Some Types of Chemical Bonds close to 99% of the weight of a living cell is composed of just four elements: carbon (C), hydrogen (H), nitrogen (N), and oxygen (O). Almost 50% of the atoms are hydrogen atoms; about 25% are carbon, and 25% oxygen. Apart from water (about 70% of the weight of the cell) almost all components are carbon compounds. Carbon, a small atom with four electrons in its outer shell, can form four strong covalent bonds withotheratoms.But most importantly,carbon atoms can combine with each other to build chains and rings, and thus large complex molecules with specific biological properties.

A. Compounds of hydrogen (H), oxygen (O), and carbon (C)

Four simple combinations of these atoms occur frequently in biologically important molecules: hydroxyl (—OH; alcohols), methyl (—CH 3 ), carboxyl (—COOH), and carbonyl (C=O; aldehydes and ketones) groups. They impart to the molecules characteristic chemical properties, including possibilities to form compounds.

B. Acids and esters

Many biological substances contain a carbon– oxygen bond with weak acidic or basic (alkaline) properties. The degree of acidity is expressed by the pH value, which indicates the concentration of H + ions in a solution, ranging from 10 –1 mol/L (pH 1, strongly acidic) to 10 –14 mol/L (pH 14, strongly alkaline). Pure water contains 10 –7 moles H + per liter (pH 7.0). An ester is formed when an acid reacts with an alcohol. Esters are frequently found in lipids and phosphate compounds.

C. Carbon–nitrogen bonds (C—N)

C—N bonds occur in many biologically important molecules: in amino groups, amines, and amides, especially in proteins. Of paramount significance are the amino acids, which are the subunits of proteins. All proteins have a specific role in the functioning of an organism.

D. Phosphate compounds

Ionized phosphate compounds play an essential biological role. HPO 4 2– is a stable inorganic phosphate ion from ionized phosphoric acid. A phosphate ion and a free hydroxyl group can form a phosphate ester. Phosphate compounds playan important role in energy-rich molecules and numerous macromolecules because they can store energy.

E. Sulfur compounds

Sulfur often serves to bind biological molecules together, especially when two sulfhydryl groups(—SH)reacttoformadisulfidebridge(— S—S—). Sulfur is a component of two amino acids (cysteine and methionine) and of some polysaccharides and sugars. Disulfide bridges play an important role in many complex molecules, serving to stabilize and maintain particular three-dimensional structures.

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