Phosphatidic acids consist of all of the structures listed, except for choline. Choline is a nitrogen-containing salt with an alcohol group. It is a precursor to the neurotransmitter acetylcholine and some classes of phospholipids including sphingomyelin and phosphatidylcholine.

Unsaturated fatty acids lead to lower LDL levels ("bad cholesterol") than saturated fatty acids. Thus, unsaturated fatty acids are healthier than saturated fatty acids. Plant oils do contain more unsaturated fatty acids than saturated fatty acids, so they are much healthier than animal fats - also note that cholesterol is largely an animal-derived lipid and is almost exclusively found in animal fats (however some research shows that very low levels of cholesterol may be found in some plant products). Saturated fatty acids also lead to higher LDL levels ("bad cholesterol"), which accelerate heart disease.

Lipid-soluble vitamins are soluble in lipids (fats). These vitamins are absorbed into the body and then stored in body tissues. Water-soluble vitamins, by contrast, are soluble in water and do not store for long periods of time in the body; they are easily eliminated through urine. There are 4 lipid-soluble vitamins. These are vitamins A, D, E, and K. In contrast, thw water-soluble vitamins are B complex and C.

Glycolipids, phospholipids, and cholesterol are all major components of membranes.  Free fatty acids, however, are not found freely in membranes.  Free fatty acids are the tails in phospholipids, sphingolipids and other lipid derivatives, but not cholesterol.

Lipids are nonpolar substances, meaning that they are hydrophobic ("water-fearing"). Water, meanwhile, is polar. A common statement to remember in biochemistry is "like dissolves like." This means that polar substances will dissolve other polar substances, while nonpolar substances will dissolve other nonpolar substances. Polar and nonpolar substances do not mix; thus, lipids and water cannot mix.

The key to this question is realizing that unsaturated fatty acids contain one or more double bonds, while saturated fatty acids contain no double bonds. From that information, you can make inferences about most of the other answer choices. Because of the double bonds within an unsaturated fatty acid, there are fewer hydrogens attached to the carbon molecules. Additionally, the double bonds result in a bent/kinked structure. Furthermore, this bent/kinked structure results in a lower boiling point. This structure disrupts packing, and reduces the van der Waals interactions, thus reducing the boiling point. Finally, fatty acids do not contain amino groups, so this is the correct answer.

The amount of cholesterol within a membrane has a unique effect on the fluidity. At low temperatures, it increases fluidity. At high temperatures it decreases fluidity. The best way to remember this is to think of cholesterol as maintaining optimal fluidity in a membrane. With higher temperatures alone, fluidity increases, and cholesterol acts to counter that increase.  The opposite is true at low temperatures; fluidity decreases, and cholesterol acts to oppose that decrease.

Saponification is the general term for a chemical reaction between an acid and a base to form a salt. This process can be used to make soap (the salt) if one mixes an oil or fat (the acid) with lye (the base). Triglycerides (triesters) are the main materials that are saponified. We can saponify triglycerides by treating them with a strong base (such as lye), which accelerates cleavage of the ester bond to release the fatty acid and glycerol. Soap can then be precipitated by a salting out process.

Phosphoacylglycerols, glycolipids, and cholesterol are all part of cell membranes. Phosphoacylglycerols and glycolipids are major components of cell membranes. Glycolipids mainly have a communicative role in which they act as markers for cell recognition. They also provide stability for the cell and help form tissues. Cholesterol helps maintain the fluidity of cell membranes, along with securing important proteins in the membrane.

Triacylglycerols, however, are not found in cell membranes. These are triesters formed by esterification of three fatty acids to glycerol. Oils are triacylglycerols that are liquids at room temperature, while fats are triacylglycerols that are solids at room temperature. Triacylglycerols are stored forms of energy for living systems.

The less double bonds there are (sites of unsaturation) and the more carbons there are in a fatty acid, the higher the melting point for a fatty acid. This is due to the difference in the three-dimensional chemical shape of such fatty acids - saturated fatty acids have linear hydrocarbon chains, which allows them to stack closely together and maximize intermolecular forces within the chains whereas unsaturated fatty acids contain a "kink" in their hydrocarbon chains, which does not allow two unsaturated fatty acids to associate very closely to one another, thereby not maximizing intermolecular forces of attraction between them. This means that the saturated fatty acids require more energy to overcome the higher quantity of intermolecular forces in order for a phase change to occur than do unsaturated fatty acids. Based on this reasoning, the 20:0 fatty acid (which has 20 carbons and zero double bonds) is expected to have the highest melting point.