Explore the six major families of carbon-based molecules that run human life: carbohydrates, lipids, proteins, nucleic acids, vitamins, and hormones. Each section connects structure to physiological function and metabolism.
Energy and structure
Carbohydrates, also called saccharides, are the most abundant biomolecules on Earth. In the human body they provide rapid energy, store glucose, and contribute to structural and recognition systems.
Simple sugars contain a carbonyl group and several hydroxyl groups. The main human hexoses share the formula C6H12O6 but differ in arrangement.
Glycolysis converts glucose into pyruvate in the cytosol. Under aerobic conditions, pyruvate feeds acetyl-CoA production and the citric acid cycle.
Glycogen. Its branched architecture allows rapid glucose release in liver and muscle.
Membranes and energy reserves
Lipids are hydrophobic or amphiphilic organic molecules. They store long-term energy, form membranes, insulate tissues, and act as precursors for signaling molecules.
Fatty acids are carboxylic acids with hydrocarbon chains, usually 4 to 24 carbons long.
Phospholipids are amphiphilic: a hydrophilic phosphate head and hydrophobic fatty-acid tails. In water they self-assemble into bilayers, the structural basis of cell membranes.
Cholesterol is a steroid membrane component and a precursor for steroid hormones, bile acids, and vitamin D. Fatty acids are degraded in mitochondria by beta-oxidation.
They contain both a water-loving phosphate head and water-repelling hydrocarbon tails.
Structure becomes function
Proteins are polymers of amino acids. Their biological role depends on their sequence, folding, interactions, and cellular context.
Each amino acid has an alpha carbon bonded to an amino group, a carboxyl group, hydrogen, and a variable R group that determines chemical behavior.
A peptide bond is a covalent amide bond formed by condensation between the carboxyl group of one amino acid and the amino group of the next. The reaction releases water.
amino acid + amino acid -> peptide + H2O| Level | Meaning |
|---|---|
| Primary | Linear amino-acid sequence encoded by DNA. |
| Secondary | Local alpha helices and beta sheets stabilized by hydrogen bonds. |
| Tertiary | Full 3D fold of one polypeptide chain. |
| Quaternary | Assembly of multiple subunits, as in hemoglobin. |
The peptide bond, a planar covalent amide linkage.
Information chemistry
DNA and RNA store, transmit, and express genetic information. Their polymers are built from nucleotides.
Each nucleotide contains a phosphate group, a pentose sugar, and a nitrogenous base.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Structure | Usually double-stranded | Usually single-stranded |
| Main role | Genetic storage | Genetic expression |
Base pairing follows complementarity: A pairs with T in DNA, A pairs with U in RNA, and G pairs with C.
Uracil. RNA uses A, U, G, and C.
Micronutrient chemistry
Vitamins are essential organic micronutrients required in small amounts. They do not provide energy directly, but many act as coenzyme precursors or regulatory molecules.
Vitamins A, D, E, and K dissolve in lipids, depend on dietary fat for absorption, and can accumulate in the liver or adipose tissue.
Vitamin C and the B-complex vitamins dissolve in water. Most are not stored extensively and require regular intake.
Vitamin D, produced when UVB radiation converts a cholesterol-derived precursor.
Chemical messengers
Hormones are chemical messengers released by endocrine tissues into the bloodstream. They regulate growth, metabolism, reproduction, stress response, and homeostasis.
Peptide hormones usually trigger membrane receptors, G proteins, tyrosine kinases, or second messengers such as cAMP and calcium. Steroid hormones cross membranes and regulate gene expression through intracellular receptors.
Blood glucose regulation is a classic negative-feedback loop.
The same carbon-based principles explain membrane permeability, receptor location, signal speed, half-life, and the difference between rapid metabolic responses and long-term gene regulation.
They typically regulate transcription inside the cell, so the response depends on gene expression and protein synthesis.
System view
Human organic chemistry is not six isolated chapters. Biomolecules share carbon skeletons, functional groups, energy carriers, and regulatory feedback loops.
Carbohydrates and lipids feed ATP production through glycolysis, beta-oxidation, acetyl-CoA, the citric acid cycle, and oxidative phosphorylation.
Nucleic acids encode proteins; proteins catalyze DNA replication, transcription, translation, and most metabolic reactions.
Vitamins support enzyme chemistry, while hormones coordinate cell behavior across tissues and maintain homeostasis.