Lecture 5 - LIPIDS

Lipids

• Lipids are biological molecules that are insoluble in water but soluble in nonpolar solvents.
• They have a wide spectrum of compositions and structures defined in terms of physical properties like water solubility.

Characteristics of Lipids

• Lipids are waxy, greasy, oily compounds found in both plants and animals.
• They serve various purposes such as energy storage, structural components (like cell membranes), and insulation against the cold.

Fatty Acids

• Fatty acids are the building blocks of lipids and are responsible for the fatty or oily characteristics of lipids.
• They have long, nonpolar hydrocarbon tails and a hydrophilic carboxyl group.
• Fatty acids can associate in aqueous solutions to form micelles, which are important for transporting insoluble lipids in the blood and the actions of soaps.
• Fatty acids usually have straight chains with 10 to 20 carbon atoms and even numbers of carbon atoms (counting the carboxyl carbon).
• They can be saturated (containing only single bonds) or unsaturated (containing double bonds) with different melting points based on chain length and saturation level.

Essential Fatty Acids

• Essential fatty acids, like linoleic acid and linolenic acid, cannot be synthesized by the body and must be obtained from the diet.

Triglycerides

• Triglycerides are esters composed of three molecules of fatty acids connected to a glycerol molecule, forming a structure called a triglyceride triacylglycerol.
• Hydrolysis breaks down triglycerides into fatty acids and glycerol, while saponification with strong bases forms carboxylate salts of fatty acids like soaps.

Waxes

• Waxes are simple lipids containing fatty acids joined to long-chain alcohols, with higher melting points and hydrophobic properties.

Phosphoglycerides and Sphingolipids

• Phosphoglycerides are major components of cell membranes, related to phospholipids, and contain amino alcohols like choline, ethanolamine, or serine.
• Sphingolipids are complex lipids that contain sphingosine instead of glycerol, with important compounds like sphingomyelins found in nerve tissue.

Glycolipids and Steroids

• Glycolipids are sphingolipids containing carbohydrates and are abundant in brain tissue, sometimes referred to as cerebrosides.
• Steroids are a class of lipids soluble in nonpolar solvents, characterized by a cyclic structure held together by ester linkages.

Cholesterol and Bile Salts

• Cholesterol is an essential component of cell membranes and a precursor to various hormones, but high levels in the blood can lead to atherosclerosis.
• Bile salts help emulsify fats in the digestive system, leading to better digestion, while issues like gallstones can arise from imbalances in bile composition.

Steroid Hormones and Prostaglandins

• Steroid hormones, like sex hormones and glucocorticoids, play important roles in regulating various bodily functions.
• Prostaglandins are cyclic compounds involved in processes like reproduction, blood clotting, and inflammation, with aspirin working to inhibit their production for relief from inflammation and fever.

Fatty Acid Structure

• Most fatty acids found in nature have an even number of C atoms (usually 14-24).
• Fatty acids can be saturated or unsaturated, with mono- or poly-unsaturated bonds.
• Marine organisms have an odd number of fatty acids.
• Fatty acids can be named in at least three ways: Systematic (octadecanoic acid), Common Name (stearic acid), and Symbol (18:0).
• The most common saturated fatty acid in nature is stearic acid (18:0) and palmitic acid (16:0).
• Saturated fatty acids are very flexible due to rotation around each of the C-C bonds.
• Unsaturated fatty acids are slightly more abundant in nature, especially in higher plants.
• Double bonds found in fatty acids are nearly always found in cis configuration, causing a bend or kink in the fatty acid chain.