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Nobody during this century can communicate with larger authority at the growth of rules in biology than Ernst Mayr. And no booklet has ever validated the lifestyles sciences so firmly within the mainstream of Western highbrow background as "The development of organic suggestion. " Ten years in coaching, it is a paintings of epic proportions, tracing the improvement of the most important difficulties of biology from the earliest makes an attempt to discover order within the variety of existence, to trendy examine into the mechanisms of gene transmission.
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Quaternary Structure. When two or more polypeptide chains associate to form a functional protein, the individual chains are referred to as subunits of the protein. The subunits need not be the same. Hemoglobin, for example, is a protein composed of two α-chain subunits and two βchain subunits. A protein’s subunit arrangement is called its quaternary structure. In proteins composed of subunits, the interfaces where the subunits contact one another are often nonpolar, and play a key role in transmitting information between the subunits about individual subunit activities.
Because the R groups that distinguish the various amino acids play no role in the peptide backbone of proteins, a protein can consist of any sequence of amino acids. 7 Levels of protein structure. The amino acid sequence of a protein is called its (a) primary structure. Hydrogen bonds form between nearby amino acids, producing (b) fold-backs called beta-pleated sheets and coils called alpha helices. These fold-backs and coils constitute the protein’s secondary structure. A globular protein folds up on itself further to assume a threedimensional (c) tertiary structure.
Special-function amino acids have unique individual properties; methionine often is the first amino acid in a chain of amino acids, proline causes kinks in chains, and cysteine links chains together. Each amino acid affects the shape of a protein differently depending on the chemical nature of its side group. Portions of a protein chain with numerous nonpolar amino acids, for example, tend to fold into the interior of the protein by hydrophobic exclusion. Proteins Are Polymers of Amino Acids In addition to its R group, each amino acid, when ionized, has a positive amino (NH3+) group at one end and a negative carboxyl (COO–) group at the other end.