Phospholipids

Phospholipida

For medical students2 min readUpdated 2026-10-10

Phospholipids are complex lipid molecules characterized by pronounced amphipathic properties. They serve as a crucial building block of biological structures, forming the bilayers of cell membranes and ensuring the transport of insoluble substances in the bloodstream.

Cellular BasisForm the bilayer structures of biological membranes

TransportForm a monolayer on the surface of lipoproteins

Nervous SystemSphingomyelins build myelin sheaths

AmphipathicityContain both hydrophilic and hydrophobic regions

Nature and Molecular Structure

The primary defining feature of phospholipids is their amphipathicity. This property stems from the molecule being structurally divided into two poles with opposite physicochemical characteristics:

Hydrophobic region: Formed by fatty acid hydrocarbon chains. This part of the molecule avoids contact with water.
Hydrophilic region: Comprises a phosphoric acid residue attached to an amino alcohol or an amino acid. This part readily interacts with aqueous environments.

This dual nature of the molecule allows phospholipids to form complex spatial structures within the body.

Supramolecular Structures

Depending on environmental conditions and functional requirements, phospholipids organize into specific spatial forms:

Bilayer structures. This is the classic organizational form underlying virtually all cell membranes. Molecules arrange themselves in two rows such that their hydrophobic tails are tucked inward, while their hydrophilic heads face the aqueous environment (the cell interior and extracellular space).
Monolayers. Phospholipids can form a single hydrophilic layer on the surface of lipoproteins. This shell is critical for the circulatory system, allowing hydrophobic lipids to be safely and efficiently transported through the bloodstream while isolating them from plasma.

Classification of Phospholipids

Depending on the polyhydric alcohol core of the molecule, phospholipids are divided into two major groups:

Glycerophospholipids. The backbone of these molecules is the three-carbon alcohol glycerol.
Sphingolipids. These compounds are based on the complex amino alcohol sphingosine. They are widely distributed in cell membranes. A key feature of sphingolipids is that in outer cell membranes, they serve as integral components of cellular antigens.

Sphingophospholipids (Sphingomyelins)

Sphingomyelins are a specific and highly important subclass of sphingolipids.

The structural scheme of sphingomyelin consists of sphingosine attached to a fatty acid residue, a phosphate group, and choline.

Their biological role is immense, as they are major structural components of neural tissue membranes. Their primary localization in the human body includes:

Myelin sheaths of neurons (providing electrical insulation for rapid nerve impulse conduction).
Gray matter of the brain.

Mnemonic

To quickly remember the components of sphingomyelin, use the mnemonic SFPC: Sphingosine + Fatty acid + Phosphate group + Choline.

Frequently asked questions

What are the main classes of glycerophospholipids?

The main representatives of glycerophospholipids, classified by the hydrophilic head group attached to the phosphate, include: phosphatidylserine, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol 4,5-bisphosphate.

Phosphatidylcholine — major membrane component.
Phosphatidylserine — involved in apoptosis and blood coagulation; serves as an essential cofactor for protein kinase C activation.
Phosphatidylethanolamine — accumulates in cells during injury.
Phosphatidylinositol 4,5-bisphosphate — a key signaling molecule; cleaved by phospholipase C to yield DAG and IP3.

Which enzymes hydrolyze phospholipids in tissues and the GI tract?

Phospholipid hydrolysis and cleavage involve several enzymes:

Phospholipase A — a pancreatic lipolytic enzyme acting on phospholipids, requiring trypsin for activation.
Phospholipase C — cleaves membrane phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate and diacylglycerol.
Acid phosphatase — hydrolyzes phosphoric acid esters; localized in the sperm acrosome, where it participates in breaking down oocyte plasma membrane phospholipids during fertilization.
Cell injury triggers the activation of endogenous hydrolases, including lipases and phospholipases.

What is the role of phosphatidylinositol in intracellular signaling?

Phosphatidylinositol 4,5-bisphosphate is the substrate for the inositol phospholipid signaling pathway. During G protein-coupled receptor activation, phospholipase C is activated. It hydrolyzes phosphatidylinositol 4,5-bisphosphate to form:

Inositol 1,4,5-trisphosphate (IP3) — binds to receptors on the endoplasmic reticulum membrane, opening calcium channels and increasing cytosolic Ca2+ concentration.
Diacylglycerol (DAG) — synergistically with Ca2+ and phosphatidylserine, activates protein kinase C.

Subsequent protein kinase activation and protein phosphorylation alter cellular protein activity.

What does the 'amphipathic properties' of phospholipids mean?

It means the molecule contains both a hydrophobic region (fatty acid tails) and a hydrophilic region (phosphoric acid residue with an amino alcohol or amino acid).

What role do phospholipids play in lipoproteins?

They form a hydrophilic monolayer on the surface of lipoproteins, which is essential for transporting hydrophobic lipids through the blood.

How do glycerophospholipids differ from sphingolipids?

Glycerophospholipids are built upon a glycerol backbone, whereas sphingolipids are based on the amino alcohol sphingosine.

Where are sphingomyelins predominantly localized?

They are concentrated in neural tissue: they are the main components of neuronal myelin sheaths and are present in the gray matter of the brain.

Go deeper

Mechanism of cell membrane bilayer formation
Structure and functions of glycerophospholipids
Role of sphingolipids in cellular antigen formation
Transport of hydrophobic lipids within lipoproteins