Lipids as an Energy Molecule

Energy Yield of Lipids

• Lipids yield more energy than glucose or protein.
  • 9 kilocalories per gram vs 4 kilocalories per gram.
• More chemical bonds mean more potential energy.

Transport and Breakdown

• Products of fat digestion are transported via the lymphatic system as chylomicrons.
• Endothelial enzymes in the lymphatic system break triglycerides into fatty acids and glycerol.

Oxidation of Lipids

• Triglycerides are the primary lipids oxidized for energy.
• Lipid oxidation involves separate pathways for glycerol and fatty acids.

Glycerol Pathway

• Glycerol is a three-carbon molecule.
• Converted to Glyceraldehyde 3-Phosphate (G3P).
• Participates in glycolysis, yielding about half the ATP compared to glucose.

Fatty Acid Pathway

• Long hydrocarbon chains broken down into two-carbon acetyl CoA fragments.
• Beta oxidation is the process of breaking down fatty acids.
  • Involves higher energy investment than glucose metabolism.
  • Occurs at elevated body temperature.

Energy Storage and Utilization

• Lipogenesis: Synthesis of triglycerides when excess energy is available.
• Glucose is easily converted to fat through acetyl CoA.
• Lipolysis: Breakdown of stored fat into glycerol and fatty acids for fuel.

Essential Nutrients

• Lipids and proteins are essential; carbohydrates are not strictly necessary.
• Essential fatty acids are required (e.g., linoleic acid).

Metabolic Processes

• Gluconeogenesis and lipolysis often occur together.
• Cortisol stimulates the breakdown of lipids.
• Lack of oxaloacetate leads to ketogenesis and production of ketone bodies.
  • May cause metabolic acidosis and ketosis.
  • Kussmaul breathing helps mitigate acidosis.

Structural Role of Lipids

• Phospholipids: Essential for cell membranes and myelin.
• Cholesterol: Component of cell membranes and precursor for steroid hormones.
  • Synthesized in the liver.
  • Used to form bile salts.

Transport and Synthesis

• Lipoproteins transport lipids in circulation.
• Acetyl CoA is a key intermediate in lipid metabolism.
  • Converts into fatty acids, cholesterol, and serves as a reactant in the Krebs cycle.

Summary Flowchart

• Lipolysis produces glycerol and fatty acids.
• Glycerol enters glycolysis; fatty acids undergo beta oxidation.
• Acetyl CoA enters the Krebs cycle or is used for cholesterol synthesis.
• Reverse reactions: Acetyl CoA for fatty acids production, glycerol synthesis from glycolysis intermediates.