Lipids: Fatty Acids & Phospholipids | Biology Class 11 - NEET PDF Download

Lipids

Note: Fat and its derivatives are known as lipids.

Important Points of Lipids

• Lipid term coined by Bloor.
• Compounds of C, H, O but the ratio of Hydrogen and Oxygen is not 2 : 1. The amount of oxygen is considerably very less.
• Lipids are insoluble in water and soluble in organic solvents like acetone, chloroform, benzene, hot alcohol, ether etc.
• Lipids occur in protoplasm as minute globules.
• Lipids do not form polymer.
• Lipids provide more than double energy as compared to carbohydrates.
• In animals, fat is present in subcutaneous layer and work as food reservior and shock-absorber.
• Lipid require less space for storage as compared to carbohydrate because lipid molecule is hydrophobic and condense.  
• Animal store maximum amount of food in the form of lipid.
• Lipid provides maximum amount of metabolic water as compared to carbohydrate and protein on oxidation.
• Lipids are not strictly macromolecules.   

 (A) Simple Lipid or Neutral Fats 

• These are esters of long chain fatty acids and alcohol. In majority of simple lipids, the alcohol is a trihydroxy sugar alcohol i.e. glycerol.
• Three molecules of fatty acid linked with one molecule of glycerol. The linkage is called ''ester bond''. Such type of lipids called Triglycerides. Three molecules of water are released during formation of triglycerides (dehydration synthesis).
• Similar or different fatty acids participate in the composition of a fat molecule. Simple lipids contain two types of fatty acids. 

(i) Saturated Fatty Acids: are those in which all the carbon atoms of hydro-carbon chains are saturated with hydrogen atoms.
e.g. Palmitic acid – CH₃(CH₂)₁₄ – COOH

        Stearic acid – CH₃(CH₂)₁₆ – COOH

(ii) Unsaturated Fatty Acids: are those in which some carbon atom are not fully occupied by hydrogen atoms.

e.g. Oleic acid – CH₃(CH₂)₇ CH=CH(CH₂)₇COOH

         Linoleic acid – CH₃(CH₂)₄ –(CH=CH–CH₂)₂ –(CH₂)₆ –COOH

          Linolenic acid–CH₃–CH₂ –(CH=CH–CH₂)₃ –(CH₂)₆ –COOH

Polyunsaturated Fatty Acids with more than one double bonds in their structure.
e.g. Linoleic acid, Linolenic acid, Arachidonic acid, Prostagladins (derived from archidonic acid).

• Unsaturated fatty acid also called as essential fatty acids because no animal is able to synthesize them.
• Simple lipids with saturated fatty acid remain solid at normal room temperature e.g. fats
• Simple lipids with unsaturated fatty acids remain liquid at room temperature e.g. oils.
• Saturated fatty acids are less reactive so they tend to store in body and cause obesity.
• Unsaturated fatty acids are more reactive so they tend to metabolise in body and provide energy.
• Oils with poly unsaturates are recommended by physicians for persons who suffer from high blood cholesterol or cardio-vascular diseases. This is because increasing the proportion of poly unsaturated fatty acids to saturated fatty acids, without decreasing the fats in the diet tend to lower the cholesterol level in blood.

Waxes: are monoglycerides with only one molecule of fatty acid attached to a long chain monohydroxy alcohol. Waxes are more resistant to hydrolysis as compared to triglycerides. Waxes have an important role in protection. They form water insoluble coatings on hair and skin in animals and stem, leaves and fruits of plants.

e.g.

Bees Wax (Hexacosyl palmitate)

Carnauba (Myricyl cerotate) which occurs on leaves, stem and fruits.

Spermaceti in skull of whale and Dolphin.

Cerumen or ear wax – occurs in external auditory canal.

Lanoline or cholesterol ester – occurs in blood, sebum and gonadal ducts as lubricating agent.

It is also obtained from wool of sheep.

(B) Conjugated or Compound Lipids 

(i)   Phospholipids or Phosphatide or Phospholipins = 2 Molecules of fatty acid + Glycerol + H₃PO₄ + Nitrogenous compound
Phospholipids are most abundant type of lipids in protoplasm.

Phospholipids have both hydrophilic polar end (H₃PO₄ and nitrogenous compound) and hydrophobic non polar end (fattty acids). Such molecules are called amphipathic. Due to this property, phospholipids form bimolecular layer in cell membrance.

Some biologically important phospholipids are as following :

(a) Lecithin or Phosphatidyl choline

• Nitrogenous compound in lecithin is choline.
• Lecithin occurs in egg yolk, oil seeds and blood.
• In blood lecithin functions as carrier molecule. It helps in transportation of other lipid.

(b)  Cephalin – Similar to lecithin but the nitrogenous compound is ethanolamine, cephalin occurs in nervous tissue, egg yolk and blood platelets. 

(c)  Sphingolipids or sphingomylins similar to lecithin but in place of glycerol it contains an amino alcohol sphingosine.

Sphingolipids occur in myelin-sheath of nerves, other examples of phospholipids are phosphatidyl serine, phosphatidyl inositol, plasmologens.

(ii)  Glycolipid – 2 fatty acid + sphingosine + galactose

eg.  Cerebroiside which occurs in white matter of brain –

Gangliosides – These occur in nerve ganglia and spleen. These also contain N-acetyl neurominic acid and glucose beside other compounds.

(iii) Derived Lipids – Lipid derived from simple or conjugated lipid. Derived lipids are complex in structure.

They are insoluble in water and soluble in organic solvents.  

(A)  Steroids – Steroids exhibit tetracyclic structure called ''Cyclo pentano perhydrophenanthrene nucleus ''

 On the basis of functional group, steroids are of two types –

(a)   Sterols – Alcoholic steroids e.g. cholesterol – Cholesterol abundantly occurs in brain, nervous tissue, Adrenal gland and skin. Cholestrol is a parent steroid. Several other biologically important steroids are derived from cholesterol. 7-dehydro cholesterol which occurs in skin is a provitamin. On exposure to ultraviolet radiation, it transforms in cholecalciferol i.e. vitamin D.

Cholesterol is also called ''most decorated micromolecule in biology''.

Ergosterol – It occurs in oil seed, fungi like ergot and yeast. Ergosterol is precursor of another form of Vitamin D-Ergocalciferol.

Coprosterol – Occurs in faecal matter. It forms decomposition of cholesterol by colon bacteria.   

Bile acid – Bile juice contains different types of steroid acids. E.g. cholic acid, Lithocholic acid etc. They help in emulsification of fats.

(b)  Sterones – Ketonic steroids, for e.g. sex hormones, Adreno corticoids, ecdyson hormone of insects, Diosgenin obtained from yam plant (Dioscorea), is used in manufacture of antifertility pills.   

(B)  Chromolipid = It is also called terpene.

Most complex lipid in protoplasm.

Chromolipids composed of repeated isoprene units.

 
Example : Carotenoids, vitamin A, E, K, Natural Rubber (Polyterpene).

Special Points

Prostaglandins – It is Derived lipid. Prostaglandins are derivatives of PUFA (Polyunsaturated fatty acid) They are helpful in contraction of uterus and fallopian tube, blood clotting, Muscle contraction.