Lipids

Lipids:Fuel Storage and Building Material 

Definition:"Lipids are non-polar organic molecules that are insoluble in polar water and soluble in non-polar organic solvents like ether, alcohol and chloroform"

Lipids are fats and fatlike substances. They are molecules of low 

polarity; consequently, they are virtually insoluble in water but 

are soluble in organic solvents, such as acetone and ether. 

Lipids are a somewhat loosely defined group of molecules with 

one main chemical characteristic: They are insoluble in water. 

Storage fats such as animal fat are one kind of lipid. Oils such 

as those from olives, corn, and coconut are also lipids, as are 

waxes such as beeswax and earwax. Even some vitamins are 

lipids! Lipids have a very high proportion of nonpolar carbon–

hydrogen (C—H) bonds, and so long-chain lipids cannot fold 

up like a protein to confine their nonpolar portions away from 

the surrounding aqueous environment. Instead, when they are 

placed in water, many lipid molecules spontaneously cluster 

together and expose what polar (hydrophilic) groups they have 

to the surrounding water, while confining the nonpolar (hydrophobic) parts of the molecules together within the cluster. You 

may have noticed this effect when you add oil to a pan containing water, and the oil beads up into cohesive drops on the water’s 

surface. This spontaneous assembly of lipids is of paramount 

importance to cells, as it underlies the structure of cellular membranes. 

 Types of Lipids                                     Principal groups of lipids are neutral fats, phospholipids,steroids,Terpenes and waxes. 

1.Neutral Fats

The neutral or “true” fats are major fuels of animals. Stored fat is 

derived either directly from dietary fat or indirectly from dietary 

carbohydrates that the body has converted to fat for storage.                                            Fats

are oxidized and released into the bloodstream as needed to 

meet tissue demands, especially those of active muscle. 

Neutral fats include triglycerides, which contain glycerol and 

three molecules of fatty acids. Neutral fats are therefore esters, a 

combination of an alcohol (glycerol) and an acid.                                                                         

Fats are lipids built from two kinds of molecules: fatty acids and 

glycerol.                                                             Fatty Acids, are long-chain hydrocarbons with a carboxyl 

group (COOH) at one end.                           Glycerol,  is a three-carbon polyalcohol 

(three —OH groups). A fat molecule consists of a glycerol molecule with three fatty acids attached by dehydration synthesis, one 

to each carbon of the glycerol backbone. Because it contains three 

fatty acids, a fat molecule is commonly called a triglyceride (the 

more accurate chemical name is triacylglycerol). This basic structure is depicted in figure.

The three fatty acids of a triglyceride need not be identical, and often they are very different from 

one another,as in figure.

The hydrocarbon chains of fatty acids vary in length.

 Fatty acids in 

triglycerides are simply long-chain monocarboxylic acids; they 

vary in size but are commonly 14 to 24 carbons long.                                           Production 

of a typical fat by the union of glycerol and stearic acid is shown 

in Figure above. In this reaction, three fatty acid molecules are 

seen to have united with OH groups of glycerol to form stearin 

(a neutral fat) plus three molecules of water.

The most common are even-numbered chains of 14 to 20 carbons. 

The many C—H bonds of fats serve as a form of long-term energy 

storage.

Saturated Fatty Acid

If all of the internal carbon atoms in a fatty acid chain are 

bonded to at least two hydrogen atoms, the fatty acid is said to be 

saturated, which refers to its having all the hydrogen atoms possible.

Unsaturated Fatty Acid                                 A fatty acid that has double bonds between one or more pairs 

of successive carbon atoms is said to be unsaturated.                                                    

Fatty acids with one double bond are called monounsaturated , and those with 

more than one double bond are termed polyunsaturated .

Fats containing polyunsaturated fatty acids have low melting points, 

because their fatty acid chains bend at the double bonds, preventing 

the fat molecules from aligning closely with one another.

Most unsaturated fats, such as plant oils, are liquid at room temperature.figure 

Unsaturated fat is composed of triglycerides that contain one or more unsaturated fatty acids (the 

kind that have one or more double bonds). These have fewer than the maximum number of hydrogen atoms bonded to the carbon chain. 

This example includes both a monounsaturated and two polyunsaturated fatty acids. Plant fats are typically unsaturated. The many kinks 

of the double bonds prevent the triglyceride from closely aligning, which makes them liquid oils at room temperature.                                                                

Most naturally occurring unsaturated fatty acids have double bonds 

with a cis (same-side) configuration, where the carbon chain is on 

the same side before and after the double bond (as in the figure above).

When fats are partially hydrogenated industrially, this can 

produce double bonds with a trans (opposite-side) configuration, 

where the carbon chain is on opposite sides before and after the 

double bond. These are the so-called trans fats. Trans fats have 

been linked to elevated levels of low-density lipoprotein (LDL; 

“bad cholesterol”) and lowered levels of high-density lipoprotein 

(HDL; “good cholesterol”), a condition associated with an 

increased risk for coronary heart disease.

Having double bonds in its fatty acid chains changes the 

behavior of the fat molecule, because free rotation cannot occur 

about a C C double bond as it can with a C—C single bond.                                                   This 

characteristic affects the melting point of the fat—that is, whether 

the fatty acid is a solid fat or a liquid oil at room temperature. Fats 

containing polyunsaturated fatty acids have low melting point.

Fats are excellent energy -Storage Molecules  

Most fats contain over 40 carbon atoms. The ratio of energy-storing 

C—H bonds in fats is more than twice that of carbohydrates,

 making fats much more efficient molecules for storing chemical energy. On average, fats yield about 9 kilocalories 

(kcal) of chemical energy per gram, as compared with about 

4 kcal/g for carbohydrates.

Most fats produced by animals are saturated (except some 

fish oils), whereas most plant fats are unsaturated. The exceptions 

are the tropical plant oils (palm oil and coconut oil), which are saturated even though they are liquid at room temperature. An oil may 

be converted into a solid fat by chemically adding hydrogen. Most 

peanut butter is artificially hydrogenated to make the peanut fats 

solidify, preventing them from separating out as oils while the jar 

sits on the store shelf. However, artificially hydrogenating unsaturated fats produces the trans-fatty acids described previously.

When an animal consumes carbohydrate, any excess is converted into glycogen or fat and is reserved for future use. The reason many humans in developed countries gain weight as they grow 

older is that the amount of energy they need decreases with age, 

but their intake of food does not. Thus, an increasing proportion of 

the carbohydrates they ingest is converted into fat.

2.Phospholipids

Complex lipid molecules called phospholipids are among the 

most important molecules of the cell, because they form the core 

of all biological membranes.Unlike fats that are fuels and serve no structural roles in the 

cell, phospholipids are important components of the molecular 

organization of tissues, especially membranes. An individual phospholipid can be 

thought of as a substituted triglyceride, that is, a triglyceride with 

a phosphate replacing one of the fatty acids. The basic structure of 

a phospholipid includes three kinds of subunits:

1.Glycerol, a 3-carbon alcohol, in which each carbon bears a 

hydroxyl group. Glycerol forms the backbone of the 

Glycerol(alcohal)

phospholipid molecule, just as it does a fat molecule.     

                                                                       2.Fatty acids,

long chains of —CH2 groups (hydrocarbon 

chains) ending in a carboxyl (—COOH) group. Only two 

fatty acids are attached to the glycerol backbone in a 

phospholipid molecule, rather than the three in a fat.

3.A phosphate group(

—PO4

2–

)

attached to one end of the 

glycerol. The charged phosphate group usually has a 

charged organic molecule linked to it, such as choline, 

ethanolamine, or the amino acid serine.

                 The phospholipid molecule can be thought of as having a 

polar “head” at one end (the phosphate group) and two long, very 

nonpolar “tails” at the other (figure). 

We call molecules with 

both polar and nonpolar regions amphipathic. This structure is 

essential for how these molecules function, although it first appears 

paradoxical. Why would a molecule need to be soluble in water but 

also not soluble in water? The formation of a membrane shows the 

unique properties of such a structure.

                 In water, amphipathic lipids will aggregate away from water 

to form different structures. One such structure is the spherical 

micelle, with the hydrophobic portion facing inward (figure).

This is how detergent molecules work to make grease soluble in 

water, with the grease within the nonpolar interior of the micelle

(Detergents form droplets called 

micelles)

and the polar surface of the micelle soluble in water. Because of 

their long fatty acid tails, phospholipids form a more complex 

bilayer structure in water. Two layers of phospholipid line up, with 

the hydrophobic tails of each layer pointing toward one another, or 

inward, and the hydrophilic heads oriented outward (figure).

Lipid bilayers

(phospholipid molecules can arrange themselves 

into two layers, called a bilayer)

are the basic framework of biological membranes, we will discuss them in cell biology.
Example of Phospholipid 
An example is lecithin, an important phospholipid of nerve membranes
( Figure).

Lecithin (phosphatidyl choline), an important phospholipid of nerve 

membranes.

Because the phosphate group on phospholipids
is charged and polar and therefore soluble in water, and the
remainder of the molecule is nonpolar, phospholipids can bridge
two environments and bind water-soluble molecules, such as
proteins, to water-insoluble materials.
3.Steroids
Steroids are complex alcohols. Although they are structurally unlike
fats, they have fatlike properties.They are naturally occurring lipid-soluble molecules. They are composed of four fused carbon rings. It forms a rigid structure. Three of the rings are six-sided.The forth is five-sided. The four rings contain a total of 17 carbons.  

Steroids play important roles in membranes and as the basis 

for a class of hormones involved in chemical signaling.

The steroids are a large group of
biologically important molecules, including cholesterol (is an important biologically active steroid, 

most 

animal cell membranes contain the steroid cholesterol),

vitamin D3, many adrenocortical hormones, and sex hormones such as testosterone and estrogen.
4.Terpenes
Terpenes are long-chain lipids that are components
of many biologically important pigments, such as chlorophyll and
the visual pigment retinal. Rubber is also a terpene.

Terpene(citronellol)

 5.Waxes                                                                          The mixture of long chain Alkanes(with odd number of carbon from C25 to C35),alcohols, ketones, and esters of long chain fatty acids is called waxes. e.g Cutin

Waxes(cutin)form a protective covering on fruits and leaves. Waxes protect plants from water loss and abrasive damage. The zeriphytic plants have a thick layer of waxes. It reduces transpiration in these plants because these plants faces shortage of water in deserts etc.

Act as water proof material in the exoskeleton of insects.