Welcome to class of Lipids Dr. Meera Kaur

Learning objectives To understand * various lipids by source, structure and use: triglycerides, fats and oil and waxes * the fluid mosaic model to describe the movement of lipid molecule in membrane * the source and functions of some sterols: cholesterol. ergosterol etc.

Introduction Lipids (Greek: lipos, means fat or lard) - are a heterogeneous class of naturally occurring organic substances - have a distinguished functional group or structural features - are insoluble in water and highly soluble in one or more of the following solvents: ether, chloroform, benzene and acetone.This property sets them apart from proteins, carbohydrates,, nucleic acids and other biomolecules - are widely distributed in the biological world - play a wide variety of roles in plant and animal tissues

Functions of lipids Lipids are concentrated source of energy. One gram fat gives 9 K calories. It serves as a cushion for the vital organs and protects them from external shocks or injuries. Lipids are the structural materials of cells and membranes Lipids serves as insulator for our body Lipids are the carrier / reservoir of fat soluble vitamins In food preparations lipids serves as a binding agent. It also enhances the palatability of foods

Types of lipids

Storage lipids Fats and oils: –are universally used as stored form of energy by living organism. One gram of fat gives 9 K Calories –are highly reduced compound, derivatives of fatty acids –fatty acids are hydrocarbon derivatives, which are long chain carboxylic acids containing up to 24 carbon atoms –the most common fatty acids in plants and animals are the even-numbered C 16 and C 18 species such as palmitate and stearate –fatty acid are classified as saturated (where all the carbon are saturated with hydrogen) or unsaturated ( which contain one or more double bonds)

Saturated Fatty acids

Stearic acid The packing of fatty acids depends on their degree of saturation. Stearic acid is shown here in its usual extended conformation. Saturated fatty acids are tightly packed and stabilized by many hydrophobic interactions

Unsaturated Fatty acids

Oleic acid The double bond (shaded)introduces a rigid bend in the hydrocarbon tails. Fatty acids with one or several such bends cannot pack together as tightly as saturated fatty acids

Triglycerides The fats and oils found in animals and plants are triglycerides Triglycerides are: – esters of fatty acids and glycerols – simple lipids –important as the storage form of fat in the human body

Glycerol

Types of Triglycerides Simple triglycerides: They are triesters made from glycerol and three molecules of one kind of fatty acids. They are rare. Mixed triglycerides: They are triesters with different fatty acid components. Animal fats and vegetable oils are many different mixed triglycerides; e.g., Butterfat contains at least 14 different carboxylic acids.

Triglycerides

Properties of Fats and Oils… Hydrogenation of fats and oils: –The difference between fats and oils is: fats are solid in room temperature and oils are liquid in room temperature. –Hydrogenation is the process of converting liquid oil to solid fat by adding hydrogen to some of the double bond of the unsaturated carbon chain in presence of nickel as catalyst; e.g., Margarine.

Properties of Fats and Oils… Rancidity: –Triglycerides soon become rancid, developing an unpleasant odor and flavor on exposure to moist air at room temperature. Rancidity is caused by either hydrolysis of ester bond or oxidation of double bond –Hydrolytic rancidity is due to the hydrolysis of ester bond by the enzyme lipase present in the airborne bacteria. It can be prevented by storing food in refrigerator –Oxidative rancidity is caused by the ruptured of the double bond due to exposure to warmth air. As a result, aldehydes with low molecular wt. are formed releasing off flavor; e.g., Off flavor in cookies. Oxidative rancidity can be prevented by adding antioxidant (Vitamin C, Vitamin E,  -Tocopherol)

Properties of Fats and Oils Saponification of fats and oils: –Hydrolysis of fats and oils by boiling with sodium hydroxide is called saponification. This process is used to make soap (Latin: sapon means soap). Soaps are the alkali metal (Na, K or Li) salts of fatty acids. –Glycerol is an important by-product of saponification process. It is recovered by evaporating the water layer. The crude soap is then purified, and coloring and perfumes are added according to the market demands.

Waxes Waxes: Waxes are part of the lipid family. Waxes are esters of long-chain saturated and unsaturated fatty acids (having carbon atoms) with long-chain alcohol(having carbon atoms). Waxes are low- melting, stable solids which appear in nature in both plants and animals. A wax coat protects surface of many plant leaves from water loss and attack by microorganisms. Carnauba wax, a major ingredients of car wax and floor polish, comes from the leaves of a South American palm tree. Beeswax is largely myricyl palmitate, the ester of myricyl alcohol and palmitic acid Waxes also coat skin, hairs and feathers, and help keep them pliable and water-proof

Beeswax The term ‘wax’ originates in the Old English word weax, meaning the “material of the honeycomb”.

Structural lipids Chemical analysis of the isolated materials shows that lipids are the major components of most membranes. This lipids are not triglycerides but another group of compound called complex lipids. There are two types of complex lipids: –Phospho lipids –glycolipids

Complex Lipids… * Phospholipids: They are esters of phosphoric acids.There are two main types of phospholipids in cellular membranes: Phosphoglecerides: They are also known as Phosphaitdyl choline (lecithin). They are built from long chain fatty acid, glycerol and phosphoric acids. sphingomyelins: They do not contain glycerol. Instead, they contain sphingosine, a long-chain unsaturated amino alcohol. Only one fatty acid is attached to the sphingisine. Sphingomylins are found in brain and nervous tissue and in the myelin sheath, the protective coat of nerves.

3-phosphoglycerol (building block for phosphoglycerides)

General structure of a glycerophospholipid. Note the glycerol-3-phosphate backbone

Glycerophospholipids

Electron microscopy (EM) of myelinated nerve fibres. Note spirally wrapped membranes around each nerve axon. The myeline sheath may be layers thick. Its high lipid content makes it a good electrical insulator

Complex Lipids Glycolipids: A lipid molecule that contains carbohydrates, which is usually a simple sugar like glucose or galactose They are also called cerebrosides because of their abundance in brain.

Membrane lipids Phospholipids: naturally aggregate in form of bilayers (which fuse to form spherical liposomes) –glycerophospholipids –sphingophospholipids Sterols: –e.g. cholesterol (animal sterol) ergosterol( plant sterol).

Lipid bilayers The fundamental component of a biological membrane is lipid bilayer. In a vigorously shaked mixture of phosphatidyl choline and water, the lipid molecules form microscopic sphere. These lipid sphere or liposome are packages of solvent surrounded by a lipid bilayer- a two layer thick wall of phosphatidyl choline

Electron micrograph of a liposome

Diagram indicating spherical micelle (b), and effect of crowding in too many lipid molecules. The micelle could develop a water-filled centre (c), or could flatten out to collapse the hollow centre but would develop water-filled spaces (d)

Two-tailed amphipathic molecules are more cylindrical, and tend to naturally form a bilayer structure (as opposed to a micelle)

Model (snapshot) of a lipid bilayer surrounded by water

Fluid mosaic model The fluid mosaic model of membrane structure proposes that lipids of the bilayer are in constant motion, gliding from one part of their bilayer to another at high speed

The fluid mosaic model of membrane structure

Steroids Steroids are a family of lipids found in plants and animal –A steroid contains four fused carbon ring: 17 carbon atoms make the structural unit of steroids known as steroid nucleus. –Steroid nucleus is found in a number of extremely important biological molecules: cholesterol, adrenocorticoid hormones, the sex hormone and bile acids.

Cholesterol Cholesterol is found only in animal cells. A typical animal cell membrane contains about 60% phospholipids and 25% cholesterol. Cholesterol imparts rigidity to cell membrane. The higher the % of chlesterol, the more rigid the membrane. Bile acids contain cholesterol.

Cholestorol (membrane sterol)

Scanning EM of adipocytes: each contains a fat globule that occupies nearly the entire cell

Triacylglycerols are biologically useful. Spermaceti organ contains spermaceti oil, a mixture of triacylglycerol and waxes. This oil is liquid at 31degree C, and becomes solid when the temperature drops several more degrees Sperm Whales

Lipids in biomolecules