Biochemistry of Lipids

Lipids

• Lipids are biological molecules that are insoluble in water but soluble in nonpolar solvents
• They are a major way of storing chemical energy and carbon atoms in the body
• Lipids repel water; this is useful for protective coatings on plants
• They are the most diverse class of biochemicals, often defined by function rather than structure
• Fats and oils have high energy density and low density, making them good energy storage forms
• Phospholipids, glycolipids, and cholesterol are components of cell membranes
• Lipid's identity is defined by a physical property, not a specific functional group

Classification of Lipids

• Lipids are crucial in three major biochemical roles: storing energy, making up membranes, and being chemical messengers
• Saponifiable lipids include triglycerides, waxes, phospholipids, and sphingolipids
• Nonsaponifiable lipids include steroids and prostaglandins
• These are not esters and do not undergo saponification because they lack ester bonds

Saponifiable Lipids

• Saponification is the hydrolysis of lipids under basic conditions, producing soap and glycerol
• Triglycerides are esters formed from glycerol and three fatty acids. They are the primary form of stored energy
• Phosphoglycerides (phospholipids) are glycerol-based and contain fatty acids, phosphate, and sometimes an alcohol; key membrane components
• Sphingolipids, not glycerol-based, are abundant in the nervous system

Nonsaponifiable Lipids

• These lipids cannot be hydrolyzed because they lack ester bonds
• Steroids have a four-fused carbon ring structure. Key examples include cholesterol (membrane component) and steroid hormones
• Prostaglandins are fatty acid-derived molecules, playing roles in inflammation and blood flow

Fatty Acids

• Fatty acids are long, unbranched carbon chains with a carboxyl group at one end
• The long, nonpolar tails are responsible for the oily characteristics of fats
• Fatty acids are amphipathic compounds (hydrophilic carboxyl group, hydrophobic hydrocarbon tail)
• In aqueous solutions, fatty acid ions associate to form spherical clusters called micelles
• Micelle formation is crucial for lipid transport in the bloodstream

Cis- and Trans-Fatty Acids

• Cis-unsaturated fatty acids have a kink caused by cis double bonds; this leads to lower melting points (often liquid at room temperature)
• Trans-unsaturated fatty acids have a linear shape due to trans double bonds; this leads to higher melting points (often solid at room temperature)
• Saturated fats contain no double bonds and are usually solid at room temperature
• Unsaturated fats have at least one double bond and are usually liquid at room temperature

Triglycerides (Fats and Oils)

• Triglycerides are the main structural form of fats and oils. They're formed from glycerol and fatty acids.
• Glycerol has three -OH groups, bonding with fatty acids
• Esters forming Triglycerides are called triacylglycerols or triglycerides

Chemical Reactions of Fats and Oils

• Hydrolysis is the reverse of esterification, breaking down triglycerides to glycerol and fatty acids. It is a key aspect of digestion.
• Hydrogenation converts unsaturated to saturated fatty acids, resulting in harder fats.

Phosphoglycerides

• These complex lipids are composed of glycerol, fatty acids, a phosphate group, and potentially other molecules. These are major components of cell membranes.

Sphingolipids

• Sphingolipids are a key component of cell membranes, especially in nerve and brain tissue
• Instead of glycerol, they contain a sphingosine backbone. They are complex lipids containing a long-chain, unsaturated amino alcohol—sphingosine. This is bonded with a fatty acid (amide bond) and another group (phosphate group or carbohydrate).
• Sphingomyelins have a choline attached via a phosphate group

Glycolipids

• Glycolipids are a type of sphingolipid containing carbohydrates (often glucose or galactose)
• Cerebrosides are common in brain tissue

Steroids

• Steroids have a characteristic four-fused ring structure
• Cholesterol is a crucial steroid in animal cell membranes
• Bile salts are cholesterol metabolites that aid fat digestion in the intestines

Steroid Hormones

• Adrenocorticoid hormones (cortisol and aldosterone) are produced by the adrenal cortex, regulating various metabolic processes
• Sex hormones (estrogen, testosterone) are crucial for reproductive function

Prostaglandins

• Prostaglandins are derived from 20-carbon unsaturated fatty acids (like arachidonic acid)
• Prostaglandins have various roles, including influencing inflammation, pain, fever, and blood clotting

Membrane Structure

• Cell membranes have a phospholipid bilayer structure. The hydrophobic tails face inward, and the hydrophilic heads face outwards

Fluid Mosaic Model

• A model of membrane structure in which proteins are embedded in a flexible lipid bilayer

Bile Salts

• Bile salts aid lipid digestion in the intestines. They emulsify fats (breaking them down into smaller droplets) and enable the absorption of lipids.