Chemical Building Blocks of Cells
■ Three major biopolymers are present in cells: proteins, composed of amino acids linked by peptide bonds; nucleic acids, composed of nucleotides linked by phosphodiester bonds; and polysaccharides, composed of monosaccharides (sugars) linked by glycosidic bonds (see Figure 2-11).
■ Many molecules in cells contain at least one asymmetric carbon atom, which is bonded to four dissimilar atoms. Such molecules can exist as optical isomers (mirror images), designated D and L, which have different biological activities. In biological systems, nearly all sugars are D isomers, while nearly all amino acids are L isomers.
■ Differences in the size, shape, charge, hydrophobicity, and reactivity of the side chains of amino acids determine the chemical and structural properties of proteins (see Figure 2-13).
■ Amino acids with hydrophobic side chains tend to cluster in the interior of proteins away from the surrounding aqueous environment; those with hydrophilic side chains usually are toward the surface.
■ The bases in the nucleotides composing DNA and RNA are heterocyclic rings attached to a pentose sugar. They form two groups: the purines—adenine (A) and guanine (G)—and the pyrimidines—cytosine (C), thymine (T), and uracil (U) (see Figure 2-15). A, G, T, and C are in DNA, and A, G, U, and C are in RNA.
■ Glucose and other hexoses can exist in three forms: an open-chain linear structure, a six-member (pyranose) ring, and
a five-member (furanose) ring (see Figure 2-16). In biological systems, the pyranose form of D-glucose predominates.
■ Glycosidic bonds are formed between either the a or p anomer of one sugar and a hydroxyl group on another sugar, leading to formation of disaccharides and other poly-saccharides (see Figure 2-17).
■ The long hydrocarbon chain of a fatty acid may contain no carbon-carbon double bond (saturated) or one or more double bonds (unsaturated), which bends the chain.
■ Phospholipids are amphipathic molecules with a hydro-phobic tail (often two fatty acyl chains) and a hydrophilic head (see Figure 2-19).
■ In aqueous solution, the hydrophobic effect and van der Waals interactions organize and stabilize phospholipids into one of three structures: a micelle, liposome, or sheetlike bilayer (see Figure 2-20).
■ In a phospholipid bilayer, which constitutes the basic structure of all biomembranes, fatty acyl chains in each leaflet are oriented toward one another, forming a hy-drophobic core, and the polar head groups line both surfaces and directly interact with the aqueous solution.
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