Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Membrane Lipids
Lipid Composition of Membranes

Different Cell membranes vary in their Lipid Composition. The specific functional and Physical Properties of various Introduction/5.html">Eukaryotic Cell membranes are determined by the unique protein-lipid composition of each membrane. For example, Table 2 shows the lipid composition of various membranes.

Class="center">Table 2 - Lipid composition (%) of some Biomembranes. Lipid designations are given in section 2.2

As seen from the last row of the table, Phospholipids and Sphingolipids are distributed asymmetrically between the two monolayers of The Lipid Bilayer, whereas the cholesterol concentration is equal in both monolayers.

There are numerous factors determining differences in the lipid composition of various membranes.

For instance, the difference in composition between the Endoplasmic reticulum and Golgi apparatus membranes is mainly due to the fact that phospholipids are synthesized in The endoplasmic reticulum, whereas sphingolipids are synthesized in the Golgi apparatus. As a result, The ratio of sphingomyelin (SM) molecules to all phosphorylated Lipids in Golgi membranes is six times higher than in endoplasmic reticulum membranes.

In other cases, the translocation of membranes from one cellular compartment to another (via transport vesicles) can selectively enrich specific membranes with certain lipids.

Differences in lipid composition can also be driven by specialized Membrane Functions. For example, The Plasma Membrane of absorptive epithelial Cells lining the intestinal surface has two distinctly different specific regions: the apical surface, which faces the intestinal lumen and is exposed to significantly fluctuating "external" factors, and the basolateral surface, which contacts other epithelial cells and the extracellular Connective Tissue matrices underlying the epithelium (Figure 22).

Figure 22 - Structure OF THE intestinal epithelium

In such "polar" cells, the ratio of sphingolipids to phosphoglycerides and cholesterol in the basolateral membrane is 0.5:1.5:1—the same ratio found in the Plasma Membranes of non-polar cells that are not subject to significant environmental stressors.

Conversely, in the apical membrane of intestinal cells, which is exposed to harsh conditions, this ratio is 1:1:1. Such a relatively high concentration of sphingolipids in this membrane enhances its strength and stability through The formation of a hydrogen-bonding network between the hydroxyl groups (-OH) in the sphingolipid HEAD groups.

METABOLISM/35.html">Selection/41.html">Review Questions and Exercises

1. List the three Major Classes of lipids that make up biomembranes.

2. What are the Structural Features of phosphoglyceride molecules? Which phosphoglycerides are most common in nature?

3. What are the structural features of sphingolipid molecules? Which sphingolipids are most common in nature?

4. What are the structural features of steroid molecules? Which Steroids are most common in nature?

5. How can the lateral diffusion coefficient of lipids be determined using the FRAP method?

6. How did the FRAP method reveal the existence of lipid rafts in biomembranes?

7. How are phospholipids, sphingolipids, and cholesterol distributed between the monolayers of the bilayer?

8. What is the reason for the sharp increase in sphingomyelin concentration in Golgi membranes compared to endoplasmic reticulum membranes?

9. How does the ratio of concentrations of sphingolipids to phosphoglycerides and cholesterol differ between the apical and basolateral membranes of intestinal epithelial cells? What is the reason for this difference?