http://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookCELL1.html
Components of Cells
Cell Membrane (also known as plasma membrane or plasmalemma) is surrounds all cells. It: 1) separates the inner parts of the cell from the outer environment; and 2) acts as a selectively permeable barrier to allow certain chemicals, namely water, to pass and others to not pass. In multicellular organisms certain chemicals on the membrane surface act in the recognition of self. Antigens are substances located on the outside of cells, viruses and in some cases other chemicals. Antibodies are chemicals (Y-shaped) produced by an animal in response to a specific antigen.
This is the basis of immunity and vaccination.
Hereditary material (both DNA and RNA) is needed for a cell to be able to replicate and/or reproduce. Most organisms use DNA. Viruses and viroids sometimes employ RNA as their hereditary material. Retroviruses include HIV (Human Immunodefficiency Virus, the causative agent of AIDS) and Feline Leukemia Virus (the only retrovirus for which a successful vaccine has been developed). Viroids are naked pieces of RNA that lack cytoplasm, membranes, etc. They are parasites of some plants and also as possible glimpses of the functioning of pre-cellular life forms. Prokaryotic DNA is organized as a circular chromosome contained in an area known as a nucleoid. Eukaryotic DNA is organized in linear structures, the eukaryotic chromosomes, which are associations of DNA and histone proteins contained within a double membrane nuclear envelope, an area known as the cell nucleus.
Organelles are formed bodies within the cytoplasm that perform certain functions. Some organelles are surrounded by membranes, we call these membrane-bound organelles.
Ribosomes are the tiny structures where proteins synthesis occurs. They are not membrane-bound and occur in all cells, although there are differences between the size of subunits in eukaryotic and prokaryotic ribosomes.
The Cell Wall is a structure surrounding the plasma membrane. Prokaryote and eukaryote (if they have one) cell walls differ in their structure and chemical composition. Plant cells have cellulose in their cell walls, other organisims have different materials cpmprising their walls. Animals are distinct as a group in their lack of a cell wall.
Membrane-bound organelles occur only in eukaryotic cells. They will be discussed in detail later. Eukaryotic cells are generally larger than prokaryotic cells. Internal complexity is usually greater in eukaryotes, with their compartmentalized membrane-bound organelles, than in prokaryotes. Some prokaryotes, such as Anabaena azollae, and Prochloron, have internal membranes associated with photosynthetic pigments.
Terms applied to cells
Heterotroph (other-feeder): an organism that obtains its energy from another organism. Animals, fungi, bacteria, and mant protistans are heterotrophs.
Autotroph (self-feeder): an organism that makes its own food, it converts energy from an inorganic source in one of two ways. Photosynthesis is the conversion of sunlight energy into C-C covalent bonds of a carbohydrate, the process by which the vast majority of autotrophs obtain their energy. Chemosynthesis is the capture of energy released by certain inorganic chemical reactions. This is common in certain groups of likely that chemosynthesis predates photosynthesis. At mid-ocean ridges, scientists have discovered black smokers, vents that release chemicals into the water. These chemical reactions could have powered early ecosystems prior to the development of an ozone layer that would have permitted life to occupy the shallower parts of the ocean. Evidence of the antiquity of photosynthesis includes: a) biochemical precursors to photosynthesis chemicals have been synthesized in experiments; and b) when placed in light, these chemicals undergo chemical reactions similar to some that occur in primitive photosynthetic bacteria.
Prokaryotes are among the most primitive forms of life on Earth. Remember that primitive does not necessarily equate to outdated and unworkable in an evolutionary sense, since primitive bacteria seem little changed, and thus may be viewed as well adapted, for over 3.5 Ga. Prokaryote (pro=before, karyo=nucleus): these organisms lack membrane-bound organelles, as seen in Figures 5 and 6. Some internal membrane organization is observable in a few prokaryotic autotrophs, such as the photosynthetic membranes associated with the photosynthetic chemicals in the photosynthetic bacterium Prochloron. (click here to view Prochloron and other cyanobacteria at the Tree of Life Page). A transmission electron micrograph of Prochloron is shown in Figure 5.
Figure 5 Prochloron, an extant prokaryote thought related to the ancestors of some eukarypote chloroplasts. Image fom http://tidepool.st.usm.edu/pix/prochloron.gif.
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