What Is a Cell?
Living things are constructed of cells, and these most basic units of life can be unicellular (one cell) or put together into a multicellular (many cells) organism.
There are no huge cells. All are microscopic, or very small if visible to the naked eye. Cells size is limited because cells need to exchange materials with their surroundings, and surface area relative to the volume decreases as size of cell increases.
Cell Theory
A short list of scientific ‘truths’ about life called Cell Theory, goes as follows:
- 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 two major classifications: prokaryotes or eukaryotes.
Prokaryotic Cells
Prokaryotes are evolutionarily ancient. These cells are of simple construction, containing no nucleus or membrane-bound organelles. Modern day bacteria (Eubacteria) and bacteria-like Archaea are prokaryotic organisms.
Eukaryotic Cells
Eukaryotic cells (Eukarya) are more complex, having evolved from a prokaryote-like predecessor. This is a cell type that you are very familiar with, whether you know it or not. Eukaryotic cells are the tiny units of life that make up your entire body. And most of the living things that we are familiar with are also composed of eukaryotic cells; animals, plants, fungi and protists.
In addition to multi-cellular eukaryotic organisms, there is also a multitude of tiny, single-celled eukaryotes, including paramecium, amoeba and yeasts, just to name a few.
Features of Eukaryotes
Eu = “true”, karyon = “nucleus”
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.
Membrane-bound Organelles
Eukaryotic cell structure also includes internal membrane-bound organelles. Organelles, such as mitochondria and chloroplasts, are 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 carry out the many additional functions required for the cell to survive, thrive, grow and reproduce.
Additional Cell Biology Information
To learn more abouteularyotic cellular structure and function see the cell biology website Science Prof Online or additional Suite 101 Articles, including Prokaryotic & Eukaryotic Cells, Bacterial Cell Wall Structure and Parasitic Amoeba: Naegleria fowleri.
Sources
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.
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