Concepts: Lipids and nucleic acids

4.1 Lipids

Lipids are the group that does not dissolve in water. A simple fatty acid is a carboxyl group attached to a hydrocarbon chain, the R group. Palmitic acid carries sixteen carbons including the carboxyl carbon, and arachidonic acid carries twenty. A fatty acid with no double bond in its chain is saturated, and one with one or more carbon-to-carbon double bonds is unsaturated. Glycerol is a simple lipid, a trihydroxy propane with three hydroxyl groups.

Most lipids in living tissue combine glycerol with fatty acids. The fatty acids are esterified to glycerol, and the products are monoglycerides, diglycerides or triglycerides depending on how many fatty acids are attached. Triglycerides are the fats and oils, and the melting point separates the two words: oils have a lower melting point and stay liquid in winter, while fats are solid. Some lipids carry phosphorus and a phosphorylated organic compound, and these are the phospholipids of cell membranes; lecithin is one example. Neural tissue holds lipids with more complex structures, and cholesterol is the standard steroid of animal membranes.

Lipids look small compared with the macromolecules, and their mass stays under 800 daltons. They still appear in the acid-insoluble fraction because membranes break into vesicles when the tissue is ground, and those vesicles are not water-soluble. The lipid class is therefore the exception that a careful answer must mention.

4.2 Nucleic acids

A nucleotide has three parts: a nitrogenous base, a sugar and a phosphate. The bases are adenine, guanine, cytosine, uracil and thymine. Adenine and guanine are purines with a double ring, while cytosine, uracil and thymine are pyrimidines with a single ring. When a base is attached to a sugar alone, the molecule is a nucleoside; adenosine, guanosine, thymidine, uridine and cytidine are the nucleosides. Esterify a phosphate to the sugar as well and the molecule becomes a nucleotide; adenylic, thymidylic, guanylic, uridylic and cytidylic acids are all nucleotides.

Nucleic acids are polynucleotides, chains of nucleotides joined by phosphodiester bonds. The sugar decides the name: a nucleic acid with deoxyribose is deoxyribonucleic acid, DNA, and one with ribose is ribonucleic acid, RNA. DNA and RNA carry the genetic information and pass it from one generation to the next. The secondary structure of DNA, the double helix with two strands held together by hydrogen bonds between the bases, comes later in the course; for this chapter the important facts are the three parts of a nucleotide, the base names, the sugar difference and the hereditary role.

4.2.1 The chemical unity of life

The chapter closes with a point that ties everything together. Living organisms differ endlessly in form, but their chemistry is remarkably similar: the same twenty amino acids, the same five bases, the same sugars and the same lipid classes appear across the living world. The molecules are built from a small set of units, and the differences between organisms come from the order of those units. That is why a single chapter on biomolecules can describe the chemistry of every organism in the biosphere.

4.2.2 Why lipids look different

Lipids are not polymers, and their mass does not put them with the macromolecules, so they are the one class that breaks the pattern of the chapter. They still build structures, and their insolubility is the reason: a membrane works because its lipids refuse to mix with water. Phospholipids make the bilayer, cholesterol stiffens it, and triglycerides store energy in a compact form. Fatty acids differ in chain length and in the number of double bonds, and that difference changes the melting point, which is why some oils stay liquid while some fats are solid at room temperature.

4.2.3 The nucleic acid backbone

Nucleotides join into a chain through the phosphate of one unit and the sugar of the next, and the linkage is a phosphodiester bond. The sugar-phosphate backbone therefore runs along the outside of the molecule, while the bases point inward. DNA and RNA differ in that backbone sugar and in one base: DNA has thymine where RNA has uracil. The naming pair is worth fixing in memory, because a one-mark question often asks for the sugar of each nucleic acid or for the base that appears in only one of them. The polymer's order of bases is what carries the information, and that order is read in the same direction from one end to the other.

4.2.4 Why membranes need lipids

A membrane works because its lipids refuse to mix with water, so they line up into a sheet with their heads out and tails in. Proteins sit in that sheet and do the transport and signalling. Cholesterol fits between the lipid tails and keeps the sheet stable across temperatures. A lipid that dissolved in water would never build such a barrier, so insolubility is the property that makes the membrane possible.