Lipids and Their Functions

Lipids

• Lipids are water-insoluble (hydrophobic) molecules that are highly soluble in organic solvents.
• They have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.
• This dual nature allows them to form structures like micelles and vesicles crucial for cellular function.

Learning Objectives

• Explain the structural properties of lipids.
• Explain the roles of lipids.
• Classify lipids.
• Recognize fatty acids.
• Explain lipid digestion and absorption.
• Recognize steroids.
• Classify lipoproteins.
• Explain fatty acid catabolism (beta oxidation).
• Explain fatty acid synthesis.

Types of Lipids

• Fatty acids
• Waxes
• Sphingolipids
• Phospholipids
• Steroids
• Triacylglycerides
• Ceramides
• Terpenes
• Eicosanoids
• And more...

Roles of Lipids

• Widely used to store energy (needs less water than carbohydrates).
• Key components of membranes.
• Elements in signaling (e.g., hormones).
• Thermal insulation.

Fatty Acids

• Simplest lipid unit.
• Consist of a hydrocarbon chain with a carboxyl group at one end.
• No polarity; hydrophobic.
• Saturated have no double bonds, unsaturated have one or more double bonds.
• Double bonds can be cis or trans.
• Saturated fats are usually solid at room temperature.
• Unsaturated fats are usually liquid at room temperature.
• Important for energy production and membrane integrity.
• Always have a carboxyl group at one end and a hydrocarbon chain other end

Triacylglycerols (TAGs)

• Storage lipids.
• Composed of glycerol and three fatty acids.
• Primarily used for energy storage.
• Contain 3 fatty acyl bonds

Phospholipids

• Important components of cell membranes.
• Backbone of glycerol or sphingosine.
• Glycerophospholipids (e.g., phosphoglycerides) and sphingophospholipids (e.g., sphingomyelins).
• Have a polar head group and nonpolar tails, crucial for membrane structure.

Glycolipids

• Contains carbohydrate in their structure.
• Involved in cell-cell recognition and signaling.
• Backbone can be glycerol or sphingosine;
• Ceramide is a common component in glycolipids

Steroids

• Composed of four fused hydrocarbon rings.
• Hormones (e.g., estrogen, testosterone, cortisol).
• Important roles in metabolism, signaling and reproduction.
• Lipid digestion (bile acid).
• Membrane component (cholesterol).

Lipid Digestion

• Occurs in the mouth, stomach, small intestine.
• Enzymes like lingual lipase and pancreatic lipase break down fats.
• Bile salts emulsify fats.
• Products are absorbed and transported in the intestines.

Lipid Absorption

• Small lipid products enter intestinal cells directly (free fatty acids, free cholesterol, 2-monoacylglycerol).
• Fat-soluble vitamins are absorbed in the intestines.

Lipid Transport via Lipoproteins

• Transport lipids in the blood.
• Different types include chylomicrons, VLDL, LDL, HDL.
• Classified by their density.
• Composed of a core of TAGs and cholesterol esters and an outer coat that includes apoproteins, phospholipids and cholesterol.

Lipid Catabolism (Fatty Acid Oxidation)

• Process of breaking down fatty acids for energy.
• CoA activation.
• Transport to mitochondria.
• Beta-oxidation.
• Acetyl CoA enters the citric acid (TCA) cycle for further ATP generation.
• Odd number of carbon fatty acids will produce Succinyl-CoA in the final step.
• Even number of carbon fatty acids will produce Acetyl-CoA in the final step.

Lipid Anabolism (Fatty Acid Synthesis)

• Process of building fatty acids from acetyl-CoA.
• Occurs in the cytoplasm of cells.
• Stages include:
  • Acetyl-CoA transport into cytosol
  • Malonyl-CoA synthesis
  • Elongation of the fatty acid chain
• Requires NADPH as a reducing agent.
• Can produce palmitate and other fatty acids.