Structural Lipids and Fatty Acids

• Storage lipids direct their aggregation into forming a membrane bilayer.
• Aggregation is determined by hydrophilic interaction at one end and hydrophobic interaction with phospholipids.
• Examples of structural lipids are glycerolipids, sphingolipids, galactolipids, and sterols (cholesterol).

Sub-Classes of Lipids

• Fatty acids are carboxylic acids with long hydrocarbon side groups.

• These are usually found in esterified form; they are often esterified to glycerides or sphingosine.

• They are also esterified to proteins in the blood as lipoproteins and are usually not found freely in nature.

• Naturally occurring fatty acids usually have between 12-24 carbons.

• Naturally occurring fatty acids are usually even-numbered carbon atoms.

• The common chain length is C16 .

• Examples: palmitic acid, stearic acid, and linoleic acid.

• Saturated fatty acids are fully reduced as they do not contain double bonds; therefore, they can rotate freely and are flexible.

• Unsaturated fatty acids are liquid at room temperature and saturated fatty acids are solid.

• Solubility and melting point of fatty acids depend on the number of carbon atoms in their chain length.

• Shorter fatty acid chains have lower solubility and a lower melting point.

• Longer fatty acid chains have lower solubility and a higher melting point.

• Unsaturated fatty acids contain double bonds.

• These can have more unsaturated fatty acids than saturated fatty acids.

• Polyunsaturated fatty acids contain more than one double bond.

• Double bonds in poly-unsaturated fatty acids occur at intervals of three carbon atoms, so they are not a conjugated system.

• Double bonds in fatty acids are in cis configuration.

• Monounsaturated fatty acids contain a single double bond.

• Conjugated systems are found in isoprenoids.

• Saturated fatty acids are in reduced forms; they tend to rotate easily about their carbon chain, making them flexible.

• They tend to pack together, forming solid structures due to this conformation.

• Unsaturated fatty acids information tends to bend to a degree.

• Unsaturated fatty acids occur as oils with loose Van der Waals forces.

• Triacylglycerols are composed of three fatty acids esterified into a single glycerol.

• They are non-polar, completely hydrophobic, and neutral.

• Examples are triglycerides, fats, or brand neutral lipids.

• Triacylglycerides are a form of energy storage in biological membranes (adipose tissue).

• Waxes are esters of long-chain fatty acids and long-chain alcohols, which have a higher melting point than triglycerides

Structural Lipids

• Structural lipids form the core of the membrane lipid bilayer, functioning as a barrier to the passage of polar molecules and ions.
• Structural lipids are amphipathic in nature as they contain lipophilic moieties (heads) and hydrophobic moieties (tails).
• Amphipathic molecules will interact with a polar environment and the other part will interact with a non-polar environment.
• Combinations of various "heads" and "tails" result in enormous diversity in the lipid bilayer.
• Types of Structural lipids are:
  • Glycerophospholipids.
  • Sphingolipids.
  • Glycosyl glycerides.
  • Ether lipids.
  • Sterols.
• Glycerophospholipids are also called phosphoglycerides; these are derivatives of phosphatidic acids with two fatty acid moieties esterified to glycerol phosphate at C1 and C2.
• Sphingolipids derivatives of sphingosine ( also called 4-sphingenine) - a C18 amino alcohol.
• Another derivative of sphingosine is ceramides.
• In ceramides, a fatty acyl group is linked to the amino group on C2 of sphingosine via an amide linkage.
• It is the parent compound of other sphingolipids, mainly in the trabecula.
• Types of sphingolipids:
  • Sphingomyelins.
  • Cerebrosides.
  • Globosides.
  • Gangliosides (glucose head group).
• Ceramides bearing a phosphatidylcholine (phosphocholine) as head groups are classified as sphingomyelin.
• Comformation and charge distribution in sphinogomyelin are similar to phosphatidylcholine.
• They are abundant in myelin that insulates neurons.
• Cerebrosides are ceramides bearing a single sugar residue (glucose, galactose) as the polar head group on C-1 via a glycosidic linkage.
• Classified as glycolipids, they are non-ionic (i.e. they carry no charge).
• Those with glucose are mostly present in the plasma membrane of non-neural cells, those bearing galactose are present in neural cells.
• Globosides- Ceramide bearing two or more sugars as their polar head group; sugar presents are D-glucose, D-galactose, and N-acetyl-D-galactosamine.
• These are neutral at physiological pH (~7)
• Gangliosides- complex sphingolipids
• Ceramides with oligosaccharides as their head group
• One residue of N-acetylneuraminic acid (Sialic acid) at the terminal
• Negatively charged at pH ~7
• Primarily present on the outer leaflet of the plasma membrane
• Serve as recognition site or as receptors

Steroids

• General steroid skeleton is present in membranes of most eukaryotic cells and are derivatives of cyclopentanoperhydrophenanthrene (the steroid nucleus).
• It is composed of four fused rings: three wit six carbons, and the fourth with five carbons.
• Major steroids are cholesterol (present in animal cells), stigmasterol (present in plant cells), and ergosterol (present in fungi).
• Cholesterol is amphipathic containing a polar Hydroxyl group present at C3 and an alkyl group at C17

Explore structural lipids like glycerolipids and sterols, which aggregate into membrane bilayers via hydrophilic and hydrophobic interactions. Learn about fatty acids, carboxylic acids with long hydrocarbon side groups that commonly have between 12-24 carbons.