Biochemistry : Phospholipids

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #14 : Lipid Structures And Functions

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

Possible Answers:

Glycerol backbone

Ketone group

Unsaturated fatty acid

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 #1 : Phospholipids

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

Possible Answers:

liposome

bilayer

micelle

vesicle

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 #2 : Phospholipids

Which of the following is false about phospholipids?

Possible Answers:

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

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

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

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

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 #3 : Phospholipids

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

Possible Answers:

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

Phospholipids and glycolipids are far less unsaturated than free fatty acids

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

None of these

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

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.

Example Question #4 : Phospholipids

Which of the following is not a component of a typical phospholipid?

Possible Answers:

Ester group

Glycerol

Phosphate group

Carbon chain

Amino group

Correct answer:

Amino group

Explanation:

A typical phospholipid is made up of a glycerol with two fatty acid carbon chains stemming off of it. On the other side of the glycerol a phosphate group is connected, an ester group connects the tail to the head. An amino group, however, is not present.

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