All living organisms, from the simplest bacteria to the most complex mammals, are composed of a fascinating and intricate array of non-living molecules. These molecules, known as biomolecules, are the fundamental chemical components that build cellular structures and drive the myriad of processes essential for life. Chemical analysis of living tissues reveals that they are primarily composed of carbon, hydrogen, oxygen, and nitrogen, along with other elements, which combine to form four major classes of organic compounds: carbohydrates, proteins, lipids, and nucleic acids. This chapter explores the structure, properties, and functions of these essential biomolecules.

Carbohydrates, often referred to as saccharides, are a major source of energy for living organisms and also serve as structural components. They are primarily composed of carbon, hydrogen, and oxygen, typically in a ratio of 1:2:1. The basic units of carbohydrates are simple sugars called monosaccharides, with glucose being the most common and vital for cellular respiration. Two monosaccharides can link together to form a disaccharide, such as sucrose (table sugar). When many monosaccharide units are joined, they form complex carbohydrates called polysaccharides. Important polysaccharides include starch, which is the primary energy storage molecule in plants; glycogen, the equivalent energy storage molecule in animals; and cellulose, a major structural component of plant cell walls that is indigestible by most animals.

Proteins are the most abundant and functionally diverse macromolecules in living systems. They are polymers composed of repeating monomer units called amino acids. There are twenty different types of amino acids, each with a unique side chain (R group) that determines its chemical properties. Amino acids are linked together by peptide bonds to form long chains called polypeptides. The specific sequence of amino acids in a polypeptide chain constitutes its primary structure. This chain then folds into localized, repeating patterns, such as alpha-helices and beta-pleated sheets, forming the secondary structure. The overall three-dimensional shape of a single polypeptide chain is its tertiary structure, which is crucial for its function. Some proteins consist of multiple polypeptide chains, and their arrangement constitutes the quaternary structure. The functions of proteins are vast and varied; they act as enzymes (biological catalysts), provide structural support (e.g., collagen and keratin), transport substances (e.g., hemoglobin), and play roles in the immune response (antibodies) and cell signaling.

Lipids are a diverse group of hydrophobic molecules that are largely nonpolar and insoluble in water. They serve critical roles in long-term energy storage, insulation, and the formation of cellular membranes. The simplest lipids are fatty acids, which are long hydrocarbon chains that can be either saturated (no double bonds) or unsaturated (one or more double bonds). Fatty acids are typically stored in the form of triglycerides (fats and oils). A crucial class of lipids is phospholipids, which are the primary components of all biological membranes. Their amphipathic nature, with a hydrophilic head and a hydrophobic tail, allows them to form the lipid bilayer that encloses cells. Another important group is steroids, characterized by a four-ring carbon structure. Cholesterol is a key steroid that modulates membrane fluidity and serves as a precursor for steroid hormones like testosterone and estrogen.

Nucleic Acids are the macromolecules responsible for the storage, transmission, and expression of genetic information. They are polymers of nucleotides. Each nucleotide consists of three components: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. There are two main types of nucleic acids: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). DNA is a double-stranded helix that contains the genetic blueprint for the organism. The sequence of its nitrogenous bases (Adenine, Guanine, Cytosine, and Thymine) encodes the instructions for building proteins. RNA is typically single-stranded and plays several vital roles in the expression of genetic information, including acting as a messenger (mRNA), transferring amino acids (tRNA), and forming part of the ribosome (rRNA).

Finally, a special class of proteins, enzymes, act as biological catalysts. They dramatically increase the rate of biochemical reactions within cells without being consumed in the process. Each enzyme has a specific three-dimensional shape with an active site that binds to a particular substrate. The efficiency of enzymes is highly sensitive to factors such as temperature, pH, and substrate concentration. The regulation of enzyme activity is a key mechanism by which cells control their metabolic pathways.