Lipids - Biochemistry

Essential fatty acids are fatty acids that cannot be synthesized by humans; therefore, these fatty acids need to be ingested through food. Essential fatty acids are typically polyunsaturated fatty acids, such as alpha-linoleic acid. Alpha-linoleic acid is a 18-carbon unsaturated fatty acid that has a double bond on the 15th carbon atom. This cannot be synthesized in the body because humans don’t have enzymes that add double bonds past the ninth carbon atom.

Oleic acid is also an 18-carbon fatty acid; however, humans can synthesize it because it is saturated and does not have any double bonds. Vitamin A is a lipid-soluble vitamin; however, it is not a fatty acid. Recall that all vitamins are molecules that also cannot be synthesized by humans and, therefore, must be ingested via the diet.

LDL, or low-density lipoprotein, is a lipid transporter that transports lipids in the blood. LDL transports several kinds of lipids such as triglycerides and phospholipids; however, the main lipid transported by LDL is cholesterol. Recall that cholesterol is a 4-membered ring structure with a hydroxyl group attached to one of the rings. A decrease in LDL will decrease the amount of cholesterol carried by LDL which will, subsequently, increase the amount of free cholesterol found in blood; therefore, decreasing LDL will increase the amount of cholesterol found in the blood.

Prostaglandins are lipid molecules derived from arachidonic acid. Recall that arachidonic acid is synthesized from an essential fatty acid called omega-6 fatty acid; therefore, prostaglandins don’t depend on cholesterol and won’t be affected by decreased LDL levels. Heart attacks can often result from atherosclerosis, or thickening and clogging of artery walls due to build up of white blood cells (WBCs). The WBCs typically accumulate in arterial walls due to increased LDL levels; therefore, a decrease in LDL levels will decrease the risk of heart attack. Since lowered LDL levels decrease the amount of WBCs in arterial walls, there will be decreased inflammation. Recall that inflammation is caused by cytokine factors released by WBCs such as macrophages and granulocytes.

Fatty acids are long hydrocarbon chains that contain a carboxylic acid moiety on one end. Saturated fatty acids have no double bonds, whereas unsaturated fatty acids have one or more double bonds. Adding a double bond to a fatty acid will eliminate two hydrogen atoms. Knowing this information, we can deduce that a 15-carbon unsaturated fatty acid with one double bond will have two less hydrogen atoms than a saturated fatty acid of the same length. The molecular formula for the 15-carbon unsaturated fatty acid with one double bond will be , whereas the molecular formula for its saturated counterpart will be . Because of its two additional hydrogen atoms, the saturated fatty acid will have a greater molecular weight.

Geometric isomerism, or cis/trans isomerism, is characterized by the relative positions of functional groups around a double bond. A molecule is ‘cis’ if two identical functional groups are on the same side of the double bond. A molecule is ‘trans’ if the functional groups are on opposite sides. Only unsaturated fatty acids contain double bonds; therefore, only unsaturated fatty acids can participate in geometric isomerism.

Incorrect answers are corrected below:

Most lipid double bonds are conjugated nonconjugated (typically three carbon atoms apart).

Lipids of the bilayer matrix most commonly move by transverse lateral diffusion (on same side of the membrane).

Proteins and carbohydrates embedded in the bilayer impart transverse symmetry asymmetry to the membrane.

An increase in the number of double bonds increases decreases the melting point of a triacylglyceride.

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, .

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.

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.

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.

For animal cells at rest, the potential difference across plasma membranes is usually somewhere between and . It is given by the Nernst equation:

Where and are constant, is the ion's charge, and are the outside and inside concentrations respectively, and is the temperature. Therefore, the potential difference decreases as the ion's charge increases, and is not independent of the concentration gradient. It increases -- it does not decrease -- with temperature.

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.

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.

Lipids with saturated tails are able to pack into very close proximity and form a relatively stable and solid bilayer because the fatty acids tails are straight and do not take up a lot of space. Adding cis-double bonds to these tails puts kinks in the them that make it harder for lipids to pack in close proximity. By unsaturating the tails of lipids in a bilayer membrane in this way, you can make that membrane more fluid.

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.

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.

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.

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.