Transport of Lipids in the Blood

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Once the chylomicrons are in the blood, their triglyceride content is removed by the enzyme lipoprotein lipase, which is attached to the endothelium of blood vessels. This enzyme hydrolyzes triglycerides and thus provides free fatty acids and

Bile Salts Emulsify Fat Droplets

droplets fat droplets

(triglycerides) (triglycerides)

droplets fat droplets

(triglycerides) (triglycerides)

Step 1: Emulsification of fat droplets by bile salts

Step 2: Hydrolysis of triglycerides in emulsified fat droplets into fatty acid and monoglycerides Step 3: Dissolving of fatty acids and monoglycerides into micelles to produce "mixed micelles"

■ Figure 18.35 Fat digestion and emulsification. The three steps indicate the fate of fat in the small intestine. The digestion of fat (triglycerides) releases fatty acids and monoglycerides, which become associated with micelles of bile salts secreted by the liver.

The Digestive System

Triglycerides

Bile salts,

Lumen of small intestine

Fatty acids

Monoglycerides

Triglycerides

Bile salts,

Fatty acids

Monoglycerides

Lipid Digestion Micelle

Chylomicrons

To thoracic duct

Lacteal

Fatty acids Monoglycerides (

Triglycerides + Protein-|

Chylomicrons

To thoracic duct o o o O

Lumen of small intestine

Micelle Fat Absorption

Nmjiji

Lacteal

■ Figure 18.36 The absorption of fat. Fatty acids and monoglycerides from the micelles within the small intestine are absorbed by epithelial cells and converted intracellularly into triglycerides. These are then combined with protein to form chylomicrons, which enter the lymphatic vessels (lacteals) of the villi. These lymphatic vessels transport the chylomicrons to the thoracic duct, which empties them into the venous blood (of the left subclavian vein).

■ Figure 18.36 The absorption of fat. Fatty acids and monoglycerides from the micelles within the small intestine are absorbed by epithelial cells and converted intracellularly into triglycerides. These are then combined with protein to form chylomicrons, which enter the lymphatic vessels (lacteals) of the villi. These lymphatic vessels transport the chylomicrons to the thoracic duct, which empties them into the venous blood (of the left subclavian vein).

Table 18.8 Characteristics of the Lipid Carrier Proteins (Lipoproteins) Found in Plasma

Lipoprotein Class

Origin

Destination

Major Lipids

Functions

Chylomicrons

Intestine

Many organs

Triglycerides, other lipids

Deliver lipids of dietary origin to body cells

Very-low-density

Liver

Many organs

Triglycerides,

Deliver endogenously produced triglycerides

lipoproteins (VLDLs)

cholesterol

to body cells

Low-density lipoproteins

Intravascular removal of

Blood vessels, liver

Cholesterol

Deliver endogenously produced cholesterol

(LDLs)

triglycerides from VLDLs

to various organs

High-density lipoproteins

Liver and intestine

Liver and steroid-

Cholesterol

Remove and degrade cholesterol

(HDLs)

hormone-producing glands

glycerol for use by the cells. The remaining remnant particles, containing cholesterol, are taken up by the liver. This is a process of endocytosis (chapter 3; see fig. 3.4) that requires membrane receptors for the protein part (or apoprotein) of the remnant particle.

Cholesterol and triglycerides produced by the liver are combined with other apoproteins and secreted into the blood as very-low-density lipoproteins (VLDLs), which deliver triglycerides to different organs. Once the triglycerides are removed, the VLDL particles are converted to low-density lipoproteins (LDLs), which transport cholesterol to various organs, including blood vessels. This can contribute to the development of atherosclerosis (chapter 13). Excess cholesterol is returned from these organs to the liver attached to high-density lipoproteins (HDLs). A high ratio of HDL-cholesterol to total cholesterol is believed to afford protection against atherosclerosis. The characteristics of these lipoproteins are summarized in table 18.8.

Test Yourself Before You Continue

1. List the enzymes involved in carbohydrate digestion, indicating their origins, sites of action, substrates, and products.

2. List each enzyme involved in protein digestion, indicating its origin and site of action. Also, indicate whether the enzyme is an endopeptidase or exopeptidase.

3. Describe how bile aids both the digestion and absorption of fats. Explain how the absorption of fat differs from the absorption of amino acids and monosaccharides.

4. Trace the pathway and fate of a molecule of triglyceride and a molecule of cholesterol in a chylomicron within an intestinal epithelial cell.

5. Cholesterol in the blood may be attached to any of four possible lipoproteins. Distinguish among these proteins in terms of the origin and destination of the cholesterol they carry.

HPer Links of the Digestive System with Other Body Systems

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Responses

  • Tom
    What enzyme hydrolyzes triglycerides from protein?
    8 years ago
  • Coleen
    What enzyme transports lipid in the blood?
    8 years ago
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    What lipids enter the blood stream quickest?
    8 years ago
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    7 years ago
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    7 years ago
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    6 years ago
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    4 years ago
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    4 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    3 years ago
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    2 years ago
  • Marroc
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
  • j
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    2 years ago
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    2 years ago
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    2 years ago
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    2 years ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    1 year ago
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    11 months ago
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    11 months ago
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    How are fatty acids and monoglycerides absorbed into the small intestine epithelial cells?
    10 months ago
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    9 months ago
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    9 months ago
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    9 months ago
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    8 months ago
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    8 months ago
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    8 months ago
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    7 months ago
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    6 months ago
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    6 months ago
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    6 months ago
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    6 months ago
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    5 months ago
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    5 months ago
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    4 months ago
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    4 months ago
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    4 months ago
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    4 months ago
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    How are digested lipids transported to liver?
    3 months ago
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    3 months ago
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    What are porteins transported by blood?
    2 months ago
  • Carl
    HOW ARE FREE FATTY ACIDS TRANSPOTED IN BLOOD?
    2 months ago
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    2 months ago
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    2 months ago
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    2 months ago
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    1 month ago
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    1 month ago
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    1 month ago
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    1 month ago
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    15 days ago
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    How the structure of lipoproteins molecules enable lipids to be transported in blood.?
    12 days ago
  • Olli-Pekka
    How does the structure of lipoprotein molecules enable lipids to be transported in the blood?
    12 days ago
  • Kevin
    How do chylomicrons transport fatty acids?
    7 days ago

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