There are three basic “rules” that tend to be universally accepted as parts to the cell theory. These “rules” are identified below:
- Cells are the basic fundamental unit of structure, function, and organization in all living things.
- New cells are formed and come from other cells. (In other words, cells produce cells.)
- All living things are made of cells.
Some resources indicate that the following modern addendums have been added to the basic cell theory.
- The cells within individuals of the same species are basically the same in chemical composition.
- Some organisms are made of only one cell. Unicellular is the term used to define such organisms.
- Other organisms are multicellular and are made of many cells.
- Heredity information (DNA) is passed from parent to daughter cells during reproduction.
However, there are some exceptions to the cell theory. Each exception violates rule number 2 of the basic cell theory that is explained above. Three exceptions are identified below.
- There is a debate as to whether or not viruses should be considered to be alive. The debate results from the fact that viruses cannot reproduce on their own. They must invade a host cell and utilize the host’s genetic material in order to reproduce.
- The first cell did not come from another cell.
- Mitochondria and chloroplasts have their own genetic material and can reproduce independently from the cell in which they reside.
Cell theory is the scientific theory that is used to describe the properties of cells, the simplest, most basic biological unit of all living organisms. This theory was first purposed in 1838 by Schleiden and Schwann, and is based on three main principles.
- All living forms are composed of one or more cells.
- Cells can only arise from pre-existing cells.
- Cells represent the smallest form of life
Looking at these three characteristics, two main exceptions to cell theory emerge. The first exception is that of mitochondria and chloroplasts. Mitochondria are the organelle responsible for energy production in most eukaryotic cells via cellular respiration, while chloroplasts are modified organelles found in plant cells that serve the same purpose using photosynthesis. Both of these organelles are able to self replicate inside of their respective cell types. This ability to self replicate within cells, even when the rest of the cell is not in the replication phase, makes them an exception to traditional cell theory.
The other main exception to cell theory is the special case of viruses. Viruses lack the ability to generate energy on their own, and also generate no waste products, both hallmarks of living organisms. Viruses also lack the ability to replicate without first invading a host cell. This inability to replicate on their own without the presence of a host has lead viruses to traditionally be classified as non-living, and therefore an exception to cell theory. Hope that helps!