Biochemistry : Lipids

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Lipids

A molecule is found to have a 3-carbon backbone with two long hydrocarbon chains attached to two of the carbons in the backbone and another side chain on the third carbon. Which of the following could be the identity of this molecule? 

Possible Answers:

Phospholipid

Any of these could be the correct identity

Triglyceride

Sphingolipid

Correct answer:

Phospholipid

Explanation:

Most lipids are made up of a 3-carbon backbone called glycerol. The differences between lipids result from the types of chains attached to the carbons of the glycerol molecule.

The question states that the 3-carbon backbone has two hydrocarbon chains attached. Recall that fatty acids are long, hydrocarbon chains (made up of only carbon and hydrogen atoms); therefore, the molecule in this question has a glycerol molecule with two fatty acids attached. Phospholipids are composed of a glycerol backbone with two fatty acids and one phosphate group. This means the identity of this molecule could be a phospholipid.

Triglycerides are lipids that contain a glycerol molecule with three fatty acids. Sphingolipids are special lipids found in cell membranes that contain a different type of backbone called sphingosine. 

Example Question #2 : Lipids

Which of the following is an essential fatty acid?

I. Vitamin A

II. Oleic acid

III. Alpha-linoleic acid

Possible Answers:

II only

I and III

II and III

III only

Correct answer:

III only

Explanation:

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.

Example Question #3 : Lipids

An individual's lipid profile shows low levels of LDL in the blood. What can you conclude about this patient? 

Possible Answers:

The patient is at risk for a heart attack

There will be increased inflammation in the walls of arteries

There is an increased level of a free 4-membered ring structure in the blood

There is an increased level of prostaglandins in the blood

Correct answer:

There is an increased level of a free 4-membered ring structure in the blood

Explanation:

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.

Example Question #4 : Lipids

Which of the following is true regarding saturated and unsaturated fatty acids? 

Possible Answers:

A 15-carbon saturated fatty acid has the same number of hydrogen atoms as a 15-carbon unsaturated fatty acid

A 15-carbon saturated fatty acid has more oxygen atoms than a 15-carbon unsaturated fatty acid

A 15-carbon saturated fatty acid has a greater molecular weight than a 15-carbon unsaturated fatty acid

Saturated fatty acids can participate in geometric isomerism

Correct answer:

A 15-carbon saturated fatty acid has a greater molecular weight than a 15-carbon unsaturated fatty acid

Explanation:

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.

Example Question #5 : Lipids

Img 20150717 192426

What is the name of the molecule shown above?

Possible Answers:

Phosphaditic acid

Triglyceride

Phosphatidylserine

Phosphatidylethanolamine

Phosphatidylcholine

Correct answer:

Phosphatidylserine

Explanation:

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, .

Example Question #6 : Lipids

Which of the following statements concerning lipids are correct?

Possible Answers:

Cholesterol is the most common steroid produced in animals

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

Most lipid double bonds are conjugated

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

Lipids of the bilayer matrix most commonly move by transverse diffusion

Correct answer:

Cholesterol is the most common steroid produced in animals

Explanation:

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. 

Example Question #7 : Lipids

In terms of energy storage, __________ store the most energy per unit weight of any molecule in the human body.

Possible Answers:

amino acids

carbohydrates

lipids

proteins

nucleic acids

Correct answer:

lipids

Explanation:

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. 

Example Question #8 : Lipids

Which of the lipids listed below is usually the most abundant phospholipid present in a cell's membrane?

Possible Answers:

Phosphatidylinositol

Phosphatidylserine

Cholesterol

Phosphatidylcholine

Triglycerides

Correct answer:

Phosphatidylcholine

Explanation:

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. 

Example Question #9 : Lipids

Which of the following is not a correct function of cholesterol?

Possible Answers:

All of these are normal functions of cholesterol

Helps maintain rigidity of blood vessels

A component of animal cell membranes

A precursor to steroid hormones, such as vitamin D

A precursor to bile salts

Correct answer:

Helps maintain rigidity of blood vessels

Explanation:

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.

Example Question #10 : Lipids

The electric potential gradient of an ion across a plasma membrane __________.

Possible Answers:

is around  in an animal cell at rest

is independent of the concentration gradient

increases with temperature

decreases with temperature

increases as the ion's charge increases

Correct answer:

increases with temperature

Explanation:

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.

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