Lipids

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MCAT Biology › Lipids

Questions 1 - 10
1

Given the same weight of each, from which macromolecule can the most heat be generated in a combustion reaction?

Saturated fat

Polyunsaturated fat

Protein

Carbohydrate

Nucleic acid

Explanation

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.

2

Given the same weight of each, from which macromolecule can the most heat be generated in a combustion reaction?

Saturated fat

Polyunsaturated fat

Protein

Carbohydrate

Nucleic acid

Explanation

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.

3

Which of the following is false regarding steroids?

Steroids are the major component of plasma membranes

Steroids and triglycerides are both classified as the same type of macromolecule

All steroids contain at least four rings

Steroid hormones bind to receptors on the nuclear membrane

Explanation

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.

4

Which of the following is false regarding steroids?

Steroids are the major component of plasma membranes

Steroids and triglycerides are both classified as the same type of macromolecule

All steroids contain at least four rings

Steroid hormones bind to receptors on the nuclear membrane

Explanation

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.

5

Which of the following is not a lipid?

Glycine

Galactocerebroside

Linolenic acid

Prostaglandin

Explanation

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.

6

Which of the following is not a lipid?

Glycine

Galactocerebroside

Linolenic acid

Prostaglandin

Explanation

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.

7

Drain cleaners are a common household staple, used to open clogged drains in bathtubs and sinks. Human hair is a common culprit that clogs pipes, and hair is made predominately of protein. Drain cleaners are effective at breaking down proteins that have accumulated in plumbing. Drain cleaners can be either acidic or basic, and are also effective at breaking down fats that have accumulated with proteins.

A typical reaction—reaction 1—which would be expected for a drain cleaner on contact with human hair, would be as follows in an aqueous solution:

Pic_1

Another reaction that may occur, reaction 2, would take place as follows in an aqueous solution:

Pic_2

The drain cleaner in Reaction 1 was used to break down fats as well as the protein depicted. How would the fats be most different in molecular structure?

They would have much lower overall polarity in their bonds.

They would be more oxidized than the proteins.

They would be more acidic than the proteins.

They would totally lack any polar bonds.

They would be more reactive than the proteins.

Explanation

Fats are less reactive, more reduced, and less acidic than proteins. They have much lower overall polarity, but do contain polar bonds, such as between C and O.

8

Which if the following lipids is responsible for the storing of energy in the body?

Triacylglycerols

Glycerophospholipids

Sterol lipids

Eicosanoids

Glycogen

Explanation

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.

9

Which if the following lipids is responsible for the storing of energy in the body?

Triacylglycerols

Glycerophospholipids

Sterol lipids

Eicosanoids

Glycogen

Explanation

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.

10

Water often acts as a reactant or solvent in biological reactions. Which of the following cellular components would not be sufficiently solvated in the body?

Lipids

Carbohydrates

Amino acids

Nucleotides

Explanation

A molecule is solvated when it is surrounded by water molecules, and separated from the other molecules in the body. This separation is possible because of charge or polarity present in the molecule, which causes it to be attractive to water.

Lipids have very low solubility in water due to their nonpolarity. As a result, we conclude that lipids would not be properly solvated in the body, and would instead be clustered together by water molecules.

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