Atomic and Molecular structure
                                    An atom is made up of protons,  neutrons and electrons. Protons are positively charged and they denote the  atomic number. Neutrons have no electrical charge. Protons and neutrons are present  in the nucleus. Electrons are negatively charged and present in the orbits  surrounding the nucleus. Isotopes have same number of protons but different  number of neutrons. Loss or gain of electrons is seen in ions. Sharing  electrons form covalent bond in molecules.   Hydrogen bond is a type of intermolecular attraction among polar  molecules such as water.  
                                  Types of organic molecules
                                    Organic molecules are those  derived from living organisms and they all contain carbon backbones.  There are 7 major classes of organic molecules  categorized by their functional groups: alcohols, aldehydes, ketones, carboxylic  acids, amines, organic phosphates and thiols.   Four major groups of macromolecules in cells are polysaccharides,  proteins, nucleic acids and lipids.  Each  of these macromolecules is built up on smaller molecules.
                                  Acids, basis and buffers
                                    Acids  are electrolytes that release hydrogen ions in water.  Bases are electrolytes that release hydroxyl  ions in water that can combine with hydrogen ions to form water.  pH represents the concentration of hydrogen  ions [H+] in solution (pH = -log [H+]).  Buffer solutions are solutions which  resist change in pH upon addition of small amounts of acid or base. Buffers  provide a relatively stable environment for biochemical reactions to take  place.
                                  Biochemical reactions
                                  Like all other chemical reactions, biochemical  reactions also favor DG negative reactions.  Some reactions are DG positive and they  can be coupled to other DG negative reactions in order to make the total DG of the coupled reaction to be negative.  Nearly all biochemical reactions are  catalyzed by enzymes, which decrease the activation energy but do not change  the DG  value of the reaction.  Six major types  of biochemical reactions are: oxidation-reduction, group transferring,  hydrolysis, elimination or addition, isomerization and ligation of various  groups.  These reactions are catalyzed by  corresponding enzymes: oxidoreductase, transferase, hydrolase, lyase, isomerase  and ligase.