ABSORPTION OF LIPIDS

ABSORPTION OF LIPIDS

Monoglycerides, cholesterol and fatty acids from the micelles enter the cells of intestinal mucosa by simple diffusion.

From here, further transport occurs as follows:

1. In the mucosal cells, most of the monoglycerides are converted into triglycerides. Triglycerides are

also formed by re-esterification of fatty acids with more than 10 to 12 carbon atoms. Some of the

cholesterol is also esterified. Triglycerides and cholesterol esters are coated with a layer of protein, cholesterol and phospholipids to form the particles called chylomicrons. Chylomicrons cannot pass through the membrane of the blood capillaries because of the larger size. So, these lipid particles enter the lymph vessels and then are transferred into blood from lymph.

2. Fatty acids containing less than 10 to 12 carbon atoms enter the portal blood from mucosal cells and

are transported as free fatty acids or unesterified fatty acids. Most of the fats are absorbed in the upper part of small intestine. Presence of bile is essential for fat absorption.

STORAGE OF LIPIDS

Lipids are stored in adipose tissue and liver. Fat stored in adipose tissue is called neutral fat or tissue fat.

When chylomicrons are traveling through capillaries of adipose tissue or liver, the enzyme called lipoprotein lipase present in the capillary endothelium hydrolyzes triglycerides of chylomicrons into free fatty acids (FFA) and glycerol. FFA and glycerol enter the fat cells (adipocytes or lipocytes) of the adipose tissue or liver cells. Then, the FFA and glycerol are again converted into triglycerides and stored in these cells. Other contents of chylomicrons such as cholesterol and phospholipids, which are released into the blood combine with proteins to form lipoproteins. When other tissues of the body need energy, triglycerides stored in adipose tissue is hydrolyzed into FFA and glycerol. FFA is transported to the body tissues through blood.

TRANSPORT OF LIPIDS IN BLOOD – LIPOPROTEINS

Free fatty acids are transported in the blood in combination with albumin. Other lipids are transported

in the blood, in the form of lipoproteins.

LIPOPROTEINS

Lipoproteins are the small particles in the blood which contain cholesterol, phospholipids, triglycerides and proteins. Proteins are betaglobulins called apoproteins.

Classification of Lipoproteins

Lipoproteins are classified into four types on the basis of their density:

1. Very-low-density lipoproteins (VLDL): Contain high concentration of triglycerides (formed from FFA and

glycerol) and moderate concentration of cholesterol and phospholipids

2. Intermediate-density lipoproteins (IDL): Formed by the removal of large portion of triglycerides

from VLDL by lipoprotein lipase. Concentration of cholesterol and phospholipids increases because of

removal of triglycerides

3. Low-density lipoproteins (LDL): Formed from IDL by the complete removal of triglycerides. These lipoproteins contain only cholesterol and phospholipids

4. High-density lipoproteins (HDL): Contain high concentrations of proteins with low concentration of

cholesterol and phospholipids. All the lipoproteins are synthesized in liver. HDL is synthesized in intestine also.

Functions of Lipoproteins

Primary function of lipoproteins is to transport the lipids via blood to and from the tissues.

Importance of Lipoproteins

High-density lipoprotein

Highdenisty lipoprotein (HDL) is referred as the ‘good cholesterol’ because it carries cholesterol and

phospholipids from tissues and organs back to the liver for degradation and elimination. It prevents the

deposition of cholesterol on the walls of arteries, by carrying cholesterol away from arteries to the liver.

High level of HDL is a good indicator of a healthy heart, because it reduces the blood cholesterol level.

HDL also helps in the normal functioning of some hormones and certain tissues of the body. It is also used

for the formation of bile in liver.

Low-density lipoprotein

Lowdensity lipoprotein (LDL) is considered as the ‘bad cholesterol’ because it carries cholesterol and

phospholipids from the liver to different areas of the body, viz. muscles, other tissues and organs such as

heart. It is responsible for deposition of cholesterol on walls of arteries causing atherosclerosis (blockage and hardening of the arteries).

 Very-low-density lipoprotein

Verylowdensity lipoprotein (VLDL) carries cholesterol from liver to organs and tissues in the body. It is also associated with atherosclerosis and heart disease.

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