As mentioned in Section 2.4, the highest level of biological classification, the domain, is based on cell type. Bacteria are made of the simpler prokaryotic cell; protists, fungi, plants, and animals are eukaryotic. Prokaryotes are smaller than eukaryotic cells and lack any internal membrane-bound structures. Eukaryotes have organelles, which are specialized structures within the cell that are surrounded by their own membranes, almost like cells within a cell. Some organelles, such as mitochondria, even have their own DNA.
It is thought that prokaryotes are relatively primitive life-forms and that eukaryotes may have evolved from a symbiotic association in which an early form of prokaryotic cell incorporated other prokaryotes internally. For example, a large anaerobic prokaryote may have incorporated an aerobic bacterium. The latter eventually became mitochondria, the site of aerobic respiration in eukaryotes.
Table 4.1 shows some of the similarities and differences between prokaryotic and eukaryotic cell structures. Figure 4.1 illustrates a typical bacterial (prokaryotic) cell. The nuclear region is not surrounded by a membrane, as it is in eukaryotes. Photosyn-thetic bacteria include thylakoid membrane structures, an exception to the rule of not having internal membranes. Granules are structures that contain storage projects such as lipids or starches. Bacterial cells are typically about 1 mm in size.
Table 4.1 Contrast Between Prokaryotic and Eukaryotic Cells
Cell wall Cell membrane Motility
Ribosomes Membranous organelles
No peptidoglycan; some have glycoprotein or protein walls Lipid biolayer ether-linked branched hydrocarbon chains Flagella
DNA Yes Absent
Phospholipid bilayer composed of ester-linked straight hydrocarbon chains
Single circular DNA
Few: e.g., thylakoids for photosynthesis in cyanobacter
Flagella Flagella or cilia
Several linear DNA molecules
Nuclear envelope, endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes, centrioles
Chloroplasts and central vacuoles cell membrane cell wall cell membrane cell wall
granules nucleojd thylakoid membrane
Figure 4.1 Structure of a prokaryotic cell.
granules nucleojd thylakoid membrane
Figure 4.2 shows typical plant and animal (eukaryotic) cells. These cells are much more complicated. The nuclear region (nucleus), endoplasmic reticulum, mitochondria, Golgi apparatus, chloroplasts in photosynthetic organisms, and a number of other structures are all surrounded by their own membranes. Eukaryotic cells are usually much larger than prokaryotic cells, on the order of 10 mm in size.
Functionally, prokaryotes are capable of a much wider range of basic metabolic processes, including many of environmental importance. These include many processes that catalyze key pathways of the biogeochemical cycles. All chemoautotrophs (organisms that obtain energy from inorganic chemicals) are prokaryotes. These include bacteria that oxidize minerals, such as Fe(II), NH3, H2S, and others. Other major processes limited to prokaryotes are nitrogen fixation and denitrification. Furthermore, with few exceptions, only prokaryotes are capable of using electron substitutes for oxygen, such as nitrate, sulfate, or carbonate, and can live their entire life cycles in the absence of oxygen. However, prokaryotes are specialized, and none can do all of these processes, and most do very few. What specialized advantage do eukaryotes have? One answer is that only eukaryotes form multicellular organisms.
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