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plasma membranedefines the periphery ofthe cell, separating itscontents from thesurroundings. It iscomposed of lipid andprotein molecules thatform a thin, tough, pliable,hydrophobic barrier aroundthe cell. The membrane isa barrier to the freepassage ofinorganic ions and mostother charged or polarcompounds.

The nucleoid, in bacteria,

is not separated from thecytoplasm by a membrane;the nucleus, in higherorganisms, consists ofnuclear material enclosedwithin a double membrane,the nuclear envelope. Cellswith nuclear envelopes arecalled eukaryotes (Greekeu, true, and karyon,nucleus); those withoutnuclear envelopesbacterial cellsareprokaryotes(Greekpro, before).

The internal volumebounded by the plasmamembrane, the cytoplasm(Fig. 13), is composed ofan aqueous solution, thecytosol, and a variety ofsuspended particles with

specific functions.

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Sel EukariotikThe internal volume bounded by theplasma membrane, the cytoplasm(Fig. 13), is composed of anaqueous solution, the cytosol, and avariety of suspended particles withspecific functions. The cytosol is ahighly concentrated solutioncontaining enzymes and the RNAmolecules that encode them; thecomponents (amino acids andnucleotides) from which thesemacromolecules are assembled;hundreds of small organic moleculescalled metabolites, intermediates inbiosynthetic and degradative

pathways; coenzymes, compoundsessential to many enzyme-catalyzedreactions; inorganic ions; andribosomes, small particles(composed of protein and RNAmolecules) that are the sites ofprotein synthesis.All cells have a nucleus or nucleoid, aplasma membrane, and cytoplasm.

The cytosol is defined as that portionof the cytoplasm that remains in thesupernatant after centrifugation of a

cell extract at 150,000 g for 1 hour.the complete set of genes, composedof DNAis stored and replicated.Sel Prokariotik

The cytoplasm of E. coli containsabout 15,000 ribosomes, thousandsof copies each of about 1,000different enzymes, numerousmetabolites and cofactors, and avariety of inorganic ions. Thenucleoid contains a single, circular

molecule of DNA, and the cytoplasm(like that of most bacteria) containsone or more smaller, circularsegments of DNA called plasmids.

In nature, some plasmids confer resistance to toxins and antibiotics in theenvironment. In the laboratory,these DNA segments are especiallyamenable to experimentalmanipulation and are extremelyuseful to molecular geneticists.

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