Lipids are essential molecules in our bodies, playing diverse roles from energy storage to cell membrane structure. They come in various forms, including triglycerides, phospholipids, and sterols, each with unique properties and functions.
Lipid metabolism is a complex process, starting with digestion in the mouth and stomach, then moving to the small intestine. Here, bile salts and enzymes break down fats, which are then absorbed, reassembled, and transported throughout the body for energy or storage.
Lipid Structure and Classification
Types and structures of lipids
- Triglycerides store energy in adipose tissue composed of glycerol backbone with three fatty acids attached
- Phospholipids build cell membranes with phosphate group and two fatty acids on glycerol, amphipathic nature allows bilayer formation
- Sterols feature four-ring structure, cholesterol most common in humans regulates membrane fluidity
- Waxes provide protective coatings (skin, leaves, fruits) formed by long-chain fatty acids esterified to long-chain alcohols
Saturated vs unsaturated fatty acids
- Saturated fatty acids contain no double bonds between carbons, solid at room temperature (butter, coconut oil)
- Monounsaturated fatty acids have one double bond, liquid at room temperature (olive oil, avocados)
- Polyunsaturated fatty acids possess two or more double bonds, liquid at room temperature, include essential omega-3 and omega-6 (fish, nuts, seeds)
Lipid Functions and Metabolism
Functions of lipids in body
- Energy storage yields 9 kcal/g when metabolized, stored in adipose tissue for long-term reserves
- Cell membrane structure regulates permeability and fluidity through phospholipid bilayer
- Hormone production derives steroid hormones from cholesterol, regulates metabolism and reproduction
- Insulation and protection provided by subcutaneous fat for thermal regulation, visceral fat cushions organs
- Vitamin absorption aids fat-soluble vitamins (A, D, E, K) uptake in intestines
Lipid digestion and metabolism
- Digestion begins with lingual lipase in mouth, continues with gastric lipase in stomach
- Pancreatic lipase in small intestine completes triglyceride hydrolysis
- Emulsification by bile salts increases surface area for enzyme action
- Absorption occurs as fatty acids and monoglycerides form micelles, enter intestinal epithelial cells
- Reassembly into triglycerides within enterocytes
- Transport via chylomicrons through lymphatic system, enter bloodstream through thoracic duct
- Metabolism involves lipoprotein lipase breaking down chylomicrons, fatty acids enter cells for energy or storage
- Beta-oxidation breaks down fatty acids in mitochondria, produces acetyl-CoA for Krebs cycle