Biochemistry Lectures For First Year : Lipids Lecturer: Hiba A. Alasadi
Lecture No. 1
LIPIDS
Lipids are the group of organic compounds, which are insoluble in water but soluble in organic solvents like chloroform, ether and benzene.
Medical and Biological Importance
1. Serve as thermal insulator against cold and protective coating on surface of many organs.
2. Serve as a source of energy like carbohydrates and involved in transport of metabolic fuel.
3. It's an ideal form for storing energy in the human body compared to carbohydrates and protein because:
(a) Energy content of fat is higher. (b) Only fat can stored in a concentrated water free form which is not possible with carbohydrates and proteins.
4. They are structural component of cell membrane and nervous tissue.
5. Serve as precursors for the synthesis of complex molecules.
6. They are essential for absorption of fat-soluble vitamins.
7. Serve as cell-surface components concerned in cell recognition, species specificity
8. Some substrate classified among the lipids have intense biological activity, they include some of vitamins and hormones.
2. Serve as a source of energy like carbohydrates and involved in transport of metabolic fuel.
3. It's an ideal form for storing energy in the human body compared to carbohydrates and protein because:
(a) Energy content of fat is higher. (b) Only fat can stored in a concentrated water free form which is not possible with carbohydrates and proteins.
4. They are structural component of cell membrane and nervous tissue.
5. Serve as precursors for the synthesis of complex molecules.
6. They are essential for absorption of fat-soluble vitamins.
7. Serve as cell-surface components concerned in cell recognition, species specificity
8. Some substrate classified among the lipids have intense biological activity, they include some of vitamins and hormones.
Classification of lipids
Based on the composition of lipids are classified as (Fig. 1.1) into :
I. Simple lipids: Esters of fatty acids with alcohols, they are :
1. Fats: They consist of three molecules of fatty acids esterified with one molecule of glycerol. All three fats can be same or different. They are also called triglycerides or triacylglycerol . Fat are also called neutral fat.
1.1.Nomenclature of triglycerides
The carbon atoms of glycerol of triglycerides are indicated with Indo-Arabic numbers and Greek alphabets. The triglycerides are further subdivided based on type of fatty acids esterified to glycerol. They are :
a. Simple triglyceride: If all the three fatty acids esterified to glycerol are same, e.g., Tripalmitin: Glycerol is esterified to three molecules of palmitic acid. Other names are 1,2,3-tripalmityl glycerol or α,β,α1-tripalmitin .
b. Mixed triglyceride: If the three fatty acids esterfied to glycerol are different than it, e.g. 1,3-distearopalmitin is a mixed glyceride in which glycerol is esterfied with two molecules of stearic acid and one molecule of palmitic acid. Other names are 1,3-distearyl-2-palmitylglycerol or α,α1-distearopalmitin.
c. Other Noteworthy Acyl Glycerols
Diacylglycerol: In which 2 fatty acids are esterified to glycerol, e.g. 1,2-diacylglycerol and 1,3-diacylglycerol.
Monoacylglycerol: In which one fatty acid esterified with glycerol .e.g. ,1-monoacylglycerol and 2-monoacylglycerol.
1. 2. Function of Triglycerides
a. They function as storage lipids in animals and plants .
b. Adipose tissue or fat tissue under skin, abdominal cavity and the mammary
gland contain triglycerols. Entire cytoplasm of adipocyteis replaced by
triglycerols.
c. The fat stored under skin serves as energy store and as insulator against
cold.
d. Woman has more fat than men.
b. Adipose tissue or fat tissue under skin, abdominal cavity and the mammary
gland contain triglycerols. Entire cytoplasm of adipocyteis replaced by
triglycerols.
c. The fat stored under skin serves as energy store and as insulator against
cold.
d. Woman has more fat than men.
1. 3. Chemical properties of triglycerides
1.Saponification :Salts of fatty acids (Soap) are formed when fat are boiled
with bases like KOH or NaOH.
with bases like KOH or NaOH.
2.Hydrogenation :It converts unsaturated fatty acids of triglycerides into
saturated fatty acids.
saturated fatty acids.
3.Lipid peroxidation :
Peroxidation (auto-oxidation) of lipids exposed to oxygen is responsible not
only for deterioration of foods (rancidity) but also for damage to tissues in
vivo, where it may be a cause of cancer, inflammatory diseases,
atherosclerosis, and aging. The deleterious effects are considered to be caused
by free radicals (ROO•,RO•, OH•) produced during peroxide formation from
fatty acids containing methylene-interrupted double bonds, i.e., those found in
the naturally occurring polyunsaturated fatty acids. Lipid peroxidate ions a
chain reaction providing a continuous supply of free radicals that initiate
further peroxidation.
only for deterioration of foods (rancidity) but also for damage to tissues in
vivo, where it may be a cause of cancer, inflammatory diseases,
atherosclerosis, and aging. The deleterious effects are considered to be caused
by free radicals (ROO•,RO•, OH•) produced during peroxide formation from
fatty acids containing methylene-interrupted double bonds, i.e., those found in
the naturally occurring polyunsaturated fatty acids. Lipid peroxidate ions a
chain reaction providing a continuous supply of free radicals that initiate
further peroxidation.
The whole process can be depicted as follows:
Vitamin E and ascorbic acid prevent peroxide formation. They called antioxidant. Free radicals mediate lipid peroxide formation in the body.
1.2. Waxes: Esters of fatty acids with long chain alcohol, for example : cholesterol ester is wax present in the blood (lipoprotein) and cell membranes.
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