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 -- abiogenesis. 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. 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.
Eukaryotic cells also 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).
To learn more about cell biology see the web sites:
Sources
- Bauman, R. (2005) Microbiology.
- Park Talaro, K. (2008) Foundations in Microbiology.
- This article was originally published in Suite101 online magazine.
