Classification of lipid - Online Biology Notes

What are the different types of lipid?

Classification of lipid on the basis of composition:

Simple (Homolipid)	Complex (Heterolipid)	Derived lipid
i) Neutral fat Eg. triglyceride	i) Phospholipid	i) Sterol and steroids
ii) Wax E.g. Bee’s wax, Spawn whale’s wax, Carnauba’s wax	ii) Glycolipid	ii) Terpenes
	ii) Sulpholipid	iii) Carotenoids
	iv) Aminolipid	iv) Lycopenes
	v) Proteolipids	v) Carotin
	vi) Lipolipids	vi) Xanthophyll

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

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.

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.

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

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.

Name of glycerophospholipids	Name and formula of -X
Phosphatidic acid	Hydrogen
Phosphatidylcholine (lecithin)	Choline (CH₂-CH₂-N+(CH₃)₃
Phosphatidyl ethanol amine	Ethanol amine (-CH₂CH₂-NH₂)
Phosphatidyl serine	Serine (HOCH₂-CHNH₂-COOH)
Phosphatidyl inositol -4,5-bisphosphate	Myoinositol-4,5-bisphosphate

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.

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

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.

Name of spingoglycolipid	Name and formula of X
Cerebrosides Glucocerebrosidesgalactocerebrosides	glucosegalactose
Globosides	D-glucose-D-galactoseD-glucose-N-acetyl galactosamine
Gangliosides	N-acetyl muramic acid
Blood group antigen – O-antigen A-antigenB-antigen	Glu-gal-N-acetyl galactosamine-gal-fructose Glu-gal-N-acetyl galactosamine-gal-fucose Glu-gal-N-acetyl galactosamine-lactose-gal/fructose

Functions of glycolipids:

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.

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.

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.