria.
There are 2 basic kinds of cells: prokaryotic cells and eukaryotic cells. Prokaryotes are simple cells, also known as bacteria. Eukaryotes are less simple cells. All eukaryotic cells have a nucleus except red blood cells in mammals, while prokaryotes do not have one. Both kinds of cells have RNA or DNA. In general, prokaryotes( organisms with prokaryotic cell) have genetic information stored in RNA while eukaryotes have genetic information stored in form of DNA. In the prokaryote it is not separated from the rest of the cell by a membrane. In the eukaryote DNA is separate from the rest of the cell.In the eukaryotes DNA is coiled in structures called chromosomes within the nucleus. In general, all prokaryotic organisms are made of just one cell.
The only kinds of prokaryotic organisms that survived to the present are bacteria. Prokaryotic organisms evolved before eukaryotic organisms, so at one point the world consisted of nothing but prokaryotic organisms.
There are two kinds of organisms: multicellular and unicellular.
There are also 2 kinds of cells with no nucleus.
Unicellular organisms are made up of one cell. Examples of unicellular organisms are:
Unicellular organisms live without other cells to help them. So the cell does all things that it needs to live. Each cell.
- respires (uses oxygen to make sugar into energy)
- gets rid of waste
- reproduces (make more of itself)
- senses its environment
- grows (mitachondrian uses oxygen for growth)
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 than 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:
- All living things are made of cells
- The cell is the basic unit of structure and function in all organisms.
- Every cell comes from another cell that lived before it.
It was in 1838 and 1839. These ideas still are the basic ideas of cell theory.
Eukaryotic cells reproduce differently than prokaryotic cells, but both go through the same general process. Most eukaryotic cells undergo mitosis, creating exact genetic copies of themselves. Exceptions to this rule are nerve cells, which never split, and sex cells, which go through a separate process called meiosis. Prokaryotic cells reproduce using binary fission, where the cell simply splits in half without the complex steps of mitosis.
For More Information: Cell (Biology) K-12 Background Information
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)