5.3 The essentials of cell structure
Cells were discovered in 1665 by Robert Hooke who saw them in slices of cork using his seventeenth century optical microscope. Hooke coined the term ‘cell’ because he compared the compartments in the cork he saw to the small rooms in which monks lived. The word cell comes from the Latin cellula, a small room. It was another 170 years before the cell theory, that all organisms are composed of one or more cells and all cells come from pre-existing cells, was first formulated (in 1839) by Matthias Jakob Schleiden and Theodor Schwann.
The essential life-sustaining activities of an organism occur within its cells, and almost all cells contain the genetic (hereditary) information necessary for performing and regulating those activities, and for transmitting the genetic information to the next generation of cells. Prokaryotes are cells, too, and both prokaryotic and eukaryotic cells have a membrane that envelops the cell and performs the crucial task of separating the cell from its environment. This cell membrane is also selectively permeable; that is, it regulates the chemicals that enter and leave the cell. By regulating the flow of ions, the membrane adjusts the electric potential and pH of the cell. By regulating the flow of water (directly or indirectly), the membrane adjusts the volume and osmotic potential of the cell. The material within (and enclosed by) the cell membrane is a complex mix of molecules and ions called cytoplasm. The cytoplasm also contains a variety of regions specialised to carry out one or more vital functions that are bounded by their own membranes in eukaryotes; these are generally called organelles.
There are two different kinds of genetic material: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is used for long-term information storage (although this function is served by RNA in some viruses). An organism’s genetic information is encoded in the sequence of its DNA or RNA. RNA is also characteristically used for information transport within the cell, e.g. as messenger and transfer RNAs (mRNA and tRNA), and ribosomal RNAs (rRNAs) serve essentially enzymatic functions during protein synthesis.
The genetic material in prokaryotes is organised as a simple circular DNA molecule which is packaged into the nucleoid region of the cytoplasm. This region is not separated by a membrane and this is a major distinguishing feature between prokaryotes and eukaryotes. In eukaryotes the genetic material is distributed between different linear molecules called chromosomes which are housed in a nucleus surrounded by an organised nuclear membrane.
Prokaryotes do not have a nuclear membrane, and they also lack the membrane-bound organelles that characterise eukaryotic cells; however, ribosomes, and many of the other components of the protein synthesis mechanism, are present in both prokaryotic and eukaryotic cells, though there are some differences in detail (and some of those differences are important in providing drug targets for therapeutic antibiotics). In prokaryotes, the functions performed by specialised organelles in eukaryotes are performed by the plasma membrane (Table 1). Prokaryotic cells do have structurally distinct regions:
- cytoplasmic region contains the cell genome (DNA), ribosomes and various inclusions;
- appendages attached to the cell surface - pili (singular = pilus) are hair-like appendages responsible for attachment; motile bacteria have flagella but these are largely composed of the self-assembling protein flagellin and are not related to eukaryotic flagella (pilus and flagellum proteins of animal pathogens are major antigens);
- cell envelope consisting of a capsule, a peptidoglycan cell wall (an important target for therapeutic antibiotics), and a plasma membrane.
Prokaryotes also frequently carry extrachromosomal (usually circular) DNA molecules called plasmids, which often carry genes conferring additional phenotypes, such as antibiotic resistance, pathogenicity, genome transfer and metabolism of exotic substrates. Plasmids are rare in most eukaryotes, but they do occur in fungi. Also, major eukaryotic organelles (e.g. mitochondria and chloroplasts) contain a genetic architecture separate from the nucleus comprising a (small) circular DNA molecule and an independent ribosome population and protein synthetic apparatus.
|Table 1. Comparison of the main features of prokaryotic and eukaryotic cells.|
|Typical organisms||bacteria, archaea||protists, fungi, animals, plants|
|Typical size||about 1-10 µm||about 10-100 µm|
|Type of nucleus||nucleoid region; no real nucleus, no membrane enclosure (pro + karyotos = before having nucleus (literally, nuts))||true nucleus surrounded by double membrane (eu + karyotos = having true nucleus (literally, nuts))|
|DNA||circular DNA (usually)||linear DNA molecules (chromosomes) organised around histone proteins|
|RNA & protein synthesis||coupled in cytoplasm||RNA-synthesis inside the nucleus, protein synthesis in cytoplasm|
|Cytoplasmic ribosomes||Subunits = 50S & 30S||cytoplasmic = subunits of 60S & 40S; organelle ribosomes more similar to prokaryotic ones|
|Cytoplasm structure||very few structures||highly structured by endomembranes, organelles and a cytoskeleton of microtubules and microfilaments|
|Cell movement||flagella largely composed of flagellin protein||flagella and cilia (undulipodia) largely made of tubulin protein|
|Mitochondria||none (redox electron transport takes place in the plasma membrane)||one to several dozen per cell (probably originating from endosymbiotic prokaryotes). Some eukaryotes lack mitochondria; in anaerobes mitochondria are replaced by hydrogenosomes.|
|Chloroplasts||none (photosynthetic electron transport takes place in folds of the plasma membrane stacked into thylakoids in cyanobacteria)||in algae and plants (probably originating from endosymbiotic cyanobacteria)|
|Organisation||usually single cells||single cells, colonies, higher multicellular organisms with specialised cells, tissues and organs|
|Cell division||binary fission (simple division)||nucleus/chromosomal division (karyokinesis) co-ordinated with cytoplasmic division (cytokinesis) in mitosis and meiosis|
|Table modified and adapted from the English Wikipedia article entitled Cell (biology), page version ID: 148282036, dated 31 July 2007 [http://en.wikipedia.org/w/index.php?title=Cell_%28biology%29&oldid=148282036] (needs Internet connection).|
Updated December 16, 2016