What Is a Cell?
Living things are constructed of cells and can be unicellular (one cell) or multicellular (many cells).
Limits on Cell Size
Cells size is limited because cells must be able to exchange materials with their surroundings. In other words, surface area relative to the volume decreases as size of cell increases, and this limits the size of cells.
Cell Theory
Only a few hundred years ago it was believed that living things could spontaneously generate from non-living matter. We now know better. Cell theory lays out the basic rules that apply to the smallest unit of life. This cell doctrine states that:
- All organisms are composed of one or more cells.
- Cells are the basic unit of structure and function in organisms.
- All cells come only from other cells.
Two Basic Types of Cells
All cells fall into one of the two major classifications: prokaryotes or eukaryotes.
Prokaryotic Cells
Prokaryotes are evolutionarily ancient. They were here first and for billions of years were the only form of life. And even with the evolution of more complex eukaryotic cells, prokaryotes are supremely successful. All bacteria and bacteria-like Archaea are prokaryotic organisms.
Eukaryotic Cells
Eukaryotic cells are more complex, evolving from a prokaryote-like predecessor. Most of the living things that we are typically familiar with are composed of eukaryotic cells; animals, plants, fungi and protists. Eukaryotic organisms can either be single-celled or multi-celled.
Features of Prokaryotes
Pro = “before”, karyon = “nucleus”
Prokaryotes, the first living organisms to evolve, are primarily distinguished by the fact that they lack a membrane-bound nucleus. In fact, the only membrane in prokaryotic cells is the plasma membrane--the outer boundary of the cell itself. Their genetic material is naked within the cytoplasm, ribosomes their only type of organelle.
Prokaryotes are most always single-celled, except when they exist in colonies. These ancestral cells, now represented by members of the domains Archaea and Eubacteria, reproduce by means of binary fission, duplicating their genetic material and then essentially splitting to form two daughter cells identical to the parent.
Features of Eukaryotes
Eu = “true”, karyon = “nucleus”
The most noticeable feature that differentiates these more complex cells from prokaryotes is the presence of a nucleus, a double membrane-bound control center separating the genetic material, DNA (deoxyribonucleic acid), from the rest of the cell.
In addition to the plasma membrane, eukaryotic cells contain internal membrane-bound structures called organelles. Organelles, such as mitochondria and chloroplasts, are both believed to have evolved from prokaryotes that began living symbiotically within eukaryotic cells. These vital organelles are involved in metabolism and energy conversion within the cell. Other cellular organelles within eukaryotic cell structure carry out the many additional functions required for the cell to survive, thrive, grow and reproduce.
Eukaryotic cells can reproduce in one of several ways, including meiosis (sexual reproduction) and mitosis (cell division producing identical daughter cells).
Additional Cell Biology Information
To learn more about cell biology see the Virtual Cell Biology Classroom and Cells Alive.
See the Suite101 Prokaryote & Eukaryote article collection page or follow these links to further explore the differences between the cellular organisms and acellular particles. For a direct comparison of prokaryotic and eukaryotic cells, listing differences, see the article, "Difference Between Prokaryotic and Eukaryotic Cells."
Sources
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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