Biomolecules

This chapter introduces the chemistry of the organic molecules that constitute living systems. It explores the structure, classification, and function of the major classes of biomolecules. The primary focus is on understanding how the chemical structures of these compounds relate to their biological roles. By the end of this chapter, you should be able to classify and draw the structures of different types of carbohydrates, explain the formation of proteins from amino acids, and describe the components of nucleic acids.

The main sub-topics are carbohydrates, proteins, nucleic acids, lipids, and vitamins. For carbohydrates, you will learn about monosaccharides like glucose and fructose, disaccharides like sucrose, and polysaccharides like starch and cellulose. For proteins, the chapter covers amino acids, peptide bonds, and the four levels of protein structure. The section on nucleic acids explains the composition of DNA and RNA and their double helix structure. Enzymes and their mechanism of action are also introduced.

The most challenging part of this chapter is often the detailed structures of carbohydrates, including their cyclic forms (Haworth projections) and the linkages between them. The topic requires a good amount of memorization for structures, names, and specific facts like vitamin deficiencies. A solid understanding of basic organic chemistry, particularly stereochemistry (chirality, D/L notation) and functional groups, is essential for success.

Formulas for Biomolecules

General formula of carbohydrates: $C_x(H_2O)_y$ Represents hydrates of carbon. Some compounds fitting this formula are not carbohydrates, and some carbohydrates do not fit this formula.
Zwitterion formation of an amino acid: $R-CH(NH_2)-COOH \rightleftharpoons R-CH(NH_3^+)-COO^-$ In aqueous solution, an amino acid exists as a dipolar ion (zwitterion).
Peptide bond formation: $H_2N-CHR_1-COOH + H_2N-CHR_2-COOH \rightarrow H_2N-CHR_1-CO-NH-CHR_2-COOH + H_2O$ The amide bond (-CO-NH-) formed between two amino acids with the elimination of a water molecule.
Hydrolysis of sucrose: $C_{12}H_{22}O_{11} + H_2O \xrightarrow{H^+} C_6H_{12}O_6 (\text{Glucose}) + C_6H_{12}O_6 (\text{Fructose})$ Acid-catalysed hydrolysis of sucrose yields an equimolar mixture of D-glucose and D-fructose.
Glycosidic linkage in maltose: $\alpha\text{-D-Glucose} + \alpha\text{-D-Glucose} \rightarrow \text{Maltose} + H_2O$ The link is an α-1,4-glycosidic bond between two glucose units.
Nucleotide structure: $\text{Phosphate} - \text{Pentose Sugar} - \text{Nitrogenous Base}$ The three components of a nucleotide, which is the monomer unit of nucleic acids.
DNA base pairing (Adenine-Thymine): $A = T$ Adenine pairs with Thymine via two hydrogen bonds.
DNA base pairing (Guanine-Cytosine): $G \equiv C$ Guanine pairs with Cytosine via three hydrogen bonds.