Question

Two problems encountered with oral or intravenous administration of drugs are the lack of tissue specificity in the action of the drug and rapid metabolism, and therefore limited period of effectiveness, of some drugs. One attempt to circumvent these problems is the use of liposomes to encapsulate the drugs. Some drugs have a longer period of effectiveness when administered this way. Liposomes can be prepared with specific proteins to bind specific cellular membrane receptors. Liposomes are also useful as a research tool to study the properties of biological membranes since they have a similar structure and properties. Much of our understanding of biological membranes has been obtained using liposomes. Which of the following statements concerning membranes is correct? A. Microdomains, called lipid rafts, are fixed in position in membranes. B. Lipid composition of the two layers of the membrane equilibrate. C. As demonstrated with liposomes, the membrane is most fluid at the surfaces. D. An increase in the cholesterol content of a membrane increases membrane fluidity. E. Lipid transporters catalyze unidirectional movement of specific lipids from one layer to the other.

Short answer

A) Lipid rafts are stationary and fixed in position. B) The lipid composition of the two layers equilibrates. C) Liposomes are used to study membrane properties, and they show that the membrane is most fluid at the surfaces. D) An increase in cholesterol content increases membrane fluidity. E) Lipid transporters can move specific lipids unidirectionally between the bilayer's two layers. Answer: E) Lipid transporters can move specific lipids unidirectionally between the bilayer's two layers.

Step-by-step solution

Evaluating Statement A

We know that lipid rafts are cholesterol- and sphingolipid-enriched microdomains in membranes that serve as platforms for signaling molecules. These lipid rafts are dynamic and can float through the membrane, meaning they are not fixed in position. So, statement A is false.

Evaluating Statement B

The lipid composition of the two layers of the membrane, also known as leaflets, does not equilibrate because each leaflet has its own unique lipid distribution. This distribution is critical to the membrane's structure and function. Therefore, statement B is false.

Evaluating Statement C

Liposomes can indeed be used to study the properties of biological membranes. However, the statement claims that the membrane is most fluid at the surfaces, which is not necessarily true. The fluidity of a membrane can be influenced by various factors, and fluidity is not limited to the surfaces alone. Therefore, statement C is false.

Evaluating Statement D

Cholesterol has a complex effect on membrane fluidity. In general, the presence of cholesterol tends to decrease membrane fluidity by filling gaps between phospholipids. So, statement D is false, as an increase in cholesterol content does not increase membrane fluidity.

Evaluating Statement E

Lipid transporters are proteins that facilitate the movement of specific lipids between the two leaflets of membranes. They can catalyze the unidirectional or bidirectional movement of lipids, depending on the type of transporter. This movement is essential for maintaining the asymmetrical distribution of lipids between the two leaflets. As a result, statement E is the correct one, as lipid transporters can catalyze the unidirectional movement of specific lipids from one layer to the other.

Key concepts

Biological Membrane Fluidity

The fluid nature of biological membranes is a key factor in their function. This fluidity allows for the dynamic movement of membrane proteins and lipids, which is critical for various cellular processes such as signaling, transport, and cell communication.

The fluidity of a membrane is influenced by its lipid composition and temperature. Membranes composed of unsaturated fatty acids are more fluid than those composed of saturated fatty acids because the kinks in the unsaturated fatty acid tails prevent the lipids from packing tightly together. Furthermore, cholesterol is a double-edged sword in this context; it can both increase and decrease membrane fluidity depending on its concentration and the temperature.

Lipid Rafts and Fluidity

Lipid rafts are an example of membrane microdomains that are more ordered and less fluid than the surrounding membrane. They contain high concentrations of cholesterol and sphingolipids and serve as organizing centers for the assembly of signaling molecules, influencing the fluidity and functionality of the membrane.

Liposome Preparation

Liposomes are vesicles composed of one or more lipid bilayers surrounding an aqueous core, and they are widely employed in research and medicine to deliver drugs to specific body tissues. Preparing liposomes involves several steps to ensure that they are suited for their intended use.

One common method is the thin film hydration technique, which starts with dissolving lipids in an organic solvent followed by evaporating the solvent to form a thin lipid film. This film is then hydrated with a buffer, leading to the formation of liposomes. The resulting vesicles can be further processed through methods such as sonication or extrusion to obtain liposomes of desired size and homogeneity.

Targeting Specific Tissues

To target specific tissues, liposomes can be engineered with surface proteins that bind specific cellular membrane receptors. This kind of specificity ensures that the encapsulated drugs are delivered predominantly to the site of interest, reducing the side effects associated with systemic drug distribution.

Lipid Transporters

Lipid transporters are essential proteins that manage the lateral movement of lipids across the membrane bilayers. They play a crucial role in maintaining the asymmetric distribution of lipids between the inner and outer leaflets of the biological membrane—a feature important for various cellular functions, including cell signaling and maintaining a membrane's integrity.

Among these transporters, flippases and floppases are well-known types. Flippases typically consume ATP to move lipids from the outer to the inner leaflet, while floppases transfer lipids in the opposite direction. Both types of lipid transporters help sustain membrane asymmetry by ensuring that certain lipids like phosphatidylserine and phosphatidylethanolamine are maintained predominantly in the inner leaflet, and others like sphingomyelin and phosphatidylcholine in the outer leaflet.

These transporters operate in a regulated manner, facilitating unidirectional (or sometimes bidirectional) movement, and are integral to maintaining the dynamic nature of the cell membrane.