Organic Compounds Essential to Human Functioning

Core Concepts

Carbohydrates: quick fuel and short-term storage

Carbohydrates are built from carbon, hydrogen, and oxygen in roughly a 1:2:1 ratio (CH₂O). Their subunits are monosaccharides (single sugars) such as glucose, fructose, and galactose. Two monosaccharides joined by dehydration synthesis form a disaccharide — sucrose (table sugar), lactose (milk sugar), and maltose. Long chains form polysaccharides:

• Glycogen — the animal storage form, stored mainly in the liver and skeletal muscle; it is the body's rapidly mobilizable glucose reserve.
• Starch — the plant storage form (a dietary carbohydrate, not made by the human body).
• Cellulose — a plant structural polysaccharide humans cannot digest; it contributes dietary fiber.

Functions: primary fuel for cells (especially the brain, which prefers glucose), quick energy storage, and structural roles. Carbohydrates also serve as recognition markers on cell surfaces (glycoproteins and glycolipids).

Lipids: energy-dense, waterproof, and structural

Lipids are a diverse group unified by one property: they do not dissolve in water. Key classes:

• Triglycerides (fats and oils) — one glycerol backbone plus three fatty acid chains. If the fatty acids have no double bonds, the fat is saturated (straight chains, solid at room temperature, e.g., butter); double bonds create unsaturated fats (kinked chains, liquid at room temperature, e.g., olive oil). Triglycerides are the body's most concentrated energy store — commonly taught as about 9 kcal per gram versus about 4 for carbohydrates and proteins (verify caloric values against current references) — and they provide insulation and padding.
• Phospholipids — a glycerol backbone, two fatty acid tails, and a phosphate-containing head. The head is hydrophilic ("water-loving") and the tails are hydrophobic ("water-fearing"), making phospholipids amphipathic — the essential property for building cell membranes (bilayers).
• Steroids — four fused carbon rings. Cholesterol is the best known: it stabilizes cell membranes and is the precursor for steroid hormones (such as cortisol and sex hormones), bile salts, and vitamin D.
• Eicosanoids — signaling lipids derived from fatty acids that act locally (e.g., in inflammation; taught in more detail in later chapters).

Proteins: the body's workhorses

Proteins are polymers of amino acids — molecules with an amino group, a carboxyl group, and a unique R (side) group. Twenty common amino acids exist; they are joined by peptide bonds formed through dehydration synthesis. A chain of amino acids folds into a functional shape described at four levels:

• Primary structure — the sequence of amino acids.
• Secondary structure — local patterns (α-helix, β-pleated sheet) held by hydrogen bonds.
• Tertiary structure — the overall 3-D fold of one chain, stabilized by interactions between side groups.
• Quaternary structure — assembly of multiple chains (e.g., hemoglobin's four subunits).

Shape equals function: if a protein loses its shape (denaturation, from heat, pH change, or chemicals), it loses its job — think of cooking an egg white (albumin denatures from clear to white). Protein functions include catalysis (enzymes speed reactions without being consumed), structure (collagen, keratin), transport (hemoglobin carries oxygen), defense (antibodies), movement (actin and myosin in muscle), and regulation (some hormones).

Nucleic acids and ATP: information and energy

Nucleic acids are polymers of nucleotides, each made of a phosphate group, a five-carbon sugar, and a nitrogenous base. DNA (deoxyribonucleic acid) uses deoxyribose and the bases adenine, thymine, guanine, and cytosine; it stores genetic information in a double helix. RNA (ribonucleic acid) uses ribose and uracil in place of thymine; it carries and executes the instructions (see Protein Synthesis in Chapter 3). The sequence of bases is the "letters" of the genetic code.

ATP (adenosine triphosphate) is a modified nucleotide — adenine, ribose, and three phosphate groups. Breaking off a phosphate releases energy that drives cellular work (muscle contraction, active transport, synthesis). ATP is often called the cell's energy currency: it is constantly produced (mainly by cellular respiration) and spent.

The common pattern: dehydration synthesis and hydrolysis

All four families follow one rule. Dehydration synthesis links subunits by removing a water molecule (monomers → polymer). Hydrolysis breaks polymers by adding water back (polymer → monomers), which is how digestion splits food molecules so they can be absorbed. Learn this pattern once and it explains synthesis, digestion, and the energy logic of metabolism.

Common Confusions