Understanding Lipids: Structure and Function

• Lipids are biomolecules soluble in organic solvents but insoluble in water
• Lipids are defined by a physical property, not by the presence of a particular functional group
• Lipids contain many nonpolar C-C and C-H bonds and only a few polar bonds, making them water-insoluble

Lipids

• Biological compounds soluble only in nonpolar solvents
• Saponifiable lipids contain esters that hydrolyze under basic conditions, categorized by the number of components
• Simple lipids contain one or more fatty acids and an alcohol
• Complex lipids contain more than two types of components, including alcohols, fatty acids, and other components
• Nonsaponifiable lipids do not contain esters and cannot be hydrolyzed

Functions of Lipids

• Protective wax coatings exist on some plants
• Lipids are energy-rich compounds with low densities
• Lipids serve as a storage form of energy for plants and animals
• Lipids are structural components, especially in cellular membrane formation
• Biomolecules are organic compounds essential to life

Fatty Acids

• Fatty acids are portrayed with a carboxylic acid ionized group at one end and a zig-zag line representing the carbons
• The alpha carbon is the first carbon following the carboxyl carbon
• The beta carbon is the second carbon following the carboxyl carbon
• The omega carbon is the last carbon in the chain, farthest from the carboxyl group
• Fatty acids form spherical clusters called micelles in water
• Micelles are important for biological functions, like the transport of insoluble lipids in the blood
• Hydrophobic tails turn inward while hydrophilic polar heads face outward in micelles, formed from oil droplets in water
• Cis configuration creates a bend in the fatty acid chain, preventing close packing
• This leads to weaker intermolecular forces and lower melting points
• Fatty acids are liquids at room temperature
• The melting point decreases as the number of carbon double bonds increases
• The length and presence of double bonds explain fluidity of biological membranes

Essential Fatty Acids

• Essential fatty acids are needed by the body but not synthesized in adequate amounts
• Linoleic and linolenic acids are essential for humans and are obtainable from plant and fish oils
• Linoleic acid is an omega-6 fatty acid, while linolenic acid is an omega-3 fatty acid
• Linoleic and linolenic acids produce hormone-like substances that regulate functions such as blood pressure, blood clotting, lipid levels, the immune response, and the inflammation response to injury and infection
• Linoleic and linolenic acids can be converted to other omega-3 and omega-6 fatty acids

Fats vs Oils

• Fats are derived from animal sources and are solids at room temperature
• Fats primarily consist of triglycerides of long-chain saturated fatty acids
• Oils are derived from plant and fish sources and are liquids at room temperature
• Oils are composed of triglycerides that contain unsaturated fatty acids

Fats as Esters

• Fats and oils are esters containing an alcohol portion derived from glycerol and an acid portion furnished by fatty acids
• Esterification forms triglycerides or triacylglycerols
• Natural triglycerides are mixtures of different triglyceride molecules
• Hydrolysis of fats and oils is important for digestion, resulting in glycerol and fatty acids
• This process can be catalyzed by enzymes (lipases) of the digestive system
• Saponification properties of soap depend on the base used
• Sodium salts (hard salts) are found in cake soap
• Potassium salts (soft soaps) are found in shaving creams and liquid soap preparations
• Traditional soap making used animal fat as the source of triglycerides and lye (crude NaOH) or an aqueous extract of wood ash as the source of the base
• Excessive fat in the diet is a risk factor in the development of chronic disease
• It raises blood cholesterol levels
• High cholesterol leads to coronary heart disease, a leading cause of death

Phospholipids

• Phospholipids are diverse functional and structural lipids
• A phosphate group (PO4) attached to choline or another amine is substituted for one of the fatty acids attached to the glycerol
• The two fatty acids will vary, and are almost always unsaturated

Cell Membranes

• Prokaryotic cells are simple unicellular organisms lacking a nucleus or membrane-enclosed organelles
• Eukaryotic cells contain membrane-enclosed organelles, particularly a nucleus
• An organelle is a specialized structure within a cell performing a specific function
• Membrane roles include external cell membrane functions as a selective barrier and internal membranes surround some organelles, creating separate cellular compartments
• Most membranes are 60% lipid and 40% protein
• Lipids in membranes are phosphoglycerides, sphingomyelin, and cholesterol
• Lipids organize in a lipid bilayer with hydrophobic (long carbon chain) portions inside and hydrophilic (polar groups) exposed to the water environment
• When a lipid bilayer breaks and tails expose to water, the resulting repulsion causes the bilayer to reform and spontaneously seal

Steroids

• Steroids are a broad class of compounds with the same structural feature
• Steroids, like lipids, are soluble in nonpolar solvents
• Cholesterol is the most abundant steroid in the human body
• It is an essential component of cell membranes, a precursor of other steroids, and synthesized by the liver while also present in foods
• A strong correlation exists between cholesterol levels and atherosclerosis (hardening of the arteries)

Hormones

• Hormones are chemical messengers secreted by specific glands, carried through the blood to a target tissue, and trigger a particular response
• Steroid hormones include adrenocorticoid hormones and sex hormones
• Androgens (example testosterone) are male sex hormones produced by testes
• Estrogen and progesterone are female sex hormones produced by ovaries, including estradiol and estrone (the most important estrogens)

Prostaglandins

• Prostaglandins are cyclical compounds synthesized from arachidonic acid
• Prostaglandins are similar to hormones and intimately involve most body processes, all phases of reproduction, and blood clotting
• Prostaglandin E2 (PGE2) and prostaglandin F2 (PGF2) can induce labor and are used for therapeutic abortion in early pregnancy
• PGE2 in aerosol form treats asthma by opening up bronchial tubes by relaxing the surrounding muscles
• Other prostaglandins inhibit gastric secretions and treat peptic ulcers
• Fully understood prostaglandins will be useful for treating many ailments