Lipides: caractéristiques générales et types

Lipids: General Characteristics

• Lipids are a diverse group of hydrophobic organic molecules, insoluble in water.
• They are crucial components of cell membranes, energy storage, and signaling.
• Key roles include acting as structural components of cell membranes, storing energy, and serving as signaling molecules.
• Common lipids include fatty acids, triglycerides, phospholipids, and sterols.
• Fatty acids are long hydrocarbon chains with a carboxyl group at one end.

Types of Lipids

• Fatty Acids:
  • Saturated fatty acids have no double bonds between carbon atoms.
  • Unsaturated fatty acids have one or more double bonds between carbon atoms, usually cis configurations.
  • Monounsaturated fatty acids have one double bond, whereas polyunsaturated fatty acids have more than one double bond.
  • Different fatty acid chains have different melting points.
• Triglycerides (or triacylglycerols):
  • Composed of three fatty acids esterified to a glycerol molecule.
  • Major form of energy storage in animals and plants.
  • Stored in adipose tissue for long-term energy reserves.
• Phospholipids:
  • Composed of a glycerol backbone, two fatty acids, and a phosphate group.
  • Crucial components of cell membranes.
  • Amphipathic, having both hydrophobic (fatty acid) and hydrophilic (phosphate) regions enabling them to form bilayers.
• Sterols:
  • Examples include cholesterol, estrogen, testosterone.
  • Four fused carbon rings structure.
  • Important components of cell membranes and precursors for steroid hormones.
  • Cholesterol is crucial for membrane fluidity and a precursor for various steroid hormones.

Lipid Functions

• Energy Storage:
  • Triglycerides are the major energy storage form.
  • Oxidation of fatty acids yields more energy per gram than carbohydrates.
• Membrane Structure:
  • Phospholipids form the basic structure of cell membranes.
  • Cell membrane fluidity is influenced by fatty acid saturation and cholesterol levels.
• Hormone Production:
  • Steroids, derived from cholesterol, serve as hormones.
  • Examples include sex hormones (estrogen, testosterone, progesterone) and adrenal cortical hormones.
• Insulation and Protection:
  • Fats provide insulation against cold temperatures.
  • They cushion internal organs and protect them from physical shock.

Lipid Digestion and Absorption

• Lipids are digested in the small intestine.
• Bile salts emulsify fats into smaller droplets, increasing the surface area for lipase action.
• Pancreatic lipases hydrolyze triglycerides into fatty acids and monoglycerides.
• Absorbed fatty acids and monoglycerides are re-synthesized into triglycerides and incorporated into chylomicrons.
• Chylomicrons are transported through the lymphatic system and then into the bloodstream.

Lipid Metabolism

• Fatty acid oxidation is a major metabolic pathway for energy production.
• β-oxidation involves breaking down fatty acids into acetyl-CoA molecules, which enter the citric acid cycle.
• Lipogenesis is the synthesis of fatty acids from acetyl-CoA.
• Lipid metabolism is regulated by hormones, such as insulin and glucagon.
• Dysregulation of lipid metabolism can lead to various health problems like obesity and cardiovascular disease.