Lipids

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.

Fatty acids are composed of a carboxylic acid head and a long carbon chain. The carbon chain is non-polar, while the carboxylic acid head is polar. A molecule with both a polar and non-polar parts is known as amphipathic. The word amphoteric means a molecule that can act as either an acid or a base. Finally, just as a fatty acid has both polar and non-polar ends, those same ends are also hydrophilic and hydrophobic, respectively.

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.

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.

A triglyceride has three fatty acids; this molecule has two (represented by the R chains). Phosphaditic acid is the simplest of the diacyl-glycerophospholipids; its phosphate group is bonded to only to the glycerol, and nothing else, which is not the case here. Among the three other choices, all of them accurately describe this molecule as a diacylglyceride phospholipid (hence the prefix phosphatidyl-); the phosphate is attached to serine as a head group, not a choline, nor ethanolamine, .

To answer this question, let's go through each of the answer choices to see what cholesterol's functions are.

It turns out that cholesterol is an important component of animal cell membranes. It helps to maintain both the structural integrity of the membrane, as well as its fluidity.

Cholesterol also serves as a precursor for all steroid hormones. In fact, if you look through all the steroid hormones, you'll find that they all have the characteristic 4-ring structure that cholesterol has.

Cholesterol also functions as a precursor for the production of bile salts in the liver. These bile salts are subsequently stored in the gallbladder and, when needed, released into the duodenum of the small intestine to aid in the digestion of lipids.

Cholesterol can deposit into the inner lining of blood vessels, however this is not a normal function of cholesterol. Rather, this is a pathological process that leads to a condition called atherosclerosis. This, in turn, can lead to the hardening of blood vessels, as well as contribute to the formation of blood clots that can impede the flow of blood in that vessel. These clots can also become dislodged and travel throughout the circulatory system, where it can become trapped in other blood vessels. This is a dangerous situation, because it can potentially lead to heart attack or stroke.

Lipids are capable of storing the most energy per unit weight of any molecule. The complete oxidation of a fatty acid yields , compared to about for carbohydrates and proteins.

An "amphipathic" molecule is one that has both hydrophilic and hydrophobic regions. A molecule that can act as both an acid and a base is amphoteric. Stereoisomers are molecules with the same molecular formula, but a different arrangement of atoms in space - an example is D-glucose and L-glucose.

Triglycerides and cholesterol are not phospholipids, so they can be eliminated from the answer choices. Phosphatidylinositol and phosphatidylserine are present in cellular membranes, but they are typically less abundant than phosphatidylcholine, which is the correct answer.