6.1 Classification and structure of lipids

Lipids come in various forms, each with unique structures and functions. Triglycerides, the main dietary fat, store energy. Phospholipids and glycolipids form cell membranes. Fatty acids can be saturated or unsaturated, affecting their physical properties.

Sterols, like cholesterol, play crucial roles in membrane structure and hormone synthesis. Waxes provide protective coatings in nature and have industrial uses. Understanding lipid types helps explain their diverse roles in nutrition and biology.

Triglycerides and Fatty Acids

Structure and Composition of Triglycerides

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• Triglycerides consist of a glycerol backbone with three fatty acids attached through ester bonds
• Most common form of dietary lipids and the primary form of energy storage in the body
• Fatty acids are long hydrocarbon chains with a carboxyl group at one end
• Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds)

Saturated and Unsaturated Fatty Acids

• Saturated fats have no double bonds between carbon atoms in the fatty acid chain
• Saturated fats are solid at room temperature (butter, lard, coconut oil)
• Unsaturated fats have one or more double bonds between carbon atoms in the fatty acid chain
• Monounsaturated fats have one double bond (olive oil, avocados, nuts)
• Polyunsaturated fats have two or more double bonds (vegetable oils, fatty fish)
• Unsaturated fats are typically liquid at room temperature

Phospholipids and Glycolipids

Structure and Function of Phospholipids

• Phospholipids have a glycerol backbone with two fatty acids and a phosphate group attached
• The phosphate group is linked to a polar molecule (choline, serine, ethanolamine)
• Phospholipids are amphipathic molecules with a hydrophilic head and hydrophobic tails
• Phospholipids are the primary components of cell membranes forming a bilayer structure

Glycolipids and Their Roles

• Glycolipids have a glycerol backbone with one or two fatty acids and a carbohydrate group attached
• The carbohydrate group can be a single sugar or a complex oligosaccharide
• Glycolipids are also amphipathic molecules found in cell membranes
• Glycolipids play important roles in cell recognition, signaling, and adhesion

Sterols and Waxes

Cholesterol and Its Functions

• Sterols are lipids with a four-ring structure and a hydroxyl group
• Cholesterol is the most common sterol in animal tissues
• Cholesterol is an essential component of cell membranes influencing fluidity and permeability
• Cholesterol serves as a precursor for the synthesis of steroid hormones, bile acids, and vitamin D

Properties and Uses of Waxes

• Waxes are esters of long-chain fatty acids and long-chain alcohols
• Waxes have hydrophobic properties and are resistant to hydrolysis
• Waxes provide protective coatings on surfaces (plant cuticles, insect exoskeletons, animal fur)
• Waxes are used in various applications (candles, polishes, cosmetics, food coatings)