28.12
脂質の吸収
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Q1: What role do bile salts play in lipid digestion?
Bile salts, produced by the liver, mix with dietary triglycerides in the duodenum to emulsify fats by breaking down large fat droplets into smaller ones. This increases surface area for enzymatic action, allowing pancreatic lipases to more effectively hydrolyze triglycerides into fatty acids and monoglycerides for absorption.
Q2: How do micelles facilitate lipid absorption in the intestine?
Fatty acids and monoglycerides bind with bile salts and lecithin to form micelles, which pass between microvilli and come in close contact with the apical enterocyte membrane. This positioning allows fatty acids and monoglycerides to leave the micelles and enter enterocytes via simple diffusion for further processing.
Q3: What happens to fatty acids after they enter enterocytes?
Once inside enterocytes via simple diffusion, fatty acids and monoglycerides undergo re-esterification back into triglycerides. These newly formed triglycerides combine with proteins and phospholipids within the enterocytes to create chylomicrons, large lipoprotein particles that transport lipids throughout the body during the absorptive state.
Q4: How do chylomicrons enter the bloodstream?
Chylomicrons exit enterocytes by exocytosis to enter lacteals, specialized lymphatic vessels in the intestinal villi. They travel through the lymphatic system and enter the bloodstream via the thoracic duct, where they circulate to deliver lipids to tissues throughout the body.
Q5: What is the function of lipoprotein lipase in lipid metabolism?
Lipoprotein lipase, associated with capillary endothelium, hydrolyzes triglycerides in chylomicrons into free fatty acids and glycerol. These breakdown products are then used for energy production or stored as fat in tissues, making this enzyme essential for lipid distribution and utilization.
Q6: What happens to chylomicron remnants after lipoprotein lipase action?
After lipoprotein lipase hydrolyzes triglycerides, chylomicron remnants are taken up by the liver, where they are broken down. The liver repurposes their components—cholesterol, phospholipids, and proteins—for synthesis of other lipoproteins or bile acids, which are important for lipid digestion and transport.
Q7: How does lipid absorption relate to the absorptive state?
Lipid absorption occurs during the absorptive state when dietary nutrients are being actively taken up and processed. During this metabolic phase, chylomicrons transport absorbed lipids through the lymphatic and circulatory systems, making lipid absorption a key component of the body the absorptive state.