Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A. N. Ogurtsov 2011

Components of Biomolecular Complexes
Lipids and Biomembranes

Lipids are naturally occurring compounds extracted from plant or animal Tissues using nonpolar Solvents (such as ether, benzene, or chloroform) that are insoluble in Water.

They include products of the interaction between Fatty acids and alcohols (Simple Lipids), amino alcohols and Other Compounds (Complex Lipids), Prostaglandins, and isoprenoid lipids (such as carotenoids, chlorophyll, and Vitamins E and K).

Triacylglycerols are simple lipids consisting of esters formed by the trihydric alcohol glycerol and three fatty acids (Figure 55(a)). They vary in type depending on The Nature of the three fatty acid residues attached to the hydroxyl groups of glycerol.

Fatty acids also serve as the foundation of Phospholipids, which make up Plasma Membranes. Fatty acids consist of a hydrocarbon chain and a carboxyl group (COOH). They vary in length, although cellular fatty acids predominantly contain an even number of carbon atoms, typically 14, 16, 18, and 20. The principal fatty acids of phospholipids are listed in Table 2.

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Figure 55 - Diagram of bond orientation in lipids: a - triacylglycerol; b - cis-, c - trans-stereoisomeric configurations relative to the double C=C bond

Fatty acids are usually designated as Cx:y, where x is the number of carbon atoms in the chain and y is the number of double bonds. Fatty acids lacking double C=C bonds are termed saturated, whereas those containing at least one double bond are termed unsaturated.

Table 2 - Principal fatty acids of phospholipids

Arachidonic (C20:4), linoleic (C18:2), and linolenic (C18:3) fatty acids are termed essential because they cannot be synthesized in mammalian Cells and must be obtained from the diet. Two stereoisomeric configurations are possible around each double C=C bond (Figure 55(b, c), Figure 56(b)). A cis-double bond introduces a rigid bend (kink) into the otherwise flexible hydrocarbon "tail" of the fatty acid.

The primary function of triacylglycerols in living organisms is energy storage. To achieve this, they accumulate in the Cytosol of cells within storage tissues (such as subcutaneous adipose tissue) as a microdispersed emulsion of oil droplets, which can occupy nearly the entire volume of the storage Cell.

Figure 56 - Effect of a double C=C bond on fatty acid conformation: a - palmitate (ionized form of palmitic acid); b - oleate (ionized form of oleic acid)

On a per-weight basis, triacylglycerols store twice as much energy as CARBOHYDRATES. When accumulated in large amounts, they can sustain an Organism's METABOLISM/26.html">Energy Metabolism for several weeks, whereas Glycogen reserves can only supply energy for roughly a day.

However, the energy stored in carbohydrates becomes accessible for the organism to use more rapidly than that stored in triacylglycerols. Therefore, Polysaccharides (starch and glycogen) and fats (triacylglycerols) function as short-term and long-term energy storage systems, respectively.

Additionally, triacylglycerols can serve as thermal insulation and a source of metabolic water: The oxidation of triacylglycerols yields twice as much water as that of carbohydrates, a mechanism utilized by desert animals.

A thick subcutaneous layer of adipose tissue reliably protects seals, walruses, penguins, and other polar homeothermic animals from the cold.

Phospholipids are complex lipids structurally similar to triacylglycerols in that they contain fatty acids linked to glycerol. However, in phospholipids, glycerol is esterified to only two fatty acid chains rather than three, with the remaining free position occupied by a phosphate group, which in turn is linked to a polar alcohol molecule that varies among different phospholipids. The polar alcohol forms the "HEAD" of the phospholipid molecule, to which two nonpolar "tails" are attached via the glycerol backbone (Figure 57).

Figure 57 - Phosphatidylcholine, a typical phospholipid

Notably, the largest spatial structures formed within a cell are not constructed from macromolecules such as Proteins or Nucleic Acids, but rather from the aggregation of lipids into Biomembranes—a distinct liquid phase. The lipids utilized by The Cell are structured to undergo spontaneous aggregation, thereby establishing the cellular infrastructure.

Lipid molecules combine two opposing chemical properties. They simultaneously contain a polar or even charged hydrophilic atomic group that tends to dissolve in water, and one or more hydrophobic hydrocarbon chains that are excluded from water (Figure 57). Such molecules are termed amphiphilic.

The dual nature of lipid molecules drives their self-assembly into membrane structures, in which the charged (or polar) heads face the aqueous phase, while the hydrocarbon tails are packed in the interior of the membrane (Figure 58).

Figure 58 - Phospholipid (phosphatidylserine) and Cholesterol molecules driving the self-assembly of a biomembrane. Dark gray areas indicate the polar Regions of the molecules

At the same time, the primary Intermolecular Forces driving the self-assembly of lipids into biomembranes are non-specific non-covalent interactions: Van der Waals, electrostatic, Hydrogen Bonds, and hydrophobic interactions.

The most common types of lipids in nature are Phospholipids and Glycolipids. Their structure is based on a glycerol molecule containing three hydroxyl groups, which can be replaced by three other groups. Typically, two of these are fatty acids attached to glycerol via a carboxyl group. Instead of the third hydroxyl group, a phosphate group or another polar (or charged) group is attached to the glycerol. If the fatty acid tails contain unsaturated carbon-carbon bonds, rigid kinks are formed at those sites (Figure 56(6)).

Fatty acid tails containing kinks pack much less tightly into an ordered structure; therefore, a biomembrane with unsaturated hydrocarbon chains in its lipids has a lower phase transition Temperature from the physiological liquid-crystalline state to the frozen gel state compared to a biomembrane with saturated C–C bonds.

Cholesterol and other sterols have a different structure. They consist of several rigidly linked hydrophobic hydrocarbon rings whose length is comparable to that of phospholipid hydrocarbon tails. A hydroxyl group at one end provides hydrophilicity, orienting the cholesterol molecule within the membrane. Cholesterol is incorporated into membranes in varying proportions to modify their properties. Because the cholesterol molecule is rigid, its presence inhibits (restricts) the mobility of neighboring lipids, thereby increasing membrane viscosity and making it less permeable to small molecules.

Lipids are utilized by cells to construct Cellular Membrane Structures.

Biomembranes are impermeable to ions and large molecules—ranging from sugars to Polypeptides—yet they are readily permeable to water, oxygen, and nitrogen molecules. Notably, molecules containing numerous carbon atoms can also cross membranes quite easily. This is precisely why alcohol disperses rapidly throughout the body, crossing all biological barriers.

When shaken in water or aqueous solutions, lipid molecules spontaneously form micelles, in which the nonpolar hydrocarbon tails are shielded from water, while the polar heads reside On the surface of the particle, interacting with the aqueous environment (Figure 59(a)).

Figure 59 – Cross-section of three structures formed by lipids In aqueous solutions: a – micelle; b – liposome; c – planar lipid bilayer

At the interface between two aqueous phases, lipids spontaneously form bilayers.

Within bilayers, the hydrocarbon "tails" point inward, away from the aqueous phase, forming a continuous hydrophobic hydrocarbon core, while the hydrophilic "heads" positioned on the outside are immersed in the aqueous solution (Figure 59(b)).

Upon vigorous agitation, these bilayers give rise to Liposomes—closed vesicles enclosed by a continuous lipid bilayer (Figure 59(6)).

The tight packing of lipid molecules in biomembranes is maintained by a combination of hydrophobic forces from the aqueous environment and van der Waals interactions among the lipid molecules themselves. The Lipid Bilayer serves as the primary structural component of all biomembranes.

Although other molecules—such as cholesterol, glycolipids, and proteins—are present in membranes, it is the hydrophobic hydrocarbon core formed by the lipid molecules that performs the fundamental barrier function of biomembranes, which in turn ensures the compartmentalization of cellular substructures.

Review Questions

1. Write down the general formula for carbohydrates.

2. What are the Similarities and differences between ribose and deoxyribose?

3. What are the similarities and differences between α- and β-glycosidic bonds?

4. What are the similarities and differences between glycogen and Cellulose?

5. What are the similarities and differences between glycogen and starch?

6. What are Glycoproteins, and how do they differ from Proteoglycans?

7. What are the similarities and differences between glycogen and triacylglycerols?

8. List the major fatty acids found in phospholipids.

9. What are the similarities and differences between phospholipids and triacylglycerols?

10. Which intermolecular interactions drive The formation of biomembranes?