Definition :– Lipids are the organic compounds which are insoluble in water but are soluble in organic solvents like chloroform, ether and benzene. These are esters of fatty acids and are capable of esters which are useful for living organisms.

Classification of Lipids:- Lipids are classified in 3 groups

• Simple lipid: These are esters of fatty acids with various alcohols. Based on the type of alcohol, simple lipids are further divided
  1. Neutral fats/ triacylglycerol/ triglycerides: These are esters with fatty acids glycerol. since these are uncharged, they are also known as neutral fats. A fat in liquid form is known as oil, like vegetable oil, mustered oil etc. Example of neutral fats is tripalmitin.
  2. Waxes: These are esters of fatty acids with monohydric long chain alcohols. these alcohols have high molecular weight. These compounds are of no use for human metabolism but are present in a large amount in nature. They are helpful for the protection purpose of leaves, stem and fruits. Example lanolin, bees-wax, spermaceti oils. They are also used in pharmaceutical, cosmetics industries.
• Complex or compound lipid:- These are esters of fatty acids with alcohols having prosthetic groups. These are again divided according to the prosthetic group present in the lipid
  1. Phospholipids: these lipids contains fatty acids, an alcohol and a phosphoric acid residue. These lipids have nitrogen containing bases and other substituents. Phospholipids are more further divided according to the alcohol present-
     • glycerophospholipids:- These lipids have alcohol glycerol. Examples include: phosphatidylserine, plasmalogens, lysophospholipid etc.
     • Sphingophospholipids:- The alcohol present in these lipids is sphingosine. example- sphingomyelins.
  2. Glycolipids:- It contains fatty acids, sphingosine alcohol and carbohydrate residue with nitrogen base. They do not contain phosphate group. Example- cerebrosides, gangliosides.
  3. Lipoproteins:- Lipoproteins are formed from the combination of lipid with a prosthetic group protein.
• Derived Lipids:- Derived lipids are the products obtained from hydrolysis of simple and compound lipids which posses the characteristics of lipids. eg- steroids.

Physical properties of Lipids

• Lipids are either liquid or non-crystalline solid at room temp
• Insoluble in water
• pure fats and oils are colorless, odorless and tasteless
• No ionic charges
• Pure fats and oils are energy rich molecules
• Soluble in organic solvents like chloroform, ether.

Chemical properties of Lipids

1. hydrolysis:- hydrolysis of lipid such as triacylglycerol is done through lipases enzyme during the digestion of fat which results in fatty acids and glycerol.
2. Saponification:- hydrolysis of fat by an alkali is known as saponification. the products formed are glycerol and alkali salts of fatty acids which are known as soaps.
3. Hydrogenation:- hydrogenation of unsaturated fat in the presence of a catalyst is known as hardening. It is mainly used for converting the liquid fats.
4. Peroxidation:- peroxidation of lipids exposed to oxygen is responsible not only for breaking of foods but is also used for damage to tissues where it may cause cancer.
5. Rancidity:- When any oil or fat produces un-pleasent odor, they are termed as rancidity. Rancidity is caused after oxidation and hydrolysis. Oxidative rancidity occurs in triacylglycerol containing unsaturated fatty acids.

Functions of lipids

1. Fats and oils are used as storage form of energy in living organisms.
2. Lipids majorly form cell membranes. lipids like phospholipids, glycolipids play important role in cell by forming impermeable barriers.
3. Lipids acts as thermal insulator in the subcutaneous tissue
4. Lipids also helps in the absorption of fat soluble vitamins.

Multiple choice questions (MCQs)

1. What are lipids?
A. Organic compounds

B. Inorganic compound

C. Both

D. None

2. Lipids are classified in how many types?
A. 2

B. 3

C. 4

D. 5

3. In which solvent, lipids are insoluble?
A. Chloroform

B. Ether

C. Water

D. All of the above

4. Which type of alcohol is present in neutral fats?
A. Glycerol

B. Monohydric acid

C. Sphingosine

D. None of the above

5. Which type of alcohol is present in waxes?
A. Glycerol

B. Monohydric acid

C. Sphingosine

D. None of the above

6. Which of the following type of lipid has no metabolic use?
A. Neutral fats

B. Phospholipids

C. Sphingolipids

D. Waxes

7. Which type of prosthetic group is present in glycolipids?
A. Carbohydrate

B. Protein

C. phosphoric acid

D. none of the above

8. Match the following lipid and its example-
a. Neutral fats 1. sphingomyelins
b. Waxes 2. gangliosides
c. Glycolipids 3. lanolin
d. Sphingophospholipids 4. Vegetable oil

9. Steroids is the example of which type of lipid?
A. Glycerophospholipids

B. Waxes

C. Derived lipids

D. Sphingophospholipids

10. Which of the following is the function of lipid?
A. Works as storage form of energy
B. Formation of cell membrane
C. Absorption of fat soluble vitamins
D. All of the above

11. Which type of lipid has prosthetic group as proteins?
A. Glycolipids

B. Lipoproteins

C. Derived lipids

D. Phospholipids

12. Which of the following statement is not true?
A. Lipids are classified in 3 types
B. Waxes are used for the protection purpose
C. Lipoprteins is the combination of simple and compound lipids
D. Plasmalogens is the example of glycerophospholipids

13. Neutral fats are also known as?
A. Triglycerides

B. Triacylglycerol

C. Lipoprotein

D. Both A and B

14. Which of the following is not the component of lipids?
A. Aldehyde

B. Alcohol

C. Esters

D. Fatty acid

15. Plasmalogens is the example of which type of lipid?
A. Neutral fats

B. Waxes

C. Derived lipids

B. None of the above

ANSWERS:-
1. Organic compound
2. 3
3. Water
4. Glycerol
5. Monohydric acid
6. Waxes
7. Carbohydrate
8. a – 4 b – 3 c – 2 d – 1
9. Derived lipids
10. All of the above
11. Lipoproteins
12. Lipoproteins is the combination of simple and compound lipids
13. Both A and B
14. Aldehyde
15. None of the above

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REFERENCE:- Pankaja Naik- Biochemistry; 4^th edition; page no:- 33-35.