Architecture and Properties of the Lipid Bilayer
A biological membrane is a complex system based on a double lipid layer (bilayer). Lipids and membrane proteins are held together exclusively by non-covalent interactions.
The main feature of membrane lipids is their amphipathicity. The molecule has:
- A polar hydrophilic "head", which 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, determining the fluidity of the structure and the ability of proteins to change conformation.
The main functions of lipids:
- Structural: forming the basis of the membrane.
- Microenvironment formation: creating optimal conditions for the function of integral and peripheral proteins.
- Regulatory: influencing the activity of enzymatic systems.
- Anchoring: fixing surface proteins.
- Signaling: transmitting 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 residues and a phosphoric acid residue. An important nuance: the second carbon atom of glycerol is typically bound to a polyunsaturated fatty acid.
Various hydrophilic molecules can attach to the phosphate group, forming specific classes:
- Phosphatidylcholine — a basic structural element of membranes.
- Phosphatidylserine — an obligate cofactor for protein kinase C activation, also playing an important role in apoptosis and blood coagulation.
- 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 residue to its amino group forms ceramide — the precursor molecule for all complex sphingolipids.
Depending on the attached polar "head", they are divided into:
- Sphingophospholipids (sphingomyelins). The polar part contains a phosphoric acid residue and choline (or ethanolamine or serine). They are particularly abundant in the nervous system structures.
- Glycolipids. The polar group is a carbohydrate residue. They are subdivided into cerebrosides (containing a linear mono- or oligosaccharide, e.g., galactose) and gangliosides (containing a branched oligosaccharide with the mandatory inclusion of N-acetylneuraminic acid — NANA).
Cholesterol and Fluidity Regulation
Cholesterol is a vital component of animal cell membranes. Its maximum content is characteristic of the plasma membrane (the molar ratio with other lipids varies from 0.3 to 0.9).
The cholesterol molecule consists of a rigid hydrophobic nucleus with a hydrocarbon chain and a small polar hydroxyl group (-OH). Cholesterol inserts into 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:
- Decreases fluidity and increases membrane rigidity by restricting the mobility of fatty acid chains.
- Reduces the possibility of lateral diffusion of proteins.
- Performs an adaptive function: with an excessive increase in fluidity (e.g., due to lipid peroxidation or the action of lipophilic substances), the proportion of cholesterol increases compensatorily.
Signaling Function: The Example of $PIP_2$
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 critical components:
- Diacylglycerol (DAG) — remains in the membrane and directly activates protein kinase C.
- 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.