Summary: Episode 20: Welcome to the MCAT Podcast series, where Doctor Dan will cover the actual science material required for the MCAT. Starting off with an overview of Biological Sciences topics, we'll get increasingly more specific as time passes. Biology for the MCAT Classes of Organic Molecules Four major classes of organic molecules found in living organisms are carbohydrates, fats, proteins, and nucleic acids. Condensation Reactions Though these classes of molecule have different structure and function, they are built up of many similar building block molecules bonded together. In each case, building block molecules are combined by the removal of water, and this is called "condensation reactions." Condensation reactions are reversible. The complex organic molecules can be hydrolyzed into the simpler building blocks molecules with the addition of water. The basic building block molecules of carbohydrates are the simple sugars or monosaccharides. Disaccharide Bonds When two simple sugars are bonded together, a disaccharide is formed. When many simple sugars are bonded together in long chains, a polysaccharide is formed. Starch, glycogen, and cellulose are examples of polysaccharides. The carbohydrates are an important energy source for all organisms. Lipids, the fats, and fat-like substances tend to be insoluble in water. Fats are made up of two building block molecules – glycerol and fatty acids. Phospholipids are derived from the fats. They are important constituents of cell membranes. Peptide Bonds The basic building block molecules of the proteins are amino acids. Amino acids are bonded together to form a protein by condensation reactions. The resulting bond is the peptide bond and the chains produced are polypeptide chains. The primary structure of each protein is the sequence and type of amino acids making up the polypeptide chains. Because hydrogen bonds form between one amino acid and another, the chain assumes a stable regular shape known as the secondary structure. These regular molecules may in turn be folded into complicated globular shapes by weak attractions between the different R groups within the chain, thus forming the tertiary structure of the protein. Nucleotide Bonds Some globular proteins are made up of two or more polypeptide chains held together by weak bonds. The way these chains fit together determines the ordinary structure. Because the conformation of a protein depends on weak bonds, it is easily altered causing a change in biological function. The building block unit of nucleic acids is the nucleotide, which is made up of a five carbon sugar attached to a phosphate group and to a nitrogen-containing base. Nucleotide units are joined together through condensation reactions between the sugar of one nucleotide and the phosphate group of the next. There are four different nucleotides in each nucleic acid. It is the different sequences of the nucleotides that encode their hereditary information. The two types of nucleic acids, DNA and RNA, differ in their basic make up and in the number of strands in the molecule. We will be discussing this in greater detail later. Free Energy and Enzymes Chemical reactions that release free energy are exothermic or exergonic. Reactions that require the addition of free energy are endothermic or endergonic. In living systems, an exothermic reaction is usually coupled with an endothermic reaction. Although exothermic reaction proceeds spontaneously, initiating a reaction may require an activation energy. Chemical reactions can be speeded up by heat, by increasing the concentrations of the reactants, or by providing the appropriate catalyst. In living systems, the catalysts are enzymes. Most enzymes are highly specific and each can interact only with those reactants or substrates that fit spatially and chemically into the active site of the enzyme. Since the formation of the enzyme
