Biology  5/e   Raven/Johnson
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Proteins: Proteins are polymers containing up to 20 different kinds of amino acids. Amino acids are classified into five different chemical groups with diverse properties. Because of the diversity, usually large number, and differing arrangements of amino acids, proteins occur in great variety.

Formation of lipid bilayers: A phospholipid is a triglyceride with one of the fatty acid chains replaced by a phosphate group, forming the "head" of the molecule. The unmodified portion of the molecule forms the "tail." In water, the nonpolar tails of nearby phospholipids aggregate, forming two layers of tails pointed toward each other--a lipid bilayer.

Distinction between prokaryotic and eukaryotic cells: Eukaryotic DNA is packaged in membrane bound structures called nuclei, which are lacking in prokaryotic cells. The interior of eukaryotic cells is compartmentalized by membranes whereas, compartmentalization is lacking in prokaryotic cells. Both prokaryotic and eukaryotic cells may have cell walls. However, prokaryotic cell walls are chemical complexes, while those of eukaryotic cells are composed of cellulose. Eukaryotes have the ability to reproduce sexually: prokaryotes cannot. Some eukaryotes are multicellular with individual cells specializing and interacting. Prokaryotes may aggregate but continue to function independently.

Evolution of eukaryotes: Eukaryotes are believed to have evolved from prokaryotes. At first they were simply larger, with thicker cell walls, and had membrane-bound structures such as nuclei. Other structures such as mitochondria and chloroplasts are believed to have evolved through endosymbiosis with smaller organisms capable of the functions of these structures.

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