Lipids (الليبيدات)

Study Notes

Lipids are a heterogeneous group of compounds related to fatty acids.
They are soluble in nonpolar solvents and insoluble in water.
They are an important source of energy.
They provide a medium for the dissolution of fat-soluble vitamins.
The body uses them as thermal insulation under the skin and around internal organs.
Lipoproteins play a role in transporting blood lipids and are involved in the composition of cell membranes and cytoplasm.
Lipids are present in the body as protoplasmic fats (part of the protoplasm structure) and as reserve fats located in adipose tissue.
Protoplasmic fats form the structural framework of cells, and their amount in a particular tissue or organ remains relatively constant, even in the case of diseases.
Reserve fats are variable; in cases of fasting or starvation, the body loses a significant portion of its reserve lipids.

Lipids are categorized into simple, complex, and derived lipids.
Simple lipids include fatty acids or their aldehydes, along with sterols. They further subdivide into:
- Neutral fats: These are triacylglycerols and other acylglycerols.
- Waxes.
Complex lipids contain additional non-lipid groups. They subdivide into:
- Phospholipids: Contain phosphorus in their molecules.
- Plasmalogens (acetals of phosphatidylethanolamines)
- Sphingolipids
- Glycolipids (glycosphingolipids)
- Sterols.
Derived lipids include compounds derived from the aforementioned materials.

Fatty acids are rarely found free in cells and tissues.
They possess a long hydrocarbon chain and a terminal carboxyl group.
The carboxyl carbon is numbered 1, the next carbon 2 (or a), the next 3 (or b), and so on.
Fatty acids are obtained through the hydrolysis of fats.
They typically have an even number of carbon atoms derived from an even number of two-carbon units.
Their structures are typically straight-chain.
They can be saturated (e.g., palmitic acid) or unsaturated, containing one or more double bonds (e.g., oleic acid).
Unsaturated fatty acids have lower melting points than saturated fatty acids with the same number of carbon atoms.

Essential fatty acids play a critical role as an energy source for higher animals and plants.
Their cells can store these acids in significant quantities as triacylglycerols.
Important examples include linoleic acid, linolenic acid, and arachidonic acid.
The body cannot synthesize them and must obtain them through the diet.
They are necessary for growth, pregnancy, and lactation, and are also precursors to prostaglandins.

Fatty acids are named by adding "-oic" to the name of the hydrocarbon chain, specifying the position of the double bond if present.
Saturated fatty acids lack double bonds.
Unsaturated fatty acids are further categorized as:
- Monounsaturated fatty acids: Contain one double bond.
- Polyunsaturated fatty acids: Contain multiple double bonds (e.g., prostaglandins).

Saturated fatty acids have a variable structure due to free rotation around single bonds, often appearing elongated.
Unsaturated fatty acids have bends owing to the double bonds and typically exhibit a 'cis' configuration, with an angle of 30 degrees with the hydrocarbon chain.
Long-chain fatty acids (16 or more carbons) are insoluble in water, but their alkali salts form a stable emulsion with water.
Fatty acids do not absorb visible or invisible light, but polyunsaturated ones can be converted into conjugated double bonds via heating and KOH, absorbing light between 230 and 260 nm, allowing for their quantitative measurement.

Neutral Lipids:
- Waxes: Composed of long-chain fatty acid esters with long-chain alcohols.
- Triacylglycerols (TAGs): Esters of glycerol with three fatty acids.
Complex Lipids (containing additional groups):
- Phospholipids: Includes various phospholipids like phosphatidylcholine and phosphatidylethanolamine.
Sphingolipids: Includes sphingomyelins, cerebrosides, and gangliosides.
Glycolipids: Contain sugar moieties.
Sterols: Including cholesterol and other steroid-like compounds.

Specific fatty acids have varying sources: some are found in various animal fats, others uniquely in certain plant seeds or nuts.

Lipoproteins are lipid-protein complexes responsible for transporting lipids in the blood.
They have variable density depending on the lipid composition.
Separating them using density difference is a method to analyze their structure.

Chromatography methods (like thin-layer chromatography (TLC) and gas chromatography (GC)) are important tools for isolating and identifying different lipids.

Biological membranes are primarily composed of phospholipids, arranged in a bilayer.
The hydrophilic (water-loving) heads of phospholipids face outward, interacting with the aqueous environment.
The hydrophobic (water-fearing) tails face inward, preventing direct contact with water.
The amphipathic nature of phospholipids is crucial to membrane structure and function.