lipid digestion

Definition

Target type: biologicalprocess

The whole of the physical, chemical, and biochemical processes carried out by living organisms to break down ingested lipids into components that may be easily absorbed and directed into metabolism. [GOC:go_curators]

Lipid digestion is a complex process that begins in the mouth and continues through the small intestine. The process involves breaking down dietary fats into smaller molecules that can be absorbed by the body.

**1. Oral Cavity:**
- **Mechanical Digestion:** Chewing and the grinding action of teeth physically break down large fat globules into smaller ones, increasing their surface area for enzymatic digestion.
- **Lingual Lipase:** The tongue secretes a small amount of lingual lipase, an enzyme that begins the breakdown of triglycerides into diglycerides and fatty acids. However, this initial digestion is limited due to the short time food remains in the mouth.

**2. Stomach:**
- **Gastric Lipase:** The stomach secretes gastric lipase, another enzyme that acts on triglycerides, but its activity is also limited by the acidic environment of the stomach.
- **Mechanical Mixing:** The stomach's churning motion further mixes food with gastric juices, promoting emulsification. Emulsification involves breaking down large fat droplets into smaller ones, increasing their surface area for enzymatic digestion.

**3. Small Intestine:**
- **Bile Emulsification:** The liver produces bile, which is stored in the gallbladder and released into the small intestine. Bile salts are amphipathic molecules with both hydrophobic and hydrophilic regions. They surround fat droplets, breaking them down into smaller, suspended droplets, creating an emulsion. This process increases the surface area available for enzymatic digestion.
- **Pancreatic Lipase:** The pancreas secretes pancreatic lipase, the primary enzyme responsible for lipid digestion. Pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids.
- **Other Enzymes:** The small intestine also produces other enzymes, such as cholesterol esterase and phospholipase A2, which break down cholesterol esters and phospholipids, respectively.

**4. Absorption:**
- **Micelles:** The breakdown products of lipid digestion, including monoglycerides, fatty acids, cholesterol, and phospholipids, are incorporated into micelles. Micelles are small, spherical structures formed by bile salts.
- **Small Intestine Absorption:** Micelles transport these lipid breakdown products to the brush border of the small intestine, where they are absorbed into the intestinal cells (enterocytes).
- **Resynthesis:** Inside the enterocytes, monoglycerides and fatty acids are resynthesized into triglycerides. Triglycerides are then packaged with cholesterol and phospholipids into chylomicrons, lipoprotein particles that transport dietary fats in the bloodstream.

**5. Transport:**
- **Lymphatic System:** Chylomicrons enter the lymphatic system, which transports them to the bloodstream.
- **Bloodstream:** The bloodstream carries chylomicrons to various tissues and organs, where they are broken down and their contents used for energy production, cell membrane synthesis, and other metabolic processes.'
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