STRUCTURE AND ACTIVITIES OF CELLS 



Proteins are the most abundant group of organic compounds in the cell. All 

 simple proteins contain carbon, hydrogen, oxygen, and nitrogen; in some, 

 sulfur and iodine occur. Proteins may be conjugated with carbohydrates to 

 form glycoproteins, with lipids to form lipoproteins, with nucleic acids 

 (see below) to form nucleoproteins, with metal ions (Fe "*", Fe , Cu^ , 

 Zn"*"*^, Mn^^) to form metalloproteins. Proteins are very large molecules 

 with representative molecular weights as follows: insulin, a hormone, 12,000; 

 pepsin, a digestive enzyme, 34,500; egg albumin, 40,000; hemoglobin, a 

 respiratory pigment, 68,000; and myosin, a contractile fiber, 850,000. There 

 are at least 25 different amino acids (Fig. 2.8/), which are combined in 

 varying proportions and sequences to form the uncounted kinds of protein 

 molecules. In the formation of a simple protein, the different kinds of con- 

 stituent amino acids (R-CH-NHg-COOH) are linked together by bonding 

 between the nitrogen atom of the amine group (NH^) and the carbon atom 

 in the carboxyl group (COOH); this is known as the peptide linkage (Fig. 

 2.8G). Two amino acids so linked are called a dipeptide (Fig. 2.8A^; 

 longer chains are tri- and polypeptides. These chains are loosely coiled, 

 and adjacent strands are held together by hydrogen bonds to make a very 

 complex chemical structure. Chemists have, however, succeeded in determin- 

 ing the exact kinds and sequences of the amino acids present in a few proteins; 

 the pancreatic hormone insulin is an example. 



It should be noted that amino acids contain both a basic (NH^) and an 

 acidic (COOH) group. In aqueous solution, these groups ionize, and the 

 amino acid becomes an electrically polarized molecule, carrying both a posi- 

 tive and a negative charge (Fig. 2.8/). Although the peptide linkages in a 

 polypeptide chain involve most of the ionizable groups, the terminal NH, 

 and COOH groups ionize when a protein is in solution, as many are within 

 the cell. Depending on the acidity of the solvent, proteins may carry both 

 a positive and a negative charge and behave as though electrically neutral. 

 On the other hand, they may be partially dissociated and behave either as 

 positive ions or as negative ones. 



The proteins of the cell, together with certain lipids, play a major role in 

 maintaining the morphological features of cells, such as membranes. Different 

 kinds of animals are characterized by specific proteins which endow them with 

 their unique qualities. Proteins are extremely sensitive to changes in their 

 surroundings such as heat and degree of acidity. They may undergo modi- 

 fication, involving the breaking of the hydrogen bonds of the molecule, which 

 results in loss of capacity to take part in cellular reactions. 



Nucleic acids are a group of complex organic compounds present in small 

 amounts in the cell. The structural unit of the nucleic acids is the nucleotide, 

 of which only nine different ones have been isolated from the nucleic acids. 

 These nucleotides are composed of a nitrogenous base, a pentose sugar, and 

 phosphoric acid (Fig. 2.8L); they are linked (Fig. 2.8./V) in varying sequence 

 to form the very large molecules of nucleic acid. Depending on the kind 

 of pentose sugar present in the molecule, there are two classes of nucleic 



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