Biochem 9.1 Energy Storage Lipids Overview

Energy Storage Lipids

• Lipids are the fourth major class of biological macromolecules, following proteins, carbohydrates, and nucleic acids.
• Lipids are hydrophobic, meaning they are insoluble in water.
• Lipid molecules have few or no polar functional groups, interacting with water through weak London dispersion forces.
• Their aggregation in aqueous solutions maximizes water entropy, driven by hydrophobic interactions.
• Energy storage lipids aggregate into lipid droplets to minimize the effect on osmolarity.
• Lipids can be used as an energy source for most cells.
• Lipid aggregation pulls lipids out of solution, similar to the precipitation of ionic salts.
• A small amount of lipid remains solvated to maintain the equilibrium constant.
• Lipids separate into a distinct liquid phase, rather than forming a solid.
• This minimizes the effect individual lipid molecules have on osmolarity, allowing cells to maintain osmolarity while storing large energy reserves.

General Properties of Lipids

• Lipids are defined by their hydrophobicity and water insolubility.
• This is due to the limited polar functional groups in these molecules compared to their size.
• Water molecules form a solvation layer around lipids, but water molecules' rotational freedom is restricted.
• This limits water-lipid interaction, favoring lipid aggregation in aqueous solutions.

Fatty Acids

• Fatty acids are carboxylic acids with an aliphatic hydrocarbon group.
• The carboxyl group is hydrophilic, while the hydrocarbon tail is hydrophobic.
• The combined hydrophilic and hydrophobic properties make them amphipathic.
• A typical fatty acid has a 16-carbon chain.
• Carbon numbering in fatty acids starts from the carboxylic acid, with the Greek letter "a" denoting the second carbon (e.g., carbon 2 is the a-carbon).
• The final carbon in a fatty acid chain is called the omega-carbon (ω carbon).
• Fatty acids can vary in the number of carbons and degrees of unsaturation.
• Fatty acids with 7-12 carbon chains are medium-chain fatty acids, and those with 6 or fewer carbons are short-chain fatty acids.
• Most important fatty acids in human metabolism have an even number of carbons.
• Fatty acids can be saturated (no carbon-carbon double bonds) or unsaturated (with at least one).
• Unsaturated fatty acids typically have cis double bonds, creating a kink in the hydrocarbon tail.
• These kinks affect the physical state of fatty acids (e.g., solids or liquids).
• Fatty acids can have multiple double bonds, but these are typically not conjugated (separated by at least one CH₂ group).
• Fatty acids are often classified by their w (omega) numbering of the nearest double bond to the end of the fatty acid chain.

Triacylglycerides (Triglycerides)

• Triglycerides are the primary energy storage lipid. This is because they store large amounts of energy per unit volume/mass.
• Fatty acids are esterified to a glycerol backbone to form triacylglycerols (triglycerides).This makes the carboxylate group more hydrophobic, and hence aggregate into lipid droplets.
• Triglycerides are much less polar than fatty acids, making them more hydrophobic and easily stored in lipid droplets.
• Glycerol in triglycerides is prochiral, becoming chiral only when the esterified fatty acids on carbon 1 and 3 differ.
• Triglycerides are stored in adipose tissue (fat tissue).
• When needed, triglycerides are hydrolyzed into free fatty acids (FFAs) and glycerol.
• FFAs are transported in the bloodstream bound to carrier proteins (e.g., albumin), reaching target tissues.
• Intact triglycerides are transported via lipoprotein particles (e.g., chylomicrons, HDL, LDL), which are surface-bound and delivered to the correct tissue.

Explore the characteristics and properties of energy storage lipids, the fourth major class of biological macromolecules. This quiz will cover their hydrophobic nature, aggregation in aqueous solutions, and the role of fatty acids. Learn how lipid structure influences their function in biological systems.