lipids

1. Lipids
Digestion and Absorption

2. Lipids are chemically a heterogeneous
group of compounds
Their common property is insolubility in
water and solubility in organic solvents
Secondly, they are actually or
potentially related to fatty acids

3. Unlike carbohydrates and proteins, lipids
can be stored in large quantities
Stored lipids constitute an important
reservoir of energy
About four-fifths of the stored energy is in
the form of lipids

4. of lipids is
The major storage form
triglycerides (triacylglycerols)
Triglycerides are stored in specialized
cells called adipocytes
A tissue rich in adipocytes is known as
adipose tissue

5. The other lipids present
in the body are:
Phospholipids
Glycolipids
Cholesterol
Free fatty acids
Fat-soluble vitamins

6. All the lipids except vitamins can be
synthesized in the body
However, all these lipids are present in
diet also
The dietary lipids have to be digested and
absorbed

7. Major dietary lipids are triglycerides,
phospholipids and esterified cholesterol
These have to be hydrolysed before their
absorption from the alimentary tract
Digestion and absorption

8. Bile also plays an important role in
digestion and absorption of lipids
Digestive juices which participate in
digestion of lipids are:
Saliva
Pancreatic juice
Succus entericus

9. Saliva contains a lipase which is secreted
by the dorsal surface of tongue
Food stays in mouth for a very short period
But salivary lipase remains active even in
the acidic environment of stomach
Salivary lipase converts triglycerides into
diglycerides and free fatty acids

10. Salivary lipase acts
preferentially on:
Triglycerides containing
short-chain fatty acids
The ester bond at
position 3

11. CH ‒
O ‒
C ‒
R
2
CH2‒O ‒
C ‒
R
3
Salivary lipase
O
II
CH2‒O ‒
C ‒
R
1
O
II
O
II

12. Upon reaching small intestine, food comes
in contact with pancreatic juice
Pancreatic juice contains a lipase which is
far more powerful than salivary lipase
However, the hydrophobic nature of lipids
poses a problem

13. Being hydrophobic, lipids are insoluble in
the aqueous environment of the gut
They are present in the gut as large
globules
Pancreatic lipase can act only at the
surface of fat globules

14. For efficient digestion, the lipids need to be
emulsified
Emulsification is breaking up of fat
globules into small droplets
Bile salts, present in bile, play a key role in
emulsification

15. Bile salts are powerful emulsifying agents
They have both hydrophilic and hydro-
phobic domains
The hydrophobic portion dips into the fat
globule
The hydrophilic domain attracts water

16. OH
OH
HO
Sodium glycocholate (bile salt)
II
O
C−NH−CH2−COONa
Hydrophilic
portion
Hydrophobic
portion

17. Peristaltic movement of the gut breaks up
fat globules into small droplets
Presence of bile salts on their surface
prevents their re-association
Phospholipids which are amphipathic
molecules also help in emulsification

18. Emulsification greatly increases the
surface area for pancreatic lipase to act on
Pancreatic lipase requires co-lipase to
become active
Co-lipase is a protein present in pancreatic
juice

19. Pancreatic lipase can easily hydrolyse the
ester bonds at positions 1 and 3
The ester bond at position 2 is resistant to
the action of pancreatic lipase
Pancreatic lipase converts triglycerides
into 2-monoacylglycerol and fatty acids

20. CH ‒
O ‒
C ‒
R
2
CH2‒O ‒
C ‒
R
3
O
II
CH2‒O ‒
C ‒
R
1
O
II
O
II
CH2‒ OH
CH ‒
O ‒
C ‒
R
2
CH2‒ OH
2-Monoacylglycerol
O
II
1
+
R3‒
COOH
2 H2O

R ‒COOH

Pancreatic
lipase
Triacylglycerol

21. Pancreatic juice also contains phospho-
lipase A2 and cholesterolesterase
Phospholipase A2 hydrolyses the ester
bond at position 2 of glycerophospholipids
Cholesterol esterase hydrolyses the ester
bond of esterified cholesterol

22. Phospholipase A2
CH‒
O‒
C‒
R2
CH‒
OH
CH2‒
O‒
‒
Choline
H2O
CH2‒O‒‒Choline
R2‒
COOH
Hydrolysis of a glycerophospholipid
O
II
CH2‒
O‒
C‒
R1
O
II
O
II
CH2‒
O‒
C‒
R1

23. H2O
R‒
COOH
HO
Hydrolysis of esterified cholesterol
O
II
R‒
C‒
O
Cholesterol esterase

24. Succus entericus contains a powerful
phospholipase
This causes complete hydrolysis of the
phospholipids
Glycerol, fatty acids, phosphate and
inositol/nitrogenous bases are liberated

25. Thus, the major products
of digestion of lipids are:
Glycerol
Cholesterol
Fatty acids
Monoglycerides

26. Digested lipids are passively absorbed into
the mucosal cells of the small intestine
Being hydrophobic, they can easily pass
through the lipid bilayer of cell membrane
Emulsification also helps in their absorption

27. Lipase Bile salts
Lipase Bile salts
Fat
globule
Micelles
Mucosal
cells

28. The products of digestion are re-combined
in the mucosal cells
Re-combination produces triglycerides,
esterified cholesterol and phospholipids
These are combined with a small amount
of some proteins (apolipoproteins)
The products are chylomicrons, a type of
lipoproteins

29. Apoprotein
Chylomicron
Free cholesterol
Phospholipids
Triglycerides
Cholesteryl
esters

30. The chylomicrons enter the lymphatic
vessels (lacteals)
They reach the systemic circulation via
thoracic duct
The lipids present in chylomicrons are
taken up by various tissues