Classification of lipid on the basis of composition:

I. Simple lipids:

They are further divided into neutral fat and oil and wax.

i. Fat and oil:

• Fat and oil are triglycerides, a triacylglycerol (TAG) in which 3 fatty acids are linked with one glycerol molecule by ester bond.
• TAG containing same type of fatty acids is called simple TAG, whereas those containing different types of fatty acids are called mixed TAG.
• Most naturally occurring fat and oil are mixed type.
• Fatty acid commonly occurring are C₁₆, C₁₈, type i.e. palmitic acid, steric acid and oleic acid.
• Triglyceride occupy 98% of diet lipid. They are the storage form of energy and usually form fat depot.Fat contains mainly saturated fatty acids therefore they are liquid at room temperature.
• Fat and oil are non-polar and hydrophobic in nature because the -OH group of glycerol and -COOH group of fatty acids are involved in ester bond formation and is not available for H-bonding.
• Functions:
  • Fat and oil provide energy in cell. Oxidation of one gram of fat release 9.3kcal energy.
  • They are important source of energy when carbohydrates are unavailable in cell.
  • They have insulating effect during cold.
  • They are stored in seed as fat depot and help during germination of seed.
  • In sperm whale, presence of triglyceride gives buoyancy (upthrust).
  • They are storage form of energy mainly formed in- subcutaneous layer.

ii. Wax:

• R₁-OH (monohydroxy alcohol) + HO-OCR₂ (fatty acids)à R₁-O-C=OR₂ (wax) + H₂O
• CH₃-(CH₂)₂₈-CH₂OH (myricyl alcohol) + CH₃-(CH₂₎₁₄-COOH (palmitic acid) à CH₃-(CH₂)₂₈-CH₂-O-C=O-(CH₂)₁₄-CH₃ (myricyl palmitate) (Bee’s wax).
• Waxes are the ester of fatty acid with high molecular weight monohydroxy alcohol.
• Different types of alcohols and FA are found in different naturally occurring waxes.
• Examples: bee’s wax, myricyl alcohol, palmitic acid, carnauba’s wax, tetracosanol, tetra triacosanol.
• Waxes are more solid and hydrophobic than fat and oil due to large hydrocarbon portion.
• Waxes are secreted by sebaceous gland, preen gland (below feather of bird), bees, sheep’s wool, sperm of whale etc.
• Functions:
  • They act as source of energy in some marine moss. E.g. planktons
  • Due to their water repelling property and smooth textures.
  • They are used for preparation of cosmetic products and boot polish.

II. Complex lipids:

• They are further divided into:
  • Phospholipids
  • Glycolipids

1. Phospholipid:

• They are further divided into two types:
  • Glycerophospholipid
  • Spingophospholipid

i. Glycerophospholipid:

• Glycerophospholipid consists of glycerol in which two fatty acids are linked with -OH group glycerol by ester bond and third -OH group of glycerol is linked with phosphate group which in turn linked with head group substituent.
• Head group substituents are different in different glycerophospholipid.

ii. Spingophospholipids: (Spingomylein):

• Spingophospholipid contain an amino alcohol called spingosine instead of glycerol.
• In spingophospholipid, one fatty acid is linked with -NH₂ group of spingosine by peptide bond and -OH group of spingosine linked with PO₃^—with in turn linked with head group substituents.
• Spingophospholipid (ceramide):
• Head group are different in spingolipid.
• If the head group in choline then it is called spingomylein.

Function of Phospholipids:

• Phospholipids are structural components of cell membrane. They make lipid bilayer of cell membrane.
• Lecithin helps in transport and metabolism of other lipids in animal.
• Lecithin deficiency causes deposition of large amount of fat in liver causing fatty liver.
• Phosphatidyl inositol-4,5-bisphosphate help in regulation of cell structure and metabolism.
• Platelets aggregating factor help in aggregation of platelets and release of serotonin from platelets.
• Helps in enzyme catalysis, ETS cycle.
• Phosphatidyl ethanol amine (cephalin) play important role in cell division, cell fusion etc.
• Phosphatidyl serine is a flippase enzyme.

2. Glycolipids:

• They are of two types:
  • Glyceroglycolipids
  • Spingoglycolipids

i. Glyceroglycolipids:

• In glyceroglycolipid, two fatty acids are linked with glycerol by ester bond and their OH group of glycerol is linked with carbohydrate head group.

ii. Spingoglycolipids:

• It consists of spingosine instead of glycerol in which -NH₂ group is linked with fatty acids by peptide bond and -OH group is linked with carbohydrate head group.
• Head group is different in different spingoglycolipid.

Functions of glycolipids:

• They are structural component of cell membrane.
• O, A, B antigen on RBC surface determine blood group.
• Help in signal transduction.
• Role in growth and tissue differentiation as well as carcinogenic.
• Gangliosides are present in brain (6%).

Sulpholipids:

• They are sulfate ester of glycolipid.
• Found in chloroplast, chromophore of bacteria.

Aminolipids:

• Mostly found in bacteria in outer and inner membrane, it is also called as mololipid. E.g. lipid containing serine.
• -(CH₂)₆-CH₂-CHOH-CH₂-C=O-NH-CHCH₂OH-COOH
• Found in Serectia.
• Lipid containing glycine: iso-3-hydroxyheptadecanoic acid linked to glycine.
• Proteolipid: contain protein attached to lipid.

III. Derived lipids:

• They are the hydrolyzed product of simple and composed lipids with various type of other compounds such as alcohol, ketone, vitamin D, sex-hormone steroid, terpenes, carotenoids.

Steroid:

• Sterane ring (Cyclopentanoperhydrophenanthrene).
• Sterane is parent compound.
• All steroids are derivatives of sterane. They are more hydrophobic than other lipids.
• E.g. cholesterol, sex hormone, Vit.D.

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