Carbohydrates, Lipids, Proteins and Nucleic Acids

Carbohydrates contain C, H and O, usually in the ratio ${\text{CH}}_2\text{O}$. Monosaccharides are single sugars such as glucose, fructose and galactose (all ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$, but arranged differently) and the five-carbon ribose and deoxyribose of nucleic acids. Two monosaccharides joined by a glycosidic bond form a disaccharide: glucose + glucose gives maltose, glucose + fructose gives sucrose (table sugar), and glucose + galactose gives lactose (milk sugar). Long chains are polysaccharides. In water, glucose forms a ring, and the –OH on carbon 1 can point below the ring (alpha glucose) or above it (beta glucose). That tiny difference changes everything. Alpha links make chains that coil into compact helices, ideal for storage. Beta links force every second glucose to flip over, producing straight chains that lie side by side and hydrogen bond into tough fibres. Human enzymes can hydrolyze alpha links but not beta links, which is why we digest starch but not the cellulose in lettuce (it passes through as dietary fibre). Cows and termites can only digest cellulose because microbes in their guts make the enzyme cellulase.

• What unites lipids is that they are hydrophobic: mostly nonpolar C–H bonds, so they do not dissolve in water. They are not built from repeating monomers, so they are not true polymers.
• Triglycerides (fats and oils): one glycerol joined to three fatty acids by three ester bonds, releasing three water molecules. They store about 37 kJ/g, more than twice the roughly 17 kJ/g of carbohydrates or proteins, which is why animals store long-term energy as fat. Fat also insulates and cushions organs.
• Saturated fatty acids have only single C–C bonds, so every carbon holds as much hydrogen as possible. The straight tails pack tightly, so saturated fats (butter, lard) are solid at room temperature. Unsaturated fatty acids have one or more C=C double bonds, usually in the cis form, which puts a kink in the tail. Kinked tails cannot pack closely, so unsaturated fats (olive oil, canola oil) are liquid.
• Trans fats are unsaturated fats whose double bonds are in the trans form, which keeps the tail nearly straight. Most came from partially hydrogenating vegetable oils to make them solid. They raise LDL ("bad") cholesterol and heart disease risk, and Canada banned partially hydrogenated oils in foods in 2018.
• Phospholipids: glycerol + two fatty acids + a phosphate-containing head. The head is polar (hydrophilic) and the two tails are nonpolar (hydrophobic), so the molecule is amphipathic. In water, phospholipids spontaneously form a bilayer with heads facing the water and tails hidden inside: the basis of every cell membrane.
• Steroids: four fused carbon rings with different side groups. Cholesterol stabilizes animal membranes and is the starting material for steroid hormones such as testosterone, estrogen and cortisol.
• Waxes: a long fatty acid joined to a long-chain alcohol. Extremely hydrophobic, they waterproof leaves (the cuticle), feathers and insect exoskeletons; bees build combs from beeswax.

Every amino acid has the same core: a central carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group (side chain). The 20 amino acids differ only in their R groups, which can be nonpolar (hydrophobic), polar, acidic (negative) or basic (positive). A condensation reaction between the carboxyl of one amino acid and the amino group of the next forms a peptide bond, so a chain always has a free amino end (N-terminus) and a free carboxyl end (C-terminus). A polypeptide folds into a precise 3D shape, and in proteins shape determines function: an enzyme's active site, an antibody's binding site and a channel's pore all depend on it. Heat, extreme pH, heavy metals or high salt concentrations break the hydrogen bonds, ionic bonds and hydrophobic interactions that hold the shape. The protein unfolds and stops working: it is denatured. The peptide bonds survive, so the primary structure is unchanged, but the function is lost; a cooked egg white cannot be uncooked. A change in primary structure can be just as serious. In sickle cell disease, one glutamic acid in the beta chain of hemoglobin is replaced by valine, and that single hydrophobic R group makes hemoglobin molecules stick together into fibres that distort red blood cells.

A nucleotide has three parts: a five-carbon sugar, a phosphate group and a nitrogenous base. Nucleotides join when the phosphate on the 5' carbon of one sugar condenses with the –OH on the 3' carbon of the previous one, forming a phosphodiester bond. The result is a sugar–phosphate backbone with bases sticking out, and a strand with a 5' end and a 3' end. DNA uses deoxyribose (missing one –OH) and the bases adenine, thymine, guanine and cytosine; it is usually double-stranded, with A hydrogen bonded to T and G to C, and it stores genetic information. RNA uses ribose and replaces thymine with uracil; it is usually single-stranded and carries out the instructions (mRNA, tRNA, rRNA). ATP (adenosine triphosphate) is a single RNA-type nucleotide: adenine + ribose + three phosphates. The three negatively charged phosphates repel each other, so hydrolyzing the last one to form ADP and inorganic phosphate releases about 30.5 kJ/mol, which the cell uses to power its work.