Lipids are nonpolar organic molecules that are insoluble in polar water but soluble in nonpolar organic solvents like ether, alcohol, and chloroform. Lipids show hydrophobic behavior. The behaviour is associated with the chemical structures of lipids. Lipids have high proportion of C-1-I bonds. C-1-I is non-polar in character. Phospholipids and cholesterol are lipids. They are important constituents of cell membranes. The most common lipids in plants and animals are fats.
(a) Fats (Oils)
Fats are esters of fatty acids and glycerol. Fats build cell parts and supply energy for cellular activities. Fats have following properties:
- Fats molecules are composed primarily of carbon, hydrogen, and
oxygen atoms. Some figs also contain small amounts of phosphorus and nitrogen. They contain a much smaller proportion of oxygen than carbohydrates. For example the formula of the fat tristearin is C5,F111006.
- The building blocks of fat molecules are fatty acids and glycerol. Fatty acids contain long hydrocarbon chains bonded to carboxyl (COOH) groups. Glycerol is a three-carbon alcohol, with each carbon bearing a hydroxyl (OH) group. Three fatty acid molecules combine with one glycerol molecule. They are attached to the three carbon atoms in the glycerol backbone.
- The fats with three fatty acids are called triglyceride neutral fat or triacylglycerol. The glycerol portion of every fat molecule is the same. But there are many kinds of fatty acids. Therefore, there are many kinds of fats. Fatty acid molecules differ in the length of their carbon chains aud in the ways the carbon atoms combine. The most common are e :en-numbered chains of 14 to 20 carbons.
Stearic acid Glycerol Stearin
(3 mol) (1 mol) (1 mol)
Ct2H33C0 OH H 0—CH2 C1711350 00 –C H2
0435C0 OH 4. HO—OH CI7H33C00 —CH + 3H20
C17H33C0 OH H 0—CH2 C17H33C00 CH2
0 CH2-0—C— (CH2 h2— CH3
H3C —(CH2 )14—C—O PI 0
Fig: Formation of lipids
Types of fats
There are two types of fats:
- Unsaturated fats: The% have double bonds. Their chains bend at the double bonds. So the fat molecules cannot align closely w ith one another. Therefore. they have low melting points. Thus the fat may be fluid at room temperature. A liquid fat is called oil. Most plant fats are unsaturated. Fatty acids with one double bond are monounsaturated. They fatty acids with numerous double bonds are polyunsaturated.
- Saturated fatty acids: They do not have double bonds. Animal fats are often saturated. They occur as hard or solid fats. In this case. the carbon atoms join by- single carbon-carbon bonds. Each carbon atom binds to man> hydrogen atoms.
; :J 111;111’111
H-C-O-C-C C-C C–C C C C C C C C C C C H
! :I II
OHHHHHHHHHHHHH H H H H
H C-0-C-C-G-C-C-C -C- C-C -C-C- c-a-a-a-a-c-O-H I
H C-O-C-C-C-C-C-C C C-C -C-C -C-C- a- – c – — – H
(a) Sleanc acid
Lb( Oleic acid
|Fatly ac ‘Cs
HHHHHHHHHHHHHHHHH 0% C-C-GCCCGGG-CLCC-GCOCCH /
0 • HHHHHHHHHHHHHHHH
C -C C C C C C C CC C- C C C G c C C H
HO H H HHHhi: HHHHHHHH
A phospholinids molecule is similar to a fat molecule. It contains a gly cern] portion and fatty acid chains. Rut phospholipids have only two laity acid chains. The nitrogen containing groups replace the third chain. There are two parts of phospholipids:
- Ilead: 1he polar phosphate and nitrogen groups are soluble inwater (hydrophilic). They form the –head– of the molecule.
• Tail: The insoluble (nowlar. hydrophobic) fatty acid portion forms the –tail.”
Phospholipids hinie double tendency. They are soluble at one end and insoluble at the other end. So they are the major structural components of cell membranes.
CM, Water-soluble end
)Cm, C m, Hie
CH, H2 Nig –In\
\CH PS ti/ N?C„M
pahnitoyiKr\ti, y Oleoyl
group,’ A group
Fat-soluble end 1C142 HA,
fig: Pluithrilipht Lecithin
- (c) Steroids
They are naturally occurring, lipid-soluble molecules. They are
composed of four fused carbon rings. It forms a rigid stnicture. Three of the rings are six-sided. The fourth is five-sided. The four rings contain a total of 17 carbons. Cholesterol is an important biologically active steroid. Similarly vitamin D is also an important steroid.
The mixture of long chain alkanes (with odd number of carbon from C25 to C.15), alcohols, ketones, and esters of long chain fatty acids is called waxes, e.g. cutin. Waxes form protective coating on fruits and leaves. Waxes protect plants from water loss and abrasive damages. The xerophytic plants a thick layer of waxes. It reduces transpiration in these plants.
Functions of lipids
Lipids perform following functions:
I. Lipids are hydrophobic compounds. Therefore, they form components of the cell membrane.
- Lipids act as storage compounds like 11ils.
- They have high proportion of C – H bonds and low proportion of oxygen. Thus lipids store double amount of energy as compared
to carbohydrates. •
- Some lipids provide insulation against atmospheric heat and cold. They also act as Or ater proof material. For example, waxes in the exoskeleton of inects. A vkax cutin forms an additional protective layer on the cuticle of epidermis of some plant organs like leaves, fruits, seeds etc.
Some lipids like steroids, cholesterol performs important functions in the bodies of plants