Nature and molecular structure
The main distinguishing feature of phospholipids is their amphiphilicity. This property is due to the fact that the molecule is structurally divided into two poles with opposite physicochemical characteristics:
- Hydrophobic part: formed by fatty acid radicals. This region of the molecule "avoids" contact with water.
- Hydrophilic part: includes a phosphoric acid residue attached to an amino alcohol or amino acid. This part, conversely, interacts well with the aqueous environment.
It is precisely this dual nature of the molecule that predetermines the ability of phospholipids to form complex spatial structures in the body.
Supramolecular structures
Depending on environmental conditions and functional tasks, phospholipids organize into strictly defined spatial forms:
- Bilayer structures. This is the classic form of phospholipid organization, underlying absolutely all cell membranes. The molecules align in two rows so that their hydrophobic tails are hidden inside the layer, while the hydrophilic heads face the aqueous environment (inside the cell and into the intercellular space).
- Monomolecular layers. Phospholipids are capable of forming a single hydrophilic layer on the surface of lipoproteins. This shell is critical for the functioning of the circulatory system: it allows hydrophobic lipids to be safely and efficiently transported with the bloodstream, isolating them from the plasma.
Classification of phospholipids
Depending on which polyhydric alcohol forms the basis of the molecule, phospholipids are divided into two large groups:
- Glycerophospholipids. In this case, the carbon skeleton of the molecule is formed by the trihydric alcohol glycerol.
- Sphingolipids. These compounds are based on the complex amino alcohol sphingosine. They are ubiquitous in cell membranes. An important feature of sphingolipids is that in the outer cell membranes, they act as an integral component of cell antigens.
Sphingophospholipids (sphingomyelins)
Sphingomyelins are a specific and extremely important subgroup of sphingolipids.
The structural diagram of sphingomyelin is as follows: the molecule is assembled from sphingosine, to which a fatty acid radical, a phosphate group, and choline are attached.
The biological role of sphingomyelins is enormous, as they are the main structural components of nervous tissue cell membranes. Their predominant localization in the human body includes:
- Myelin sheaths of neurons (acting as electrical insulation, ensuring rapid nerve impulse propagation).
- Gray matter of the brain.
Briefly: answers to topical questions
What are the main representatives (classes) of glycerophospholipids?
The main representatives of glycerophospholipids, classified by the type of hydrophilic group attached to the phosphate, are: phosphatidylserine, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol 4,5-bisphosphate.
- Phosphatidylcholine is a major membrane component.
- Phosphatidylserine is involved in apoptosis and blood clotting; it is an essential cofactor for protein kinase C activation.
- Phosphatidylethanolamine is listed among glycerophospholipids that accumulate in the cell upon damage.
- Phosphatidylinositol 4,5-bisphosphate is a key component of signaling pathways; under the action of phospholipase C, it is cleaved to form DAG and IP3.
Which enzymes carry out the hydrolysis of phospholipids in tissues and the GI tract?
Hydrolysis or cleavage of phospholipids in the cited sources is associated with the following enzymes:
- Phospholipase A is a lipolytic pancreatic enzyme; it acts on phospholipids and requires the participation of trypsin for activation. Prophospholipase is also mentioned as a lipolytic enzyme activated by trypsin.
- Phospholipase C cleaves membrane phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate and diacylglycerol.
- Acid phosphatase hydrolyzes phosphoric acid esters; it is localized, in particular, in the acrosome of spermatozoa, where it participates in the cleavage of oocyte plasmalemma phospholipids during fertilization.
- Upon cell damage, the activation of hydrolases—lipases and phospholipases—is described.
What is the role of phosphatidylinositol in intracellular signal transduction?
Phosphatidylinositol 4,5-bisphosphate is the substrate of the inositol phosphate signaling pathway and a key component of signaling pathways. During G-protein-coupled signal transduction, phospholipase C is activated. It hydrolyzes phosphatidylinositol 4,5-bisphosphate to form:
- Inositol 1,4,5-trisphosphate — it binds to receptors on the endoplasmic reticulum membrane, opens calcium channels, and increases the cytosolic Ca2+ concentration.
- Diacylglycerol — together with Ca2+ and phosphatidylserine, it activates protein kinase C.
Subsequently, the activation of protein kinases and protein phosphorylation lead to changes in the functional activity of proteins.
For quick memorization of sphingomyelin components, use the acronym SFCh: Sphingosine + Fatty acid + Phosphate group + Choline.
Class and exam questions
What do the "amphiphilic properties" of phospholipids mean?
This means that the molecule simultaneously contains a hydrophobic part (fatty acid radicals) and a hydrophilic part (a 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 necessary for the transport of hydrophobic lipids by the blood.
How do glycerophospholipids differ from sphingolipids?
The structural basis of glycerophospholipids is the alcohol glycerol, while the basis of sphingolipids is the amino alcohol sphingosine.
Where are sphingomyelins predominantly localized?
They are concentrated in nervous tissue: they are the main components of the myelin sheaths of neurons and are part of the brain's gray matter.
What else to study on this topic
- Mechanism of cell membrane bilayer structure formation
- Structure and functions of glycerophospholipids
- The role of sphingolipids in the formation of cell antigens
- Transport of hydrophobic lipids as part of lipoproteins
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