Students should be familiar with ionizing radiation as the type of radiation that generates free radicals.  Further, it should be understood that free radical formation causes hyper-reactivity among exisitng nucleic acid molecules and subsequent destruction.

In order to hold a base pair together, the hydrogen bond(s) between the bases must be stable. This stability depends on many factors, such as the size and shape of the bases. The adenine-thymine base pair is most stable when held together by 2 hydrogen bonds, while the cytosine-guanine base pair is most stable when held together by 3 hydrogen bonds.

Lipids are hydrophobic molecules that have low solubility in water and high solubility in nonpolar organic solvents. The following choices all describe lipid molecules, with the exception of glycine. Glycine is an amino acid and contains a carboxyl group (like fatty acid lipids), but also a amine group. These function groups make glycine hydrophilic and polar, unlike lipids.

Terpenes are special classes of lipids that are derived from isoprene units. An isoprene unit’s molecular formula is ; therefore, terpenes must contain a molecular formula that is derived from a empirical formula.

The molecular formula consists of seven isoprene units ( = ); therefore,  is a type of terpene.

Pentene is an alkene with the molecular formula .  () is derived from pentene units; however, terpenes do not have pentene units.

Of the choices provided, only energy storage in the form of tricacylglycerols makes sense. Free fatty acids do not act as the main storage form for fats, but are used to create triacylglycerols.

Triacylglycerols (triglycerides) are composed of three fatty acids and a glycerol backbone. Their primary function is to store energy. As a secondary function they can also provide thermal insulation for an organism.

Phospholipids (glycerophospholipids) are primarily used in the cell membrane, but can also be involved in signaling. Sterol lipids (steroids) are primarily used as hormones, or involved in cell signaling. Eicosanoids are a family of fatty acid involved in signaling.

Glycogen is used to store energy, but it is a carbohydrate glucose polymer. The question specifies that the answer must be a lipid.

Steroids are a type of lipid. This means that steroids are nonpolar molecules, composed of primarily of hydrogen and carbon. Recall that the interior of the plasma membrane consists of the nonpolar tails of phospholipids. Since nonpolar molecules tend to dissolve in other nonpolar molecules, steroids will be able to traverse the interior (nonpolar phospholipid tail) of plasma membranes and cross into the cell.

A steroid is a type of lipid molecule that is found extensively in living organisms. Recall that triglycerides are also a type of lipid; therefore, steroids and triglycerides are classified as the same type of macromolecule. One of the characteristics of steroids is that they have four cycloalkane rings. Three of the four rings are six-membered rings (cyclohexane) and one ring is a five-membered ring (cyclopentane). Steroids are found in several biomolecules, such as hormones. Some examples of steroid hormones include the sex hormones (estrogen, progesterone, and testosterone), aldosterone, and cortisol. Since they are nonpolar, steroid hormones can traverse the lipid bilayer and enter the cell; therefore, the steroid hormone receptors are found inside the cell (on the nuclear membrane).

A type of steroid, called cholesterol, can be found in plasma membranes but it is not the major component of the membrane. Recall that plasma membranes are primarily composed of a phospholipid bilayer; therefore, phospholipids (another type of lipid) are the major component of plasma membranes.

Steroids are a type of lipid that are characterized by their four-ring molecular structure. The four rings consist of three six-membered rings and one five-membered ring. Recall that six-membered rings are called cyclohexanes and five-membered rings are called cyclopentanes; therefore, you will most likely find a cyclohexane in a steroid.

Phosphate groups, pentose sugars, and nitrogenous bases are found in nucleotides, which are monomers that make up nucleic acids. Steroids are a type of lipid; therefore, you will most likely not find these substances in a steroid.

The correct answer is saturated fat. Rather than attempting to draw each one of these macromolecules, one should view this conceptually. In a combustion reaction, heat is generated for every carbon-hydrogen bond that is oxidized. In other words, the higher the density of C-H bonds in a compound, the more potential energy it can store. Fats, which primarily consist of only carbon and hydrogen, will contain the most energy and subsequently generate the most heat when oxidized. This is the same reason why animals and plants store energy as fat molecules.

*Note: A polyunsaturated fat has more than one double bond between its carbons, which reduces the total number of C-H bonds.