13.2 Chemical Building Blocks: Proteins, Carbohydrates, Carbon, Nucleic Acids & Lipids

The Chemical Building Blocks of Life

All cells are built from a small set of organic macromolecules assembled from simpler monomers. The PA-CAT Bulletin of Information, rev. 20240815 groups these under Chemical Building Blocks and Biological Molecules (Table 6). A representative sample item: an enzyme cleaves a disaccharide into two monosaccharides; if the disaccharide is sucrose, the products are glucose and fructose — two different monomers. Knowing which saccharide is a mono-, di-, or polysaccharide is exactly the distinction tested.

Carbon Chemistry and Functional Groups

Carbon's four valence electrons let it form up to four covalent bonds, producing long chains, branches, and rings — the skeleton of every biological macromolecule. Carbon-carbon single, double, and triple bonds, plus stable bonds to H, O, N, S, and P, give organic chemistry its diversity. Attached functional groups confer chemical personality:

• Hydroxyl (-OH): polar, makes molecules water-soluble (alcohols, sugars).
• Carboxyl (-COOH): acidic; donates H+ to become -COO- (amino acids, fatty acids).
• Amino (-NH2): basic; accepts H+ to become -NH3+ (amino acids, nucleotide bases).
• Phosphate (-PO4): energy transfer and nucleotide backbones.
• Sulfhydryl (-SH): forms disulfide bridges that stabilize protein tertiary structure.

A macromolecule's behavior — solubility, charge, reactivity — is the sum of its functional groups. The same carbon skeleton with a hydroxyl versus a carboxyl group behaves very differently, which is why recognizing these groups pays off on structure-and-function items.

Monomers, Polymers, and Dehydration vs. Hydrolysis

Macromolecules are polymers built from repeating monomers by dehydration synthesis (a water molecule is removed to form a new covalent bond). The reverse — hydrolysis — adds water to break a bond. The disaccharide sample item is hydrolysis: water is added across the glycosidic bond, splitting sucrose into glucose plus fructose. Digestion is largely hydrolysis; biosynthesis is largely dehydration. Enzymes catalyze both directions, lowering activation energy without changing equilibrium.

The Four Macromolecule Classes

Carbohydrates: Sugars and Polymers

Carbohydrates have the empirical formula (CH2O)n. Monosaccharides are single sugars: glucose, fructose, galactose (hexoses); ribose and deoxyribose (pentoses). Disaccharides are two monosaccharides joined by a glycosidic bond: sucrose = glucose + fructose (table sugar); maltose = glucose + glucose (malt sugar); lactose = glucose + galactose (milk sugar). Polysaccharides are long chains: starch (plant energy storage, glucose polymer), glycogen (animal energy storage in liver and muscle), cellulose (plant structural, glucose with beta linkages humans cannot digest), and chitin (arthropod exoskeleton and fungal walls, contains nitrogen). The glycosidic linkage geometry (alpha vs. beta) determines whether humans can digest the polymer: starch has alpha linkages digestible by amylase; cellulose has beta linkages that human enzymes cannot cleave.

Returning to the sample item: sucrose is the disaccharide; fructose is a monosaccharide (one of the two monomers); starch is a polysaccharide (many glucose units). An enzyme that cleaves a disaccharide performs hydrolysis of a single glycosidic bond.

Proteins: Amino Acids to Folded Machines

Proteins are polymers of 20 amino acids, each with a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable R group (side chain). R-group chemistry — nonpolar, polar, acidic, or basic — drives folding. Peptide bonds link amino acids into primary structure (sequence); hydrogen bonds form secondary alpha helices and beta sheets; R-group interactions (hydrophobic collapse, salt bridges, hydrogen bonds, disulfide bridges) produce tertiary 3-D shape; multiple polypeptides assemble into quaternary structure (e.g., hemoglobin's four subunits). Function depends entirely on folded shape — denaturation by heat, pH, or salt destroys activity. A single amino acid substitution in hemoglobin (Glu to Val) causes sickle-cell disease, illustrating how primary sequence dictates function.

Nucleic Acids and Lipids

Nucleic acids (DNA, RNA) store and express genetic information. A nucleotide has three parts: a nitrogenous base, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a phosphate. Phosphodiester bonds form the sugar-phosphate backbone; complementary base pairing (A-T, A-U, G-C) drives the double helix. ATP, a nucleotide, is the universal energy currency.

Lipids are hydrophobic, not true polymers. Triglycerides (glycerol + three fatty acids) store energy densely. Phospholipids (glycerol + two fatty acids + phosphate group) form bilayer membranes — the hydrophilic phosphate head faces water, the hydrophobic tails face inward. Steroids (four fused rings, e.g., cholesterol, testosterone, estrogen) are signaling molecules. Fatty acids are saturated (no double bonds, straight chains pack tightly, solid at room temperature, e.g., butter) or unsaturated (one or more cis double bonds, kinked chains, liquid — oils). Trans fats have trans double bonds created by industrial hydrogenation and pack like saturated fats.

Exam Strategy

For the saccharide sample item, classify by monomer count: one = monosaccharide, two = disaccharide, many = polysaccharide. For macromolecule identification, match the monomer to the polymer (amino acid to protein, nucleotide to nucleic acid, monosaccharide to carbohydrate). Lipids are the outlier — no single repeating monomer, grouped by hydrophobicity. For protein items, remember that sequence dictates fold and fold dictates function.