端木·宇 2008-6-19 22:26
Types of Cells
There are two major types of cells: prokaryotes and eukaryotes. Eukaryotic cells, whose name derives from the Greek [i]eu[/i], meaning “good,” and [i]karyon[/i], “kernel” or “nucleus,” have a nucleus and membrane-bound organelles. Prokaryotic cells, whose name derives from the Greek [i]pro[/i],meaning “before,” contain neither nucleus nor organelles. As the namesimply, prokaryotic cells are less evolutionarily advanced thaneukaryotic cells.
[b] Prokaryotes[/b]
Prokaryotes include some of the mostprimitive forms of life: bacteria and blue-green algae (also known ascyanobacteria). Prokaryotic organisms are generally single-celled.
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Prokaryotes have a cell membrane, and they are made up of generally undifferentiated fluid, called the [b]cytoplasm[/b],in which floats a circular ring of DNA that controls the functioning ofthe cell. Prokaryotes maintain their shape through a cytoskeleton andhave ribosomes that float in the cytoplasm. In addition, someprokaryotes have a special type of cell wall made of a protein-sugarcombination called peptidoglycan. A few prokaryotes possess whipliketails called [b]flagella[/b] that help propel the cells through water.
Though less complex and less efficient thaneukaryotes, prokaryotes are hardy because of their simplicity. They areable to survive environmental extremes that would kill higher lifeforms.
[b] Eukaryotes[/b]
All living things besides bacteria andcyanobacteria consist of eukaryotic cells, which are larger andstructurally more complex than prokaryotic cells. Like prokaryotes,eukaryotes are surrounded by a lipid bilayer cell membrane and havecytoplasm and ribosomes. However, unlike prokaryotes, eukaryotes alsocontain [b]organelles[/b] and a defined [b]nucleus[/b] containing DNA.
Eukaryotes benefit enormously from thepresence of membrane-bound organelles. Each organelle creates anadditional compartment in the cell that can specialize in particularactivities or processes, increasing productivity as a result. Thestructure of eukaryotic cells and the specific functions of the variousorganelles are often tested by the SAT II Biology.
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opqr801 2008-11-30 03:36
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