Lipids Overview

Lipids

• Lipids are a group of water-insoluble organic molecules extracted from tissues using nonpolar solvents.
• Lipids are not polymers.
• Lipid oxidation releases a large amount of energy.
• Key biological functions of lipids include energy storage, heat insulation, structural components of cell membranes, and communication.

Types of Lipids

• Fatty acids: Can exist free or as esters (TAGs). Found in high levels in plasma during fasting; circulate bound to albumin.
• Triacylglycerols (TAGs): Also known as triglycerides. Consists of a glycerol head group attached to 3 fatty acid chains. Stored as cytoplasmic lipid droplets.
• Phospholipids: Major components of cell membranes. Two forms: phosphoglycerides and sphingolipids.
• Sphingolipids: Based on sphingosine (C18 amine alcohol) instead of glycerol. Fatty acids and R groups are added. Functions include cell recognition, signaling, and the formation of lipid rafts. Sphingolipids are a major class of glycolipids in humans.
• Steroids: Lipids with a common carbon skeleton consisting of four fused carbon rings. Different steroids are created by varying functional groups attached to the rings. Cholesterol is the principal steroid synthesized by mammals and is a precursor for other steroids.

Fatty Acids

• Fatty acids exist either freely or as esters (TAGs).
• Fatty acids (FAs) are found at high levels in plasma during fasting, and circulate bound to albumin.
• Fatty acids have long hydrocarbon chains of varying lengths (4-40 carbons), and are amphipathic.
• Common fatty acids include C16 and C18 in plants and animals.
• Fatty acids can be saturated (single bonds between carbons) or unsaturated (one or more C=C double bonds). Unsaturated fatty acids have a "bend" in their structure because of the double bonds, reducing their melting point (Tm).
• The carboxyl head group is polar and hydrophilic, whereas the fatty acid chain is nonpolar and hydrophobic.

Triacylglycerols (TAGs)

• 90% of dietary lipid intake is in the form of TAGs.
• The liver produces TAGs, transporting them to tissues in lipoproteins.
• TAGs are hydrolyzed to free fatty acids (FFAs) and glycerol by the enzyme lipase.
• TAGs are fats, and fatty acids in TAGs can be the same (homogenic) or different (heterogenic).

Phospholipids

• Phospholipids are a major component of cell membranes. They are composed of glycerol + 2 FAs + a phosphate group.
• When the R group in a phospholipid is choline, it is called glycerophosphatidyl choline.

Steroids

• Cholesterol is the principal steroid synthesized by mammals, synthesized in the liver and obtained from the diet. Cholesterol is also a component of eukaryotic cell membranes and is the precursor for other steroids.
• Cholesterol synthesis involves a rate-limiting step using HMG-CoA Reductase.
• Steroids are lipids with a common carbon skeleton, consisting of four fused carbon rings. Different steroids are created by the different functional groups added to the rings.

Cholesterol Synthesis

• The average total body cholesterol is ~150g, 90% of which is part of cell membrane structure.
• The rate-limiting step in cholesterol synthesis is HMGCoA reductase.

Lipid Transport

• Dietary lipids are absorbed from the intestine to the liver (exogenous).
• Lipids are then transported from the liver to peripheral tissues (endogenous).
• This process is carried out by apolipoproteins (large, spherical complexes).
• The core of complexes is composed of triglycerides and cholesterol esters, with a surface layer of phospholipids and cholesterol. Examples are CM, VLDL, LDL, and HDL.

Lipoproteins

• Chylomicrons have the lowest % weight protein, and highest % weight triacylglycerols.
• VLDLs are the second highest in triacylglycerols.
• LDLs are highest in cholesterol esters.
• HDLs have the highest protein to lipid ratio, and are highest in density.

Summary

• Lipids consist of hydrophobic organic compounds.
• Fatty acids are long chain carboxylic acids that can be saturated or unsaturated.
• Triglycerides (TAGs) contain 3 esterified fatty acids to glycerol.
• Phospholipids include a phosphate group instead of one fatty acid.
• Sphingolipids are built on a sphingosine backbone.
• Cholesterol is the primary steroid produced in mammals, synthesized in the liver, and obtained from the diet.
• Dietary lipids are metabolized and transported as lipoproteins.

This quiz covers the fundamentals of lipids, including their chemical nature, types, and biological functions. Learn about fatty acids, triacylglycerols, phospholipids, and sphingolipids, alongside their roles in energy storage and cell membranes.