Lipids

Lipids

Lipids are a large and diverse group of naturally occurring organic compounds that share some general properties.

General Properties of Lipids

1. Insoluble in water.2. Soluble in non-polar organic solvents e.g.

ether, acetone,..3. Contain carbon, hydrogen, and oxygen;

sometimes contain nitrogen and phosphorus.4. On hydrolysis they give fatty acids.5. Take part in plant and animal metabolism.

Functions of Lipids

1. Energy storage, mobilization, and utilization.2. Cell differentiation and growth.3. Cell membrane structure. 4. Signal transmission.5. Hormone synthesis.6. Bile acid synthesis.

Fatty Acids

Fatty acids are the building blocks of lipids; they are straight-chain organic acids.

All fatty acids have an even number of carbon atoms.

Fatty acids are important sources of fuel because, metabolized, they yield large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. In particular, heart and skeletal muscle prefer fatty acids

Structure of Fatty Acids

• Fatty acids consist of one carboxyl group (-COOH) attached to a non-polar hydrocarbon tail.

• General Formula: CH3(CH2)n -COO-

Saturated Fatty Acids

Non-Saturated Fatty Acids

Types of Fatty Acids I- Saturated fatty acids: • Contain only single bonds between carbon atoms.• Have high melting points.• At room temperature they are fats (solid form).

II- Unsaturated fatty acids: • Contain one or more double bonds between carbon atoms.• Have low melting points.• At room temperature they are oils (liquid form).• They are subdivided into: Monounsaturated (one double

bond) and Polyunsaturated (many double bonds) fatty acids.

Saturated and Non-Saturated Fatty Acids

Essential Fatty Acids

• Fatty acids which are essential for the complete nutrition of the human body.

• There are twenty different needed fatty acids in your body, but they are all made from two:

- Linoleic acid (omega-6 fatty acid) found in seed oils like corn, peanut, cotton seeds,..

- Linolenic acid (omega-3 fatty acid) found in seed oils like soybeans, walnuts,..

• These two fatty acids are essential. You must get them from the foods you eat because your body cannot manufacture them. Thus, essential fatty acids are the building blocks for all the other fats in your body.

• Linoleic acid and Linolenic acid essential fatty acids are especially important for making prostaglandins (hormone) in your body.

• They also play a role in controlling your blood pressure, your heart, your kidneys, your digestive system, and body temperature.

• They are important for allergic reactions, blood clotting and making other hormones.

Classification of Lipids

Lipids are divided into three main types:

1. Simple.2. Complex.3. Precursor & derived lipids.

I- Simple Lipids

• Simple lipids are esters of fatty acids.• When hydrolyzed they yield fatty acids and

alcohol.

• If the simple lipid yield 3 fatty acids and glycerol it is called: fat or oil.

• If the simple lipid yield one fatty acid and high molecular weight alcohol it is called: wax.

II- Complex Lipids

When hydrolyzed they give (yield) one or more fatty acids, an alcohol, and other compounds.

Examples of Complex Lipids: Phospholipids, Glycolipids, and lipoproteins.

III- Precursor and Derived Lipids

Precursor lipids Precursor lipids are compounds produced when

simple and complex lipids are hydrolyzed to produce fatty acids, glycerol, and other alcohols.

Derived lipids Derived lipids are formed by metabolic

transformation of fatty acids to steroids, ketone bodies, lipid-soluble vitamins,..

e.g: Prostaglandins, Steroids,..

Simple Lipids: FATS AND OILS

Triglycerides Triglycerides (TAG, Triacylglycerol) are major

form of fat stored by the body. A triglyceride consists of three molecules of fatty acid combined with a glycerol. Triglycerides come from the food we eat as well as from being produced by the body

Triglycerides

Glycerol 3 Fatty Acids FAT (TAG)

TAG

TAG

Triglycerides

• TAG that contains one type of fatty acid is called simple TAG, whereas TAG that contains 2 or 3 types of fatty acids is called mixed TAG.

Fatty acids forming TAG can be saturated, unsaturated, or a combination.

Uses of Fats in the Body

1. Energy source: fats produce more than double the energy produced by proteins and carbohydrates.

2. Storage: a mean to store food in the body for energy and to protect internal organs.

3. Keeps our bodies warm in cold weather.4. Present in cell structure and nerve tissues.

Physical Properties of Fats and Oils

1. Pure fats are generally white solids and oils are generally yellow liquids.

2. Tasteless and odorless.3. Fats and oils are insoluble in water but are

soluble in organic liquids like benzene, acetone,..

4. Do not pass through membranes.5. Are lighter than water.

oil Fat

oils are insoluble in water

oils are lighter than water

Chemical Reactions of Fats

1. React with Iodine.2. Hydrolysis.3. Saponification.4. Hydrogenation.5. Rancity.

I- Reaction with Iodine

Unsaturated fats and oils will react with

Iodine but saturated fats and oils will not.

II- Hydrolysis

Fats will hydrolyze (break down) to form glycerol and free fatty acids when they are treated with enzymes, acids, or bases.

3

III- Saponification

Saponification is the heating of a fat with a strong base (e.g. NaOH or KOH) to produce glycerol and the salt of fatty acid. The Na or K salts of a fatty acid are called Soaps.

FAT + NaOH or KOH SoapHeat

Saops

Saponification

VI- Hydrogenation

Hydrogenation is the process of converting vegetable oils to fats by the addition of hydrogen in the presence of a catalyst.

In this process some but usually not all of the double bonds in oils have been reduced by the addition of H2 e.g. Margarine.

Hydrogenation

Margarine

V- Rancidity

When fats are left at room temperature for a short period of time they develop unpleasant odor and taste i.e. they become Rancid. This occurs due to 2 reactions: oxidation and hydrolysis of fat.

Complex Lipids: PHOSPHOLIPIDS

Structure of Phospholipids Phospholipids are complex fat in which one

fatty acid has been replaced by phosphoric acid (phosphate group) and one of several nitrogen-containing molecules. Phospholipids are found in all tissues in the human body, especially brain, liver, and spinal tissue and also in cell membranes.

When hydrolyzed they give (yield) one or more fatty acids, an alcohol, and other compounds

Types of Phospholipids

According to the alcohol part, phosphospholipids are divided into:

1. Phosphoglycerides: contain glycerol. E.g. Lecithin, cephalins.

2. Phosphosphingosides: contain sphingosine. E.g. sphingomyelins.

Properties of Phospholipids

Amphipathic: Amphipathic lipids are molecules that have

non-polar (hydrophobic) end or tail and a polar or ionic (hydrophilic) end or head.

Complex Lipids: GLYCOLIPIDS

They are sugar containing lipids also called cerebrosides because they are found in large amounts in the brain tissue.

Structure of Glycolipids Glycolipids are composed of one fatty acid, one

sphingosine molecule, and a galactose sugar. There are many types depending on the type of fatty acid present.

Complex Lipids: LIPOPROTEINS

Lipoproteins are lipids associated with special proteins to give plasma lipoproteins.

Examples of Lipoproteins• Chylomicrons: 90% lipids and 10% protein.• Very Low Density Lipoproteins (VLDL): 60% lipids and 40%

protein.• Low Density Lipoproteins (LDL): 8% lipids and 92% protein.• High Density Lipoproteins (HDL): 5% lipids and 95%

protein.

The Cell Membrane

Cell membranes are composed of 40-50% lipids, 50-60% proteins, cholesterol, and a small amount of carbohydrate. These components vary in amounts depending on the type of cells.

Functions of the Cell Membrane1. Give structure and protection to the cells.2. Hold up proteins that transport molecules

across the cells.