Biochemistry – Lecture Section I

Biochemistry: the study of the chemical reactions of life.

Life involves the most complex chemistry known to man. For example, the chemical composition of the human body is known down to the microgram, but we are unable to replicate even the simplest living thing in the lab. Knowing the chemicals involved and understanding how the reaction mechanisms fit together are two different things.

Life on Earth takes very diverse forms. To the chemist, life can be broken down into systems including these generalized hierarchical levels:

molecules
organelles
cells
tissues
organs
organisms

The Molecules of Life:

• Amino acids
• All amino acids have the general formula shown here.
• There are 20 standard amino acids.
• Proteins
• Proteins are very complicated molecules made up of amino acids. The 20 amino acids can be arranged in any order to make a polypeptide made up of thousands of amino acids. Their potential for variety is extraordinary. This variety allows proteins to function as extremely specific enzymes that function in a cell's metabolism. Proteins make up about one-half of your body's non-water mass.
• Not all proteins are enzymes, but most enzymes are proteins.

Enzymes act as catalysts in the chemical reactions in cells. A catalyst is a molecule which increases the rate of a reaction but is not the substrate or product of that reaction. A substrate is a molecule upon which an enzyme acts to yield a product. Enzymes are usually named for the substrate on which they act or the action which they perform.

• Carbohydrates
• Simply put, carbohydrates are "sugars".
• They are important metabolically. Sugars are the major energy storage molecules for living organisms.
• Simple sugars are known as monosaccharides.

Glucose

Fructose

• Sugars composed of two simple sugars are known as disaccharides.

Sucrose (composed of 1 glucose molecule and 1 fructose molecule)

• Complex carbohydrates are made by combining more and more simple sugars.

Cellulose (composed of a chain of glucose molecules)

• Lipids
• Lipids are characterized by their solubility in organic solvents rather than in water. The structure of lipids shows no common features throughout the class, although the structures tend to have more carbon-hydrogen bonds than any other class of biological molecules. Unlike proteins and nucleic acids, lipids do not necessarily form polymers. In general, lipids can be found in cell membranes and in fats.
• Cell membranes are composed of a double layer of lipid molecules. In the diagram below, the "head" of a lipid molecule is a phosphate group and the "tail" is composed of two carbon chains. The middle part of the membrane composed of the "tails" is highly hydrophobic (lacking affinity for water) as opposed to the negatively charged "heads", which are hydrophilic (having a strong affinity for water).
• Most membranes in cells are semipermeable, allowing some things to easily pass through and blocking other things. Sometimes the cell can control this process and other times it cannot. The energy involved in this process has both chemical and physical aspects and is far from completely understood. One thing is certain; membranes are a major part of cellular chemistry.
• Fats are esters formed from glycerol and fatty acids. Fatty acids are carboxylic acids with 12 to 20 carbon atoms in the chain. Some fatty acids are saturated (having only single bonds), while other are unsaturated - having as many as four double bonds in the molecule. In general, animal fats are more saturated and plant oils are more unsaturated.
• Steroids are another class of lipids. All steroids contain a tetracyclic ring system. One steroid produced by your body is cholesterol.
• Some important vitamins are lipids. Vitamins are substances used by living cells to aid enzyme reactions. Your body uses vitamins in ways ranging from the synthesis of pigments needed for good vision to the red pigment in hemoglobin that carries oxygen in the blood.
• Nucleic acids
• Measured by mass, nucleic acids are the smallest group of organic chemicals in your body. However, these large polymers are the largest single molecules in the body. Together, DNA and RNA store and transfer genetic information. The diagram at the top of this page is a very small segment of the structural formula for one DNA molecule.