CH2 CHEM PT 5 - FATS -Triglycerides, Phospholipids, Steroids, and Eicosanoids

Triglycerides

• Definition: A type of fat formed by linking glycerol to three fatty acids; the most common storage fat in the body.
• Structure:
  • Glycerol: a three-carbon backbone with an -OH group on each carbon (the left structure in the transcript).
  • Three long fatty acids attached to glycerol via ester bonds (hence a triglyceride).
  • Fatty acids: long hydrocarbon chains that are mostly nonpolar due to abundant C–H bonds; one end may appear polar due to a carboxyl group, but the overall molecule is nonpolar.
• Formation (dehydration synthesis):
  • Three fatty acids are linked to glycerol by removing water (dehydration synthesis).
  • General reaction:
    \text{glycerol} + 3\;\text{fatty acids} \rightarrow \text{triglyceride} + 3\,\mathrm{H_2O}
• Purpose and properties:
  • Energy storage: a triglyceride stores a large amount of energy.
  • Insulation and protection: provides thermal insulation and cushions organs.
• Breakdown (hydrolysis):
  • Water is added to break the ester bonds, regenerating glycerol and three fatty acids.
  • General reaction:
    \text{triglyceride} + 3\,\mathrm{H_2O} \rightarrow \text{glycerol} + 3\;\text{fatty acids}
• Transport in blood:
  • Triglycerides are nonpolar and insoluble in water, so they are transported as lipoproteins.
  • Lipoproteins coat triglycerides with polar proteins, allowing dissolution and transport in bloodstream.
• Saturated vs. unsaturated fats:
  • Saturated fats: fatty acids fully saturated with hydrogen; no C=C double bonds.
  • Structure example: a fully saturated chain often depicted as
    \mathrm{CH3- (CH2)_{n}-COOH}
  • Unsaturated fats: contain one or more C=C double bonds, creating kinks that prevent tight packing.
  • Consequences:
  • Saturated fats tend to be solid at room temperature.
  • Polyunsaturated (and monounsaturated) fats tend to be liquid at room temperature.
• Biological relevance:
  • Metabolism of saturated vs. unsaturated fats differs in the body.
• Summary: Triglycerides are nonpolar energy storage molecules that cannot dissolve in water and require lipoprotein transport in the bloodstream.

Phospholipids

• Structure:
  • Similar to triglycerides but with only two fatty acid chains and a polar head group containing phosphorus (the “P” in the transcript).
  • Polar head group is hydrophilic (water-attracting) due to the phosphate-containing group.
  • Two nonpolar fatty acid tails are hydrophobic (water-repelling).
  • Overall, phospholipids are amphipathic: have both hydrophilic and hydrophobic regions.
• Typical representation:
  • Diagrammed as a circle with two legs or a glycerol backbone with two tails and a polar head.
• Function in cells:
  • Form the phospholipid bilayer, the foundation of cell membranes.
  • In the bilayer, polar heads face outward toward water (intracellular and extracellular fluids) and nonpolar tails face inward, forming a nonpolar barrier.
• Membrane significance:
  • Creates a barrier separating the intracellular fluid from the extracellular fluid.
  • Supports differences in solute concentrations between the inside and outside of the cell.
  • In the next unit, further discussion on membrane composition and transport mechanisms.
• Comparison with triglycerides:
  • Phospholipid structure includes a polar head and two nonpolar tails, whereas triglycerides have three nonpolar fatty acid tails and no polar head.

Steroids (Steroid fats) & Cholesterol

• General idea:
  • Steroids are a class of fats with a distinctive four-ring carbon skeleton; nonpolar overall.
  • Cholesterol is the principal steroid cholesterol that serves as a precursor to other steroids and as a structural component of cell membranes.
• Structure:
  • Four fused hydrocarbon rings (steroid nucleus).
  • Not drawn in full here, but each ring contributes to a rigid, largely nonpolar framework.
• Functions and importance:
  • Hormone synthesis: Cholesterol is the precursor for steroid hormones such as estrogens, progesterone, and testosterone.
  • Hormones mentioned: estrogen, progesterone, testosterone, and cortisol (involved in long-term stress responses).
  • Membrane structure: Cholesterol helps modulate cell membrane fluidity and integrity.
• Important note:
  • While cholesterol sometimes has a negative reputation, it has essential physiological roles as a hormone precursor and membrane component.

Eicosanoids

• Definition:
  • A group of fats derived from essential fatty acids; essential fats must be obtained from the diet (cannot be synthesized by the body).
• Source and types:
  • Common essential fatty acids include omega-3 and omega-6 fatty acids.
  • Eicosanoids are derived from these essential fats; prostaglandins are a key group of eicosanoids.
• Functions and regulatory roles:
  • Prostaglandins:
  • Regulate blood clotting, mucus production in the stomach, uterine contractions, and body temperature control.
  • Leukotrienes (another type of eicosanoid):
  • Play a role in immune function.
• Significance:
  • Eicosanoids illustrate how essential dietary fats can be converted into powerful regulatory molecules with widespread physiological effects.

Visual example: phospholipid bilayer model

• The provided model represents a cell membrane comprised of phospholipids.
  • White spheres represent polar phospholipid heads (hydrophilic).
  • Red tails represent nonpolar fatty acid chains (hydrophobic).
  • Arrangement shows a phospholipid bilayer with heads exposed to aqueous environments and tails forming the nonpolar interior of the membrane.
• Takeaway:
  • This model reinforces the amphipathic nature of phospholipids and their critical role in forming a selective barrier between the cell’s interior and exterior.

Key connections and implications

• Core concepts:

  • Lipids are a diverse group of fats with distinct structures leading to different functions: energy storage (triglycerides), membrane structure (phospholipids), hormone synthesis and membrane rigidity (steroids/cholesterol), and regulatory signaling molecules (eicosanoids).
  • Nonpolar vs. polar properties dictate solubility and transport: triglycerides are nonpolar and require lipoproteins for blood transport; phospholipids create amphipathic membranes.

• Relevance to physiology:

  • Energy metabolism differences between saturated and unsaturated fats affect health and metabolism.
  • Phospholipid bilayers determine membrane fluidity and permeability, influencing transport and signaling.
  • Steroid hormones regulate reproductive, stress, and metabolic processes and are derived from cholesterol.
  • Eicosanoids link diet-derived fats to immune function, inflammation, and physiological regulation (e.g., clotting, gastric mucus, uterine activity, temperature).

• Practical implications:

  • Dietary essential fats (omega-3 and omega-6) are crucial for producing eicosanoids with diverse regulatory roles.
  • Understanding lipid transport explains why triglycerides travel in the bloodstream within lipoprotein particles rather than as free fats.

• Quick recap of the four types of fats covered:

  • Triglycerides: energy storage, nonpolar, three fatty acids on glycerol, dehydrative synthesis, hydrolysis for breakdown, transport via lipoproteins.
  • Phospholipids: two fatty acids, one phosphate-containing polar head, form cell membranes (bilayer), amphipathic.
  • Steroids (cholesterol): four-ring structure, nonpolar, hormone precursors, membrane component.
  • Eicosanoids: derived from essential fatty acids, include prostaglandins and leukotrienes, regulate numerous physiological processes.