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▲ FIGURE 2-11 Covalent and noncovalent linkage of monomers to form biopolymers and membranes. Overview of the cell's chemical building blocks and the macrostructures formed from them. (Top) The three major types of biological macromolecules are each assembled by the polymerization of multiple small molecules (monomers) of a particular type: proteins from amino acids (Chapter 3), nucleic acids from nucleotides (Chapter 4), and polysaccharides from monosaccharides (sugars). The monomers are covalently linked into polymers by coupled reactions whose net result is condensation through the dehydration reaction shown. (Bottom) In contrast, phospholipid monomers noncovalently assemble into bilayer structure, which forms the basis of all cellular membranes (Chapter 5).

are linear polymers containing hundreds to millions of nu-cleotides linked by phosphodiester bonds. Polysaccharides are linear or branched polymers of monosaccharides (sugars) such as glucose linked by glycosidic bonds.

A similar approach is used to form various large structures in which the repeating components associate by non-covalent interactions. For instance, the fibers of the cytoskeleton are composed of many repeating protein molecules. And, as we discuss below, phospholipids assemble noncovalently to form a two-layered (bilayer) structure that is the basis of all cellular membranes (see Figure 2-11). Thus a repeating theme in biology is the construction of large molecules and structures by the covalent or noncovalent association of many similar or identical smaller molecules.

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