AP Biology : Understanding Lipids

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding Lipids

Which of the following types of molecules has the most energy available per unit of carbon?

Possible Answers:

Sugars

Amino acids

Proteins

Alcohols

Fats

Correct answer:

Fats

Explanation:

Fats have an incredibly high potential to produce a lot of energy when broken down. This is because they are very saturated, which means they have a lot of bonded hydrogens. They also have a lot of carbon-carbon bonds, which have a lot of potential energy stored. When you break down a fat, especially one that has fourteen or more carbons in the chain, you release the energy from every carbon-carbon and carbon-hydrogen bond.

Comparing this to a sugar, alcohol, or protein (amino acids make up proteins), we can see that there aren't as many of these bonds to break. Proteins, in fact, require a lot of energy to break down because they have to be converted into other forms first.

Example Question #1 : Understanding Lipids

Phospholipids are amphipathic. This means that a phospholipid is __________.

Possible Answers:

both polar and nonpolar

a trigger for the immune response

only soluble in organic solvents

found only in animal cells

Correct answer:

both polar and nonpolar

Explanation:

Amphipathic molecules have both a polar and nonpolar region. This amphipathic quality allows phospholipids to create the plasma membrane in eukaryotic cells. The polar region is the phosphate head, which interacts with the aqueous cytosol and extracellular environment. The nonpolar region is the fatty acid tail, which is sequestered in the bilayer of the membrane and helps reduce the permeability to certain molecules.

Example Question #74 : Macromolecules

Cholesterol is soluble in organic solvents such as chloroform, but is insoluble in water. Based on this information, to which class of macromolecule does cholesterol belong?

Possible Answers:

Nucleic acid

Oligosaccharide 

Enzyme

Lipid

Correct answer:

Lipid

Explanation:

Water is a very polar substance that will not interact well with nonpolar macromolecules. Enzymes (proteins), oligosaccharides (carbohydrates), and nucleic acids all contain polar regions that make them soluble in aqueous environments. Lipids, however, are hydrocarbons and generally lack a polar region. Lipids would not be soluble in water, but would be soluble in nonpolar organic solvents, like chloroform. We can conclude that cholesterol is a lipid.

Example Question #3 : Understanding Lipids

What would happen to a group of lipids in a polar solvent?

Possible Answers:

There is no way to predict what would happen

The lipids would interact readily with the solvent

The lipids would precipitate out of the solvent

The lipids would group together

Correct answer:

The lipids would group together

Explanation:

Lipids are composed of hydrocarbon chains and are very nonpolar. Polar solvents interact well with polar solutes, but do not solvate nonpolar solutes. When lipids are placed in a polar solvent, they will group together to minimize surface contact with the solvent. These droplets of lipids, or micelles, act like containers for the lipid, keeping them grouped together instead of being distributed through the solvent.

The lipids do not precipitate as they are not necessarily in a solid form. Even lipids in the liquid state can form micelles.

Example Question #2 : Understanding Lipids

What are the main components of a triglyceride?

Possible Answers:

A polar head group, a glycerol backbone, and three fatty acids

A sugar-phosphate backbone and three fatty acids

A glycerol backbone and three fatty acids

A cholesterol molecule, a glycerol backbone, and three fatty acids

Correct answer:

A glycerol backbone and three fatty acids

Explanation:

Triglycerides are made up of a glycerol backbone and three fatty acids. They are commonly used to store energy within cells.

A polar head group, a glycerol backbone, and three fatty acids very nearly describes a phospholipid (phospholipids only have two fatty acids). The other answers are not compounds that are readily observed in cells.

Example Question #5 : Understanding Lipids

Where are lipids most commonly found in cells?

Possible Answers:

The cytosol

The extracellular matrix

Membranes

Organelle matrices

Correct answer:

Membranes

Explanation:

Lipids are primarily found in membranes. This includes both the plasma membrane and membranes surrounding particular organelles. Lipids are very useful in membranes because their nonpolar nature helps them act as a barrier between the cell and the outside environment.

Example Question #6 : Understanding Lipids

How many rings does a typical steroid molecule contain?

Possible Answers:

Two

Three

Five

Four

Correct answer:

Four

Explanation:

Remember that steroids are derived from cholesterol, a four-ringed structure. The four-ringed structure is a constant in all cholesterol-derived molecules. Variation between molecules comes from the final constituent on the four rings.

Example Question #5 : Understanding Lipids

Which of the following is false regarding lipids?

Possible Answers:

Triglycerides and phospholipids have a two-carbon glycerol backbone

Lipids are the major component of plasma membranes

Unsaturated fatty acids contain carbon-carbon double bonds

Lipids are precursors to several molecules in the endocrine system

Correct answer:

Triglycerides and phospholipids have a two-carbon glycerol backbone

Explanation:

Plasma membranes are mostly made up of phospholipids. Phospholipids are lipid molecules that contain both polar and nonpolar regions. This amphipathic nature of phospholipids is very important in plasma membranes. Several hormones of the endocrine system are derived from steroids (a type of lipid). Some examples of steroid derived hormones include testosterone, estrogen, and cortisol; therefore, lipids are precursors to molecules in the endocrine system. By definition, unsaturated fatty acids contain carbon-carbon double bonds.

Glycerol is a three-carbon molecule and forms the backbone of triglycerides and phospholipids. Triglycerides have three fatty acids, one attached to each of the carbons in glycerol. Phospholipids use two glycerol carbons to bind fatty acids, and the third to bind a phosphate group.

Example Question #3 : Understanding Lipids

Which of the following is not considered a lipid?

Possible Answers:

Phospholipids

Triglyceride

Glucose

Cholesterol

Steroids 

Correct answer:

Glucose

Explanation:

There are several types of lipids, including fatty acids, phospholipids, sphingolipids, and sterol lipids as well as many more.

Triglycerides fall under the category of glycerolipids and contain a three-carbon glycerol backbone with three fatty acids tails. Phospholipids contain a polar phosphate group, a glycerol backbone, and two fatty acid tails. Sterol lipids contain a four-ring base structure. Cholesterol and steroids are sterol lipids.

Glucose is a monosaccharide (carbohydrate), not a lipid.

Example Question #4 : Understanding Lipids

Which of the following is composed of a glycerol molecule bound to three fatty acids?

Possible Answers:

Steroid

Triglyceride

Carbohydrate

Phosphate

Nucleotide

Correct answer:

Triglyceride

Explanation:

Lipids are the fat molecules of biology and are frequently composed of fatty acids. Glycerol is a short 3-carbon chain that is often used to bind different fatty acids together and form lipid structures. When glycerol binds to three fatty acid chains, it forms a triglyceride. Triglycerides are used to store energy in the body.

Phospholipids, which make up the cell membrane, are formed when a glycerol binds to two fatty acids and a phosphate group. Phosphates are complex ions formed from one phosphorus atom and four oxygen atoms: . Steroids are another type of lipid, but do not contain glycerol.

Carbohydrates, proteins, and nucleic acids are the other primary classes of biological macromolecules, and generally do not contain glycerol. Nucleotides are the monomers of nucleic acids and are composed of a pentose sugar, a phosphate group, and a nitrogenous base. Carbohydrates are formed from monosaccharide monomers.

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