Lipids Overview and Functions

Lipids: Overview

• Lipids are a diverse group of biomolecules, insoluble in water, soluble in nonpolar solvents.
• The term "lipid" comes from the Greek word "lipos," meaning fat
• They are not polymers of a specific monomer, unlike proteins or DNA, due to their diverse structures.
• They serve as energy storage, structural components of biomembranes and act as surfactants, detergents, and emulsifying agents. They also provide insulation and cushion internal organs, and help in absorbing fat-soluble vitamins.

Lipid Functions

• Triglycerides: Store energy.
• Phospholipids and cholesterol: Form structural components of biomembranes.
• Steroid hormones and prostaglandins: Act as metabolic regulators.
• Amphipathic lipids: Act as surfactants, detergents, and emulsifying agents, acting as insulators in neurons.
• Lipids provide insulation against temperature changes.
• Lipids protect internal organs.
• Lipids help with the absorption of fat-soluble vitamins A, D, E, and K.
• Lipids enhance the taste and palatability of food.

Classification of Lipids: Storage

• Triacylglycerols (TAGs) / Triglycerides: Esters of glycerol and fatty acids; major storage form of fats in animals and humans.
• Structure: Composed of a glycerol molecule and three fatty acid chains.
• Saturated fatty acids: Molecules closely packed, high melting points, solid form at room temperature.
• Unsaturated fatty acids: Nonlinear chains, do not pack closely, low melting points, liquid form at room temperature.

Classification of Lipids: Unsaturated Fatty Acids

• Polyunsaturated fatty acids (PUFAs): Essential fatty acids (mammals cannot synthesize them).
• Examples: Linoleic acid (omega-6) and linolenic acid (omega-3) - important for physiological functions.
• Arachidonic acid is derived from linoleic acid and is a precursor for many prostaglandins.

Classification of Lipids: Omega fatty acids

• Omega-6 fatty acid: Linoleic acid.
• Omega-3 fatty acid: Linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) - critical for many physiological processes.

Classification of Lipids: Waxes

• Esters of long-chain fatty acids and high molecular weight monohydric alcohols.
• Examples: Beeswax, lanolin.

Classification of Lipids: Membrane Lipids

• Phospholipids: Glycerols, fatty acids, phosphate, and a polar head group; are components of biological membranes.
• Examples of phosphoglycerides: Phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, and phosphatidylglycerol.
• Sphingolipids: Sphingosine, fatty acids, phosphate, and a polar head group; important components of membranes.
• Examples of sphingolipids: Ceramide, sphingomyelin, glycosphingolipids (e.g., cerebrosides, gangliosides), and sphingolipids with complex oligosaccharides

Classification of Lipids: Nonsaponifiable Lipids

• Terpenes: Polymers of isoprene unit, e.g., fat-soluble vitamins A, D, E, and K.
• Steroids: Characterized by a tetracyclic carbon skeleton. Includes cholesterol, bile acids, steroid hormones (e.g., cortisol, testosterone, estrogen), and Vitamin D.
• Steroid alcohols: Includes cholesterol and other sterols.

Lipids in Membranes

• Fluid Mosaic Model: Membranes are a dynamic structure consisting of phospholipids and proteins.
• Membrane asymmetry: The asymmetrical distribution of phospholipids and glycolipids in the bilayer of human RBCs is noted.
• Movement in the membrane: includes diffusion (lateral and transbilayer) and rotation.
• Cholesterol is a key regulator of fluidity: Important in maintaining membrane fluidity and preventing extremes in fluidity.

Transport Across Membranes

• Chemical Transport: Describes the movement of substances across cell membranes, encompassing passive diffusion, active transport (primary and secondary), and facilitated transport.
• Different factors such as size, charge, and lipid solubility influence how effectively a substance crosses the membrane.
• Carrier-Mediated Transport: Uses protein transporters to move substances across.
• Active Transport: Uses energy against the concentration gradient, moving substances from low to high concentration.

Lipoproteins

• Lipoproteins transport cholesterol and triacylglycerols in the blood.
• LDL ("bad cholesterol"): transports cholesterol to peripheral tissues.
• HDL ("good cholesterol"): picks up cholesterol from dying cells, delivering it to the liver for excretion.