MCAT Biology : Properties of Lipids

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Properties Of Lipids

Drain cleaners 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 fats that are broken down by drain cleaners in the preceeding passage come predominately from oil secretions in skin and on hair. What is a main function of fats in the body?

Possible Answers:

To lubricate joints

To store energy in the form of triacylglycerols

To dissolve lipids in the blood

To store energy in the form of free fatty acids

To create cytoskeletal elements

Correct answer:

To store energy in the form of triacylglycerols

Explanation:

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.

Example Question #2 : Properties Of Lipids

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

Possible Answers:

Glycogen

Eicosanoids

Sterol lipids

Glycerophospholipids

Triacylglycerols

Correct answer:

Triacylglycerols

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.

Example Question #3 : Properties Of Lipids

Which of the following best describes steroid molecules?

Possible Answers:

Steroids are polar and cannot travel across the plasma membrane

Steroids are nonpolar and can travel across the plasma membrane

Steroids are nonpolar and cannot travel across the plasma membrane

Steroids are polar and can travel across the plasma membrane

Correct answer:

Steroids are nonpolar and can travel across the plasma membrane

Explanation:

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.

Example Question #4 : Properties Of Lipids

Which of the following is false regarding steroids?

Possible Answers:

Steroid hormones bind to receptors on the nuclear membrane

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

Steroids are the major component of plasma membranes

All steroids contain at least four rings

Correct answer:

Steroids are the major component of plasma membranes

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.

Example Question #1 : Lipids

Which of the following will you most likely find in a steroid molecule?

Possible Answers:

Cyclohexane ring

Pentose sugar

Phosphate group

Nitrogenous base

Correct answer:

Cyclohexane ring

Explanation:

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.

Example Question #5 : Properties Of Lipids

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

Possible Answers:

Saturated fat

Protein 

Polyunsaturated fat

Carbohydrate

Nucleic acid

Correct answer:

Saturated fat

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.

Example Question #6 : Lipids

Sexually transmitted diseases are a common problem among young people in the United States. One of the more common diseases is caused by the bacterium Neisseria gonorrhoeae, which leads to inflammation and purulent discharge in the male and female reproductive tracts.

The bacterium has a number of systems to evade host defenses. Upon infection, it uses pili to adhere to host epithelium. The bacterium also uses an enzyme, gonococcal sialyltransferase, to transfer a sialyic acid residue to a gonococcal surface lipooligosaccharide (LOS). A depiction of this can be seen in Figure 1. The sialyic acid residue mimics the protective capsule found on other bacterial species.

Once infection is established, Neisseria preferentially infects columnar epithelial cells in the female reproductive tract, and leads to a loss of cilia on these cells. Damage to the reproductive tract can result in pelvic inflammatory disease, which can complicate pregnancies later in the life of the woman.

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What is likely true of the lipid A found on the glucosamine molecule in Figure 1?

Possible Answers:

It is derived from host proteins

It is synthesized by ribosomes

It is unable to anchor in the cell membrane due to its hydrophilicity

It contains only nitrogen, carbon, and hydrogen

It is composed predominately of hydrogens and carbons

Correct answer:

It is composed predominately of hydrogens and carbons

Explanation:

Lipids are hydrocarbons, and are the most energy-rich biological macromolecules due to their heavily reduced state. This reduced state is a function of the roughly equal electronegativity between carbon and hydrogen atoms.

Example Question #111 : Macromolecules

Water is the solvent in which all chemical reactions take place for living organisms. In addition, water has a number of critical characteristics that allows it to be invaluable to life as we know it.

Which of the following statements is FALSE concerning the properties of water?

Possible Answers:

Water's strong surface tension allows certain bugs to "stand" on the water's surface.

Water's ability to form hydrogen bonds allow it to maintain a liquid state in nature.

Water solvates hydrophobic molecules, such as fatty acids, and separates them.

Water plays a major role in the breakdown of macromolecules.

Correct answer:

Water solvates hydrophobic molecules, such as fatty acids, and separates them.

Explanation:

Water is able to solvate hydrophilic compounds. Hydrophobic molecules, such as fatty acids, are typically aggregated together by water and can be separated.

Example Question #11 : Lipids

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?

Possible Answers:

They would have much lower overall polarity in their bonds.

They would be more acidic than the proteins.

They would be more reactive than the proteins.

They would totally lack any polar bonds.

They would be more oxidized than the proteins.

Correct answer:

They would have much lower overall polarity in their bonds.

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.

Example Question #112 : Macromolecules

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?

Possible Answers:

Nucleotides

Amino acids

Carbohydrates

Lipids

Correct answer:

Lipids

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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