BIOCHEMISTRY 1 BCM 201 CHAPTER 3 LIPIDS

Chemical & physical Properties of Fatty acids Lipids classifications Types of Lipids Fatty Acids Chemical & physical Properties of Fatty acids Fats and oils Lipids classifications

LIPIDS Class of biomolecule whose distinctive characteristic is their solubility behavior. Insoluble in water, but soluble in organic solvents including :- diethyl ether, chloroform, methylene chloride and acetone.( this is due its hydrophobic nature )

Types of Lipids Lipids with fatty acids ( open chain forms) Waxes Fats and oils (triglycerides) Phospholipids Sphingolipids Glycolipids Lipids without fatty acids ( cyclic forms) Steroids

Lipid : structure Open chain Long nonpolar tail with polar head Saponifiable Cylic forms/Fused Ring: Based on the steroid ring skeleton

Fatty acids A molecule with a carboxyl group as the polar end and a hydrocarbon chain as the hydrophobic end. Also been called Amphiphatic compounds because the carboxyl group ( COOH ) is hydrophilic and the hydrocarbon tail is hydrophobic.

Long-chain carboxylic acids Insoluble in water Typically 12-18 carbon atoms (even number) Some contain double bonds

An unbranched –chain carboxylic acid, mostly in even numbers of carbon. Polar hydrophilic group Non polar hydrophobic tail

Saturated and Unsaturated Fatty Acids Saturated = C–C bonds Unsaturated = one or more C=C bonds

General rules for the structures of naturally occurring fatty acids Most fatty acids have an even number of carbon atoms. The hydrocarbon chain is almost always unbranced. Most carbon-carbon bonds are single;however ,fatty acids may contain one,two, or more carbon-carbon double bonds.

Double bonds are most cis. For monounsaturated fatty acids,the double bond is usually between carbon 9 and 10. If more than one carbon-carbon double bond is present they are not conjugated but are separated by methylene unit.

Degree of unsaturation refers to the number of double bonds Degree of unsaturation refers to the number of double bonds. The superscript indicates the position of double bonds. Ex: Δ9 refers to double bond at ninth carbons atom from the carbonyls ends.

Comparison between saturated and unsaturated fatty acids Properties of Unsaturated Fatty Acids Properties of Saturated Fatty Acids Contain one or more double C=C bonds Nonlinear chains do not allow molecules to pack closely Few interactions between chains Low melting points Liquids at room temperature Contain only single C–C bonds Closely packed Strong attractions between chains High melting points Solids at room temperature

Triacylglycerols Glycerol is a simple compound that contains three hydroxyl groups. When all three of the alcohol groups form ester linkages with fatty acids, the resulting compound is a triacylglycerols.

TAGs which are solids at room temperature are rich in saturated acids and are called fats. TAGs which are liquids at room temperature are rich in unsaturated acids and are called oils. examples : oil seeds include peanut butter, corn, palm, olive and soybean.

Fats and Oils Formed from glycerol and fatty acids

Triglycerides (triacylglcerols) Esters of glycerol and fatty acids

Properties of Triglycerides Hydrogenation Unsaturated compounds react with H2 Ni or Pt catalyst C=C bonds C–C bonds Hydrolysis Split by water and acid or enzyme catalyst Produce glycerol and 3 fatty acids

Hydrogenation

Product of Hydrogenation Hydrogenation converts double bonds in oils to single bonds. The solid products are used to make margarine and other hydrogenated items.

Butter

Margerine

It is possible to convert back and forth between saturated and unsaturated fats and oils, and commercially this is done to achieve a certain texture. The fats tend to be solid at room temperature, and the oils tend to be liquids at room temperature. For some functions the liquid is more desirable, such as a cooking oil. For other functions, such as spreading margarine, the solid is more desirable, unless it is too solid. So food processing companies have found ways to partially hydrogenate vegetable oils to make them more solidified. It's also possible to dehydrogenate these solids to put double bonds back in.

The terms saturated, mono-unsaturated, and poly-unsaturated refer to the number of hydrogens attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail. Fats, which are mostly from animal sources, have all single bonds between the carbons in their fatty acid tails, thus all the carbons are also bonded to the maximum number of hydrogens possible. Since the fatty acids in these triglycerides contain the maximum possible amount of hydrogens, these would be called saturated fats. The hydrocarbon chains in these fatty acids are, thus, fairly straight and can pack closely together, making these fats solid at room temperature.

Oils, mostly from plant sources, have some double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks” in the shape of the molecules. Because some of the carbons share double bonds, they’re not bonded to as many hydrogens as they could if they weren’t double bonded to each other. Therefore these oils are called unsaturated fats. Because of the kinks in the hydrocarbon tails, unsaturated fats can’t pack as closely together, making them liquid at room temperature.

In unsaturated fatty acids, there are two ways the pieces of the hydrocarbon tail can be arranged around a C=C double bond. In cis bonds, the two pieces of the carbon chain on either side of the double bond are either both “up” or both “down,” such that both are on the same side of the molecule. In trans bonds, the two pieces of the molecule are on opposite sides of the double bond, that is, one “up” and one “down” across from each other.

Saponification and Soap Hydrolysis with a strong base Triglycerides split into glycerol and the salts of fatty acids The salts of fatty acids are “soaps” KOH gives softer soaps

Saponification

example

Learning Check L1 How would the melting point of stearic acid compare to the melting points of oleic acid and linoleic acid? Assign the melting points of –17°C, 13°C, and 69°C to the correct fatty acid. Explain. stearic acid (18 C) saturated oleic acid (18 C) one double bond linoleic acid (18 C) two double bonds

Solution L1 Stearic acid is saturated and would have a higher melting point than the unsaturated fatty acids. Because linoleic has two double bonds, it would have a lower mp than oleic acid, which has one double bond. stearic acid mp 69°C oleic acid mp 13°C linoleic acid mp -17°C

Phospholipids Is a fat derivatives, having hydrophobic and hydrophilic domains. Also referred as phosphoglycerides. Its simple form, one glycerol bonded to two fatty acids and a phosphate group. i) phosphotidic acid ii) phosphate group carrying one several nitrogen-containing molecules, choline.

A major component of the cell membranes. Phospholipids are always needed for the formation of a stable bilayer structure. Examples of phospholipids include lecithin, cephalins, phosphoinositides (in the brain), and cardiolipin (in the heart)

Functions of phospholipids: Act as building blocks of the biological cell membranes in virtually all organisms Participate in the transduction of biological signals across the membrane. Act as efficient store of energy as with triglycerides. Play an important role in the transport of fat between gut and liver in mammalian digestion. An important source of acetylcholine which is the most commonly occuring neurotransmitter substance occuring in mammals.

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Waxes Complex mixtures of esters of long chain carboxylic acids and long chain alcohols. Serve as protective coatings for both plants and animals. In plants, they coats stems, leaves and fruit. In animals, they are found on fur, feathers,and skin.

Sphingolipids Sphingolipids, like the phospholipis ,are composed of a polar head group and two nonpolar tails. The core of sphingolipids is either the long chain amino alcohol,sphingosine or its N-acyl derivative ( a ceramide ). Are found in plants and animals. Abundant in nervous system.( component of nerve cell membrans.

Steroid A group of lipids that have fused ring structure of 3 six membered rings,and 1 five-membered ring. Steroids are nonpolar. The most abundant steroid is cholesterol. Some common examples of steroids are: Vitamin D, cholesterol, estrogen and cortisone.

Functions Steroids are used to increase muscle bulk and strength and to enable longer and harder athletic training sessions. Some users take steroids for cosmetic (body image) reasons. Anti-inflammatory steroids (or corticosteroids) can reduce swelling, pain, and other manifestations of inflammation. Synthetic steroids are used in a variety of therapies to control respiratory problems, skin inflammation, ease joint inflammations, and to treat blood disorders.

Steroids increase protein synthesis, promoting growth of muscles and bones. They reduce the recovery time needed between training sessions and enable athletes to train more intensively for longer periods. Anabolic steroids increase muscle mass.

Cholesterol is the precursor for all steroids Cholesterol is the precursor for all steroids.It is a common component of animal cell membranes . It functions to help stabilize the membrane. Thus,it is crucial molecule in animals.

Steroid

Cell Membrane The membrane that surrounds the cytoplasm of a cell; it is also called the plasma membrane or, in a more general sense, a unit membrane. This is a very thin, semifluid, sheet like structure made of four continuous monolayers of molecules. The plasma membrane and the membranes making up all the intracellular membranous organelles display a common molecular architectural pattern of organization, the unit membrane pattern, even though the particular molecular species making up the membranes differ considerably.

All unit membranes consist of a bilayer of lipid molecules, the polar surfaces of which are directed outward and covered by at least one monolayer of nonlipid molecules on each side, most of which are protein, packed on the lipid bilayer surfaces and held there by various intermolecular forces. Some of these proteins, called intrinsic proteins, traverse the bilayer and are represented on both sides. The segments of the polypeptide chains of these transverse proteins within the core of the lipid bilayer may form channels that provide low-resistance path was for ions and small molecules to get across the membrane in a controlled fashion.

The best model to represent membranes is fluid-mosaic model. S The best model to represent membranes is fluid-mosaic model. S. jonathan Singer and Garth Nicholson in 1972 suggested this model ,which consist of a lipid bilayer embedded with proteins ,with some on the surface (peripheral ) and others passing through the entire bilayer ( integral ).

Fluid-mosaic model

Fluid-mosaic model

Plasma membranes are primarily lipid bilayers with associated proteins and glycolipids Cholesterol is also a major component of plasma membranes Chapter 23

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