English Lectures / lipids 1 ENG

Phospholipids

Phospholipids are complex lipids, esters of polyhydric alcohols and higher fatty acids. They contain the remainder of phosphoric acid and an additional group of atoms of various chemical nature connected with it.

Classification of phospholipids

Depending on their polyhydric alcohol, it is customary to divide phospholipids into three groups:

• glycerophospholipids

(glycerophosphatides) - contain glycerol residue

phosphatidylcholine (lecithin) phosphatidylethanolamine (cephalin) phosphatidylserine

cardiolipin

plasmalogen (ethanolamine plasmologen)

• phosphosphingolipids - contain sphingosine residue

sphingomyelins

• There are also phosphoinositides - they contain an inositol residue

Properties of phospholipids

•Phospholipids are amphiphilic substances. They consist of a polar “head”, which contains glycerol or another polyhydric alcohol, a negatively charged phosphoric acid residue and often a group of atoms that carries a positive charge, and two non-polar “tails” of fatty acid residues.

•The main feature of phospholipids is that their “head” is hydrophilic, and the “tails” are hydrophobic. This makes it possible to form a bilayer when in the water column, a double layer of phospholipid molecules, where the hydrophilic heads on both sides come in contact with water and the hydrophobic tails are hidden inside the bilayer and are thus protected from contact with water.

The biological role of phospholipids

•Phospholipids are an important part of cell membranes. They provide fluid and plastic properties of cell membranes and cellular organelles, while cholesterol provides rigidity and stability of the membranes.

•Phospholipids are involved in the transport of fats, fatty acids and cholesterol. Being more hydrophilic than cholesterol, due to the presence of phosphoric acid residues in the molecule, phospholipids are a kind of “solvent” for cholesterol and other highly hydrophobic compounds.

•Phospholipids slow down collagen synthesis and increase collagenase activity.

Glycolipids

Glycolipids are complex lipids resulting from the combination of lipids with carbohydrates. In glycolipid molecules, there are polar “heads” (carbohydrate) and non-polar “tails” (residues of fatty acids). Due to this, glycolipids (together with phospholipids) are part of the cell membranes.

Glycolipid functions

•Glycosphingolipids, which are components of the outer layer of the plasma membrane, can participate in intercellular interactions and contacts.

•Some of them are antigens, for example, xenogenic and allogeneic antigens. Similar oligosaccharide chains were found in other plasma membrane glycoproteins.

•A number of gangliosides function as receptors for bacterial toxins (e.g., cholera toxin, which triggers the adenylate cyclase activation process).

Digestion of fats in the gastrointestinal tract

The breakdown of fat into glycerin and higher fatty acids is carried out under the influence of the lipase enzyme. For the action of lipase on fat, it is necessary to preliminary emulsify it, achieved by mixing in the intestines of edible gruel with bile.

Digestion of fats in the gastrointestinal tract

1)In the oral cavity, fats do not undergo chemical changes.

2)Lipase is present in the stomach, but its activity is low due to the lack of conditions necessary for emulsification of fat. In the stomach, only emulsified fats are hydrolyzed - the fats of milk and egg yolk.

3)Basically, the digestion of fat occurs in the intestine, and primarily in the duodenum, where bile salts with powerful emulsifying action enter along the bile ducts with bile.

Digestion of fats in the gastrointestinal tract

•Bile acids form the thinnest film on fat droplets, which prevents the merging of individual droplets of fat into larger drops.

•This leads to a sharp increase in the surface of contact of fat with the enzyme lipase and, consequently, the rate of hydrolytic decomposition of fat.

•Bile acids include cholic, deoxycholic and others. In their structure, they are close to cholesterol.

Digestion of fats in the gastrointestinal tract

•In cells of the intestinal epithelium, fats, or lipoids, specific for a given animal species, are re-synthesized from the products of hydrolysis of edible fats.

•Synthesized lipids are transported to fat depots. If necessary, fats can be transferred from fat depots to the bloodstream and used by tissues as energy material.