Biochemistry : Biochemistry

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #155 : Macromolecule Structures And Functions

Which of the following is true of eicosanoids produced from alpha-linolenic acid?

Possible Answers:

Their production is increased by adding vegetable oil in the diet

A decreased amount is recommended for those with high risk of heart disease

They are considered pro-inflammatory

They produce a weak platelet aggregation response

They produce a strong platelet aggregation response

Correct answer:

They produce a weak platelet aggregation response

Explanation:

Alpha-linolenic acid (ALA) is an omega-3 fatty acid that is utilized to produce series 3 eicosanoids. Series 3 eicosanoids lead to a weak platelet aggregation response, and are considered anti or less-inflammatory. Thus, a diet rich in omega-3 fatty acids is recommended for those with heart disease risk. Omega-3 fatty acids are primarily found in fish or flax and chia seeds, not vegetable oil, which is rich in omega-6 fatty acids.

Example Question #2 : Lipid Structures And Functions

Micelle formation is a result of which of these?

Possible Answers:

Hydrophobic interactions with the carbon chain end of free fatty acids

Hydrogen bonding between water and the polar head of free fatty acids

Hydrogen bonding between water and the carbon chain end of free fatty acids

Hydrophobic interactions with the polar head of free fatty acids

Covalent bonding between fatty acids and water

Correct answer:

Hydrophobic interactions with the carbon chain end of free fatty acids

Explanation:

The hydrocarbon tail of free fatty acids is nonpolar, and so it interacts unfavorably with water molecules.  So, when free fatty acids are placed in a polar environment, the nonpolar tails are driven together and inward in order to avoid water molecules.  A micelle is formed from the circular formation of free fatty acids. 

Example Question #11 : Lipid Structures And Functions

Increased unsaturation in a fatty acid results in __________.

Possible Answers:

no change in membrane fluidity

an increase or decrease in fluidity dependent of the temperature

decreased membrane fluidity

no change with respect to membrane fluidity

increased membrane fluidity

Correct answer:

increased membrane fluidity

Explanation:

A greater degree of unsaturation results in a disruption of van der Waals forces between fatty acid chains. This is due to the "kinks" that occur within an unsaturated fatty acid. It is easy to visualize a fatty acid membrane with kinks being less tightly packed, and therefore more fluid. This increase in fluidity is independent of temperature.

Example Question #12 : Lipid Structures And Functions

Which of the following is true of an omega-6 fatty acid?

Possible Answers:

It has six double bonds located throughout the fatty acid

It has a single bond at the sixth position, counting from the carboxyl end of the fatty acid

It has a single bond at the sixth position, counting from the methyl end of the fatty acid

It has a double bond at the sixth position, counting from the methyl end of the fatty acid

It has a double bond at the sixth position, counting from the carboxyl end of the fatty acid

Correct answer:

It has a double bond at the sixth position, counting from the methyl end of the fatty acid

Explanation:

The rules for fatty acid nomenclature state that Omega 6 means that a double bond is located at the sixth position of the fatty acid - counting from the methyl end.

Example Question #11 : Fatty Acids

Which of the following statements about lipids is incorrect?

Possible Answers:

Myristic acid is saturated with 14 carbons and 0 double bonds

Linolenic acid's nomenclature is C18:29,12

Lipid double bonds are almost always cis

None of these

Biological waxes are nonpolar esters of fatty acids and long chain alcohols

Correct answer:

Linolenic acid's nomenclature is C18:29,12

Explanation:

Linoleic acid's nomenclature is C18:29,12. Linolenic acid's nomenclature is C18:39,12,15. They both have 18 carbons but differ by one extra double bond in linolenic acid. 

Example Question #1 : Glycerolipids And Triglycerides

In fat, which glycolysis metabolite is a reactant in single-step synthesis of glycerol 3-phosphate?

Possible Answers:

Phosphoenolpyruvate

3-phosphoglycerate

Dihydroxyacetone phosphate

Glyceraldehyde 3-phosphate

Pyruvate

Correct answer:

Dihydroxyacetone phosphate

Explanation:

Glycerol phosphate can be created by the glycerol phosphate shuttle. Insects use this process in their muscles for flying, because they require quick ATP synthesis. To generate that ATP, the NADH synthesized during glycolysis ends up being regenerated into , and is shuttled over to the mitochondria to participate immediately in oxidative phosphorylation. Via the glycerol phosphate shuttle, NADH reduces dihydroxyacetone phosphate directly into glycerol phosphate, in a reaction that is catalyzed by glycerol 3-phosphate dehydrogenase. The metabolite here that can be recognized from glycolysis is the dihydroxyacetone phosphate. Phosphoenolpyruvate, pyruvate, and 3-phosphoglycerate are all indeed glycolysis metabolites, but none could be easily reduced to glycerol 3-phosphate. If you guessed glyceraldehyde 3-phosphate, you’d be close, because structurally, it appears that it could also be directly reduced to glycerol phosphate. Indeed, during glycolysis, it is interconverted to dihydroxyacetone phosphate. But the glycerol phosphate shuttle uses dihydroxyacetone phosphate; hence, the answer is dihydroxyacetone phosphate.

Example Question #175 : Biochemistry

Which component of a phospholipid imparts a charge upon the macromolecule and therefore makes the head hydrophilic? 

Possible Answers:

Glycerol backbone

Unsaturated fatty acid

Ketone group

Phosphate group

Saturated fatty acid

Correct answer:

Phosphate group

Explanation:

Phospholipids are amphipathic, meaning they have an end that is hydrophobic (the fatty acid tail) and an end that is hydrophilic (the head). Phosphate groups have a negative charge, thus attracting them to water, and the presence of a phosphate group at the head of a phospholipid makes that head hydrophilic. Glycerol itself polarizes a fatty acid, but the glycerol is located in the head, not the backbone, and is not charged like phosphate.

Example Question #176 : Biochemistry

In sufficient concentrations, one-tailed phospholipids will form a __________ in solution.

Possible Answers:

micelle

vesicle

liposome

bilayer

Correct answer:

micelle

Explanation:

The polar head groups and the hydrocarbon tails will separate themselves in such a way that one-tailed phospholipids will form micelles, whereas two-tailed phospholipids will form a bilayer (liposome). 

Example Question #1 : Phospholipids

Which of the following is false about phospholipids?

Possible Answers:

The fluidity of membranes composed of phospholipids is affected by the length and degree of saturation of fatty acid tails.

Their hydrophilic tails are exposed to the interior of membranes, and their hydrophobic heads to the exterior.

When, in water, they form a sphere, it is called a micelle.

Typically, at least one tail is saturated and another tail is unsaturated.

If a phospholipid bilayer slightly breaks apart, the lipids rearrange on their own to refill the layer. 

Correct answer:

Their hydrophilic tails are exposed to the interior of membranes, and their hydrophobic heads to the exterior.

Explanation:

Phospholipids have two hydrocarbon tails, and one is indeed usually relatively saturated (has no double bonds) and the other relatively saturated (has double bonds). The degree of saturation, as well as the length, influences membrane fluidity; more cis-double bonds pack together less tightly, and decrease fluidity. In order to minimize free energy, the hydrophobic parts of phospholipids rearrange to refill a bilayer if it happens to break. In water, phospholipids can form a membrane, or a sphere called a micelle in which the hydrophobic tails pack together. Note that the tails are hydrophobic, and the heads are hydrophilic; tails are oriented toward the interior of a bilayered membrane.

Example Question #2 : Phospholipids

Why do phospholipids and glycolipids form bilayers rather than micelles when placed in an aqueous media?

Possible Answers:

Phospholipids and glycolipids are far less unsaturated than free fatty acids

Hydrophobic interactions do not drive the fatty acid hydrocarbons inwards in phospholipids and glycolipids as they do for free fatty acids

Phospholipids and glycolipids do not have hydrocarbon tails, and so there is nothing to drive them towards micelle formation

None of these

The two fatty acid chains present in glycolipids/phospholipids are too bulky to form micelles 

Correct answer:

The two fatty acid chains present in glycolipids/phospholipids are too bulky to form micelles 

Explanation:

Phospholipids and glycolipids have two hydrocarbon chains, whereas free fatty acids only have one.  The extra carbon tail, in combination with the unsaturation of these types of lipids (double bonds present) makes them far bulkier than free fatty acids.  The extra bulk disallows micelle formation, and bilayers (liposomes) form instead.

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