Biological membrane lipids

Architecture and properties of the lipid bilayer

A biological membrane is a complex system based on a lipid bilayer. Lipids and membrane proteins are held together exclusively by non-covalent interactions.

The main feature of membrane lipids is their amphiphilicity. The molecule has:

• A polar hydrophilic "head" that contacts the aqueous environment outside and inside the cell.
• Hydrophobic "tails" hidden from water within the bulk of the bilayer.

The ratio of fatty acids in the "tails" is of critical importance. Unsaturated fatty acids predominate and determine the fluidity of the structure, as well as the ability of proteins to change their conformation.

Main functions of lipids:

• Structural: formation of the membrane framework.
• Matrix-forming: creation of optimal conditions for the functioning of integral and peripheral proteins.
• Regulatory: influence on the activity of enzymatic systems.
• Anchoring: fixation of surface proteins.
• Signaling: transmission of hormonal signals into the cell.

Glycerophospholipids

This is the most common group of membrane lipids. Their chemical basis is phosphatidic acid (diacylglycerol phosphate). It consists of glycerol attached to two fatty acid radicals and a phosphoric acid residue. An important nuance: polyunsaturated fatty acid is typically bound to the second carbon atom of glycerol.

Various hydrophilic molecules can attach to the phosphate group, forming specific classes:

1. Phosphatidylcholine — the basic structural element of membranes.
2. Phosphatidylserine — an essential cofactor for protein kinase C activation, also playing an important role in apoptosis and blood clotting.
3. Phosphatidylinositol 4,5-bisphosphate ($PIP_2$) — a key substrate for generating intracellular signals.

Sphingolipids and their derivatives

Unlike glycerophospholipids, this group is based not on glycerol, but on the amino alcohol sphingosine. Attachment of a fatty acid radical to its amino group forms ceramide, the precursor molecule for all complex sphingolipids.

Depending on the attached polar "head", the following are distinguished:

• Sphingophospholipids (sphingomyelins). The polar part contains a phosphoric acid residue and choline (or ethanolamine or serine). They are particularly abundant in the structures of the nervous system.
• Glycolipids. The polar group is a carbohydrate residue. They are divided into cerebrosides (containing a linear mono- or oligosaccharide, e.g., galactose) and gangliosides (containing a branched oligosaccharide with the obligatory inclusion of N-acetylneuraminic acid — NANA).

Cholesterol and fluidity regulation

Cholesterol is an essential component of animal cell membranes. Its maximum content is characteristic of the plasma membrane (the molar ratio to other lipids varies from 0.3 to 0.9).

A cholesterol molecule consists of a rigid hydrophobic core with a hydrocarbon chain and a small polar hydroxyl group (-OH). Cholesterol is embedded in the hydrophobic zone of the bilayer parallel to the phospholipid "tails", with its -OH group oriented toward the aqueous phase (the size and charge of this group are significantly smaller than those of phospholipid "heads").

Biological role of cholesterol:

• Reduces fluidity and increases membrane rigidity by restricting the mobility of fatty acid chains.
• Decreases the potential for lateral diffusion of proteins.
• Performs an adaptive function: when fluidity increases excessively (e.g., due to peroxidation or the action of lipophilic substances), the proportion of cholesterol increases compensatorily.

Signaling function using $PIP_2$ as an example

Changes in lipid structure can serve as a powerful signal for the cell. A classic example is the hydrolysis of phosphatidylinositol 4,5-bisphosphate ($PIP_2$) by the enzyme phospholipase C.

The reaction yields two major components:

1. Diacylglycerol (DAG) — remains in the membrane and directly activates protein kinase C.
2. Inositol 1,4,5-trisphosphate ($IP_3$) — travels to the endoplasmic reticulum (ER), where it acts as a key that opens $Ca^{2+}$ channels and releases calcium into the cytosol.

Class and exam questions

What else to study on this topic

• Organization of the glycocalyx: carbohydrate components on the outer surface of the membrane
• Interaction of peripheral and integral proteins with the lipid matrix
• Mechanisms of lateral diffusion of membrane components
• Membrane adaptation during lipid peroxidation

Other topics in «Structure and Functions of Biological Membranes»

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