[CENTER]Some Types of Chemical Bonds
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Chemical bonds form between molecules and allow building of complex structures . Each atom can establish chemical bonds with
another in a defined way . Strong forces of attraction are present in covalent bonds , when two atoms share one pair of electrons . Weak forces of attraction occur in noncovalent bonds.
They play a major role in many biomolecules such as carbohydrates,lipids,nucleic acids,and proteins . Four major types of noncovalent interactions are distinguished : hydrogen bonds,
ionic interactions , vander Waals interactions , and hydrophobic effects.
Close to 99%of the weight of a living cell is composed of just four elements : carbon ©,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 , is the central chemical building block of the living world . It can form four strong covalent bonds with other atoms . 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. Functional groups with hydrogen (H),oxygen (O),and carbon ©
Four simple combinations of these atoms occur frequently in biologically important molecules : hydroxyl (—OH ; alcohols) , methyl (—CH3) , carboxyl (—COOH) , and carbonyl (C =O ; aldehydes and ketones) groups . They impart characteristic chemical properties to the molecules , 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 (see.p.38),the building blocks of proteins . Proteins have specific roles in the functioning of an organism.
D. Phosphate compounds
Ionized phosphate compounds play an essential biological role.HPO4 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 play an important role in energy-rich molecules and numerous macromolecules because they can store energy.
E. Sulfur groups
Sulfur often joins biological molecules together , especially when two sulfhydryl groups (—SH)react to form a disulfide bridge (—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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