Lipids Overview and Classification

Lipids Overview

• Lipids are water-insoluble (hydrophobic) molecules, but highly soluble in organic solvents.
• They have both hydrophilic (water-loving) and hydrophobic (water-fearing) parts, this is crucial for their function in biological systems.

Learning Objectives

• Explain structural properties of lipids
• Explain the roles of lipids
• Classify lipids (including fatty acids, steroids, phospholipids, waxes, sphingolipids, triacylglycerides, and others)
• Explain lipid digestion and absorption
• Recognize steroids (e.g., cholesterol, testosterone, estrogen)
• Classify lipoproteins
• Explain fatty acid catabolism (beta-oxidation)
• Explain fatty acid synthesis

Types of Lipids

• Fatty acids: Basic building blocks of many lipids
• Waxes: Contain long-chain fatty acids and alcohols; serve as protective coatings
• Sphingolipids: Important components of cell membranes
• Phospholipids: Key components of cell membranes, form bilayers
• Steroids: Four fused carbon rings, include hormones and cholesterol
• Triacylglycerides (Triglycerides): Store energy
• Terpenes: Important components of many essential oils and pigments
• Eicosanoids: Have diverse roles in cell signaling

Roles of Lipids

• Energy storage: Lipids store energy more efficiently than carbohydrates per gram (needs less water, more efficient energy storage)
• Membrane components: Lipids are essential components of cell membranes
• Signaling molecules: Lipids function as signaling molecules (e.g. hormones).
• Thermal insulation: Lipids provide thermal insulation in animals.

Lipid Digestion

• Digestion: Breaks down large fat molecules into smaller pieces.
• Absorption: Lipids are taken up by the intestinal cells
• Transportation: The products of digestion are transported to other parts of the body

Important Biological Lipids

• Free fatty acids
• Triacylglycerols
• Phospholipids
• Glycolipids
• Steroids

Triacylglycerols

• Triacylglycerol (TAG), also called triglyceride, is a primary energy storage form.
• Composed of glycerol and three fatty acids.
• Formed by dehydration reactions between glycerol and fatty acids.
• Crucial for energy storage in adipose tissue.

Phospholipids

• Glycerophospholipids (phosphoglycerides) and sphingophospholipids (phosphosphingolipids) depend on backbone
• Composed of glycerol, two fatty acids, and a phosphate group.
• Key components of cell membranes because of their dual nature (hydrophobic tails and a hydrophilic head region).

Glycolipids

• Glycolipids are a class of lipids present in cell membranes and are formed by a lipid backbone and carbohydrate groups.
• The carbohydrate groups are crucial for cell-cell recognition and signaling.

Steroids

• Steroids consist of four hydrocarbon rings.
• They have a crucial role as hormones (e.g., estrogen, testosterone) and components of cell membranes (e.g., cholesterol)

Fatty Acids

• Fatty acids are long carbon chains, usually 12-24 carbons in length.
• Typically have a carboxylic acid group.
• Hydrophobic hydrocarbon tail and a hydrophilic head.
• Can be saturated or unsaturated (presence of double bonds)

Lipid Metabolism

• Lipids are broken down (catabolized) for energy.
• Fatty acid oxidation occurs in mitochondria.
• Processes like beta-oxidation are involved in breaking down fatty acids for energy release.

Fatty Acid Synthesis

• Acetyl CoA transport
• Malonyl CoA synthesis
• Elongation of the fatty acid chain
• Lipids are synthesized for storage, cell membranes, or other functions.

Lipoproteins

• Important for lipid transport in the blood
• Different classes of lipoproteins (e.g, Chylomicrons, VLDL, LDL, HDL)
• Lipoproteins are complexes of lipids and proteins to transport lipids through bloodstream

Lipid Catabolism

• Glycerol is initially catabolized into G-3-P.

• Specific steps/reactions are involved depending on initial lipid source (fatty acid or glycerol)

  • Even length fatty acid chains break down to Acetyl CoA which enters the TCA cycle.
  • Odd length fatty acid chains undergo different processes to break down to final components that can enter the TCA cycle.

Lipids in the body

• Important for functions like energy storage, membrane structure, and hormone production

Digestion of dietary lipids

• Dietary lipids undergo digestion, absorption and transportation in the intestine and bloodstream
• Various enzymes and mechanisms (including emulsified breakdown) are involved in reducing lipid size for efficient absorption

Lipid Synthesis

• Synthesis of important biological lipids occurs with specific enzymes and pathways, such as fatty acid synthesis, steroid synthesis etc.