Summary

• Lipids are a heterogeneous group of compounds that are soluble in solvents like alcohol and benzene but insoluble in water
• They are the big source of energy within the body
• They are the building blocks of many higher molecular weights compounds
• Lipids are associated with certain disorders such as atherosclerosis and obesity
• One molecule of glycerol and three molecules of fatty acid come together via a condensation reaction to form a triglyceride
• Triglycerides and phospholipids are two groups of lipids
• A condensation reaction (removal of water) between a glycerol molecule and fatty acid forms an ester bond
• Hydrolysis, addition of a water molecule, of a triglyceride form fatty acids and glycerol.
• A fatty acid that has more than one double bond (=) is called a polyunsaturated fatty acid and if one double bond is present they are termed monounsaturated fatty acid
• A phospholipoid structure is where one fatty acid group of a triglyceride is substituted with a phosphate containing group
• Phospholipids are polar molecules where they have a hydrophilic phosphate ‘head’ and a hydrophobic ‘tail’ of two fatty acids making them
• In aqueous solution phospholipid molecules are the main component of the cell membrane in animal and plant cells and form a lipid bilayer of cell surface membranes

What are lipids

Lipids are the class of organic compounds which means they are carbon containing compounds. They are hydrophobic (water-fearing) in nature. They are mostly soluble in solvents such as alcohol, ethers and chloroform but insoluble in water. Lipids are the diverse group of compound that are made up of fatty acids and their derivates. They mostly include fats, oils, waxes, hormones and certain components of cell membrane.

Formation of a basic lipid

The basic structure of a lipid comprises of a glycerol and three fatty acids shown in Figure 1. When a molecule of one glycerol molecule and three fatty acid molecules (R-COOH) join together, a condensation reaction takes place (ester bond formation) where three water molecules are removed and a basic lipid is formed which is called triglyceride. The formation of a triglyceride is illustrated in Figure 1. In triglycerides the glycerol molecule (Figure 2) is the same but there are different forms of oils and fats which are due to the different types of attached fatty acids. The R group of the fatty acid group may be either polyunsaturated or saturated.

Glycerol

Glycerol’s are alcohols with three carbons and three hydroxyl (OH) molecules attached with five hydrogen molecules (chemical structure shown below). In a triglyceride structure glycerol’s are the backbone attached with three fatty acids. The bond between fatty acid and glycerol is called ester bond.

Functions of lipids

Lipids mainly include fats, oils, certain hormones, steroids and some complex molecules which makes them have a variety of functions some of which are as follows:

• They are the source of energy
• They can be easily stored in body
• The insulate the body temperature
• They protect many internal organs as a padding agent
• They are an important component of living cell; as the cell membrane is made up of a phospholipid bilayer
• Some lipids like steroids and hormones serve as chemical messengers between cells tissues and organs

Test for lipids

The test for lipids is known as the emulsion test. The process follows:

Method:

• Add 2cm3 drops of sample being tested with 5cm3 of ethanol
• shake vigorously to dissolve the solution
• Add an equal amount of water

Results:

• If a lipid is present in the solution, a cloudy-white emulsion is observed

Theory:

• Ethanol emulsifies fats
• When water is added, an emulsion of any tiny droplets refract light to make the appearance look cloudy

Classification of lipids

There is no single internationally accepted system of classification of lipids. The figure below is one of the more accepted scheme of classification for lipids (Figure 3):

Classification of lipids

Lipids are basically classified into three groups

• Simple lipids
• Compound lipids
• Derived lipids

Simple lipids

These are the esters of fatty acids with glycerol’s
Fatty acids + glycerol’s ——- Lipids

They are further subdivided into two groups
a) Triglyceride b) Waxes

a) Triglycerides

Triglycerides are the combinations of fatty acids with glycerol’s (Figure 1)
Oils are the liquid triglycerides at room temperature
Fats are solid triglycerides at room temperature
Oils and fats are most abundant lipids found in nature. It is the best kind of reserve food material in humans and source of energy. They also act as a insulator for body heat and also protect internal organs of the body as a padding material
A triglyceride molecule is formed by joining of one molecule of glycerol to three fatty acids combined with glycerol. Each fatty acid is joined and combined with the glycerol forming an ester bond in three condensation reaction.
Hydrolysis causes the breakdown of the triglyceride and produces glycerol and three fatty acids.
Triglycerides have key roles in respiration and energy storage due to its insolubility and its high carbon to hydrogen ratio
Triglycerides have low mass to volume ratio making them good molecules for storage
They do not affect water potential of cells since they are large in nature and insoluble in water

b) Waxes

These are the esters of fatty acids with alcohol (with higher molecular weight) other than glycerol. Waxes are present all over in nature. Many plant leaves have waxy coating which protects them from dehydration. These are also obtained from animal sources of which most common are bees wax and spermaceti. Spermaceti is obtained from sperms of whales and is mostly used in cosmetics and pharmaceuticals.

Compound lipids

These are the esters’ of fatty acids with alcohol and some other additional groups because of which they have many types most common are as follows:

a) Phospholipid

• They are found in animal tissues plasma membranes of animal and plant cell
• Phospholipids are the class of lipids which are made up of esters of fatty acid and glycerol’s with phosphoric acid and nitrogen base
• They form the major part of the cell membrane
• They form the lipid bilayer because of their amphiphilic character. The phospholipid molecules compose of two (‘water-repelling’) hydrophobic fatty acid “tail” and the hydrophilic (‘water-loving’) phosphate group “head” (Figure 4)
• Phospholipids are similar to lipids where a fatty acid molecule is replaced by a phosphate molecule. The fatty acids are hydrophobic and repel water whereas, phosphate molecules attract water and are known to be hydrophilic

b) Plasmogens

They occur in the heart and brain muscles

c) Lipositol

These are present in combination with phytic acid in brain, heart, kidneys and plant tissues

d) Sphingomyelin

These are the source of phosphoric acid in body tissues also present in nervous system and red blood cells

Derived lipids – Fatty acids

Fatty acids are comprised of hydrocarbon chains terminating with carboxylic group (-COOH). There are around 70 fatty acids. These are obtained from the hydrolysis of natural fats and oils. The hydrocarbon chain in fatty acid may contain single or double bond.

Those fatty acids that have single bond between carbon hydrogen and no double bonds they are called saturated fatty acids; as all the carbon atoms are attached with the maximum possible number of hydrogen atoms. Those fatty acids that have double bond (C=C) between carbon hydrogen are called unsaturated fatty acids (Figure 5).

• These are the hydrolytic products of compound lipids
• These are present in animal and plant food
• They are obtained from hydrolysis of fats
• They are hydrocarbons derivatives

Physical properties of lipids

• Mostly fats and oils are colorless, odorless where any color is present due to some external group attached
• They have less than one specific gravity
• They have low melting points
• They are poor conductors of heat and electricity
• When agitated with water in the presence of soap they form emulsions

Chemical properties of lipids

Hydrolysis

It takes place by heating a triglyceride with water at higher temperature and pressure in the presence of an enzyme called lipase which results in the formation of fatty acid and glycerol.

Sponification

When fats are boiled with strong a alkaline solution it decomposes into glycerol’s and salt of fatty acids shown in the figure below:

Rancidity

It is the unpleasant odor and taste that develop by fats upon aging due to hydrolysis of component glycerides of fats into fatty acid and glycerol.

Cell membrane

Phospholipids form cell membrane because of the arrangements of the hydrophilic phosphate group head and hydrophobic fatty acid tail. The phospholipids lined up to each other with tails inside and head facing outside due to which a double layer is formed called lipid bilayer.

Functions of phospholipids

1. Phospholipids act as a barrier in cell membrane to protect cell
2. Phospholipids in combination with proteins make the cell to be selectively permeable
3. Phospholipids are important components of cell surface membranes where they form a bilayer arrangement which allows lipid-soluble substances to be crossed easily Phospholipids take part in assembling of lipoprotein
4. The bilayer arrangement relates back to the structure of the phospholipid. Since both inside the cell and outside the cell consist of water, the phospholipids in the cell surface membrane form a double layer.
5. Where the hydrophilic (‘water-loving’) heads of the phospholipids point into the watery environment outside the membrane or inside the cell. While, the hydrophobic tails point in the middle of the membrane. This forms a hydrophobic core within the cell membrane (Figure 6).
6. Phospholipids also work as a emulsifier in body in wetting surfaces like joints and other parts of body.

Frequently Asked Questions

Where does the synthesis of lipids take place?

Lipids are synthesized in the endoplasmic reticulum present in animal cells. 

How triglycerides are made?

Triglycerides are made by an ester linkage between glycerol and fatty acids. Three molecules of fatty acids are linked with glycerol to make triglycerides. 

What are the functions of lipids?

Lipids such as triglycerides act as a source of energy. They are the major energy stores present in our bodies. Lipids as phospholipids are an essential component of membranes. 

What is the importance of phospholipids?

Phospholipids in cell membrane act as a barrier to polar and charged materials. They have a role in cell signalling. They also act as an emulsifier in different body cavities and joints. 

References:

[1]. https://alevelbiology.co.uk/notes/lipids/
[2]. https://www.britannica.com/science/lipid
[3]. https://www.medicinenet.com/script/main/art.asp?articlekey=4168
[4]. https://byjus.com/biology/lipids/
[5]. https://www.researchgate.net/publication/294873626_Producing_pork_to_meet_modern_consumer_demands/figures?lo=1
[6]. https://nsb.wikidot.com/c-9-5-5-1
[7]. https://openstax.org/books/biology-2e/pages/3-3-lipids