Lipids: Constituent Units, Structures, Practice Problems and FAQs

You must have noticed that when we wrap fried or fatty food in paper, it turns translucent. Have you ever wondered why this happens? This is due to the presence of fats or oil in the food. But what are fats and oils? Fats or oil are lipids which are macromolecules having molecular weight less than 800 Daltons. 

You must be aware that when oil is mixed with water it floats on the surface of water. Do you know why? This is because oil is insoluble in water and lighter than water, which causes it to form a thin film on the water surface. Similarly, all lipids are water insoluble, non-polar and hydrophobic, even the ones present in the living cells. Thus, they separate out in the water-insoluble fraction of a cellular extract, along with other macromolecules, because of their association with living membranes.

Lipids are not polymers like other macromolecules such as protein, nucleic acids. They consist of carbon, hydrogen and oxygen. Did you know that the fats or lipids in our body have multiple functions? They not only help the body in heat insulation, they also help in providing energy to the body during periods of starvation. In fact, you will be amazed to know that lipids can provide twice as much energy when compared to that of glucose. Come let us learn some more about these amazing macromolecules in our body.

Table of Contents

• Constituent Units of Lipids
• Classification of Lipids
• Practice Problems of Lipids
• FAQs of Lipids

Constituent Units of lipids

Lipids have a lot of diversity on the basis of different constituent units. However, the majority of the lipids have glycerol, fatty acids or sterol as the constituent units. 

Classification of Lipids

On the basis of groups present, lipids are classified into three types as follows: 

• Simple lipids - They are formed of fatty acids and alcohols. Examples include fats, oils and wax. 
• Compound or conjugated lipids - They are formed of fatty acids, glycerol and additional groups such as phosphates or carbohydrates. Examples include phospholipids, glycolipids etc.
• Derived lipids - They are lipids like substances such as sterol. Examples include prostaglandins.

Simple Lipids 

Simple lipids are fatty acid esters of alcohol and the alcohol in the bulk of lipids is glycerol. Long chain alcohol is found in waxes, whereas glycerol and fatty acids are found in fats and oils.

Glycerol

It is an alcohol, consisting of three hydroxyl (OH) groups, three carbons, and five hydrogens. It is also called trihydroxy propane because three hydroxyl groups are present. It is a colourless, sweet and highly viscous liquid.

Fatty Acid

They are carboxylic acids with a R group.

Carboxyl group forms the acid and charged part. The R group forms a hydrocarbon chain. R groups may vary depending upon the number of carbon atoms present. It could be a methyl group, ethyl group or a long carbon chain consisting of 3-19 carbon atoms. Examples include palmitic acid (16 carbons), stearic acid (18 carbon) and arachidonic acid (20 carbons). 

On the basis of types of bond present in between carbon atoms, fatty acids are of two types as follows: Saturated fatty acids and Unsaturated fatty acids

Saturated Fatty Acids

They have single bonds present in the carbon chain. For this reason, they have a high melting point. They are solid at room temperature. Mostly animal fats contain saturated fatty acids. Examples include palmitic acid, stearic acid, etc. 

Unsaturated Fatty Acids

They have one or more double bonds present in the carbon chain. For this reason, they have a low melting point. They are generally liquid at room temperature. Mostly plant fats contain unsaturated fatty acids. Examples include sunflower oil, gingelly oil, sesame oil.

On the basis of number of double bonds, unsaturated fatty acids are further of two types as follows:

• Monounsaturated Fatty Acids (MUFA): They have only one double bond present in the carbon chain. For example: oleic acid, linoleic acid
• Polyunsaturated Fatty Acids (PUFA): They have two or more double bonds present in the carbon chain. For example: linolenic acid, arachidonic acid

Did you know that polyunsaturated fats include omega-3 and omega-6 fats? These are essential fatty acids that the body needs for brain function and cell growth. Oils derived from plants such as olive oil, sesame oil, etc., are rich in PUFA (polyunsaturated fatty acids).

Glycerides

Glycerides are esters of fatty acids and glycerol. The carboxylic group (-COOH) of the fatty acid undergoes an esterification reaction with the hydroxyl (-OH) group of the alcohol (glycerol) to form an ester bond by the elimination of water.

Based on of the number of fatty acid chains present, glycerides are of three types as follows:

On the basis of fatty acid group present, triglycerides can be of two types: Fats and Oils

Wax

They are esters of long chain fatty acid and long chain alcohol. They form a water insoluble coating. They are found on hair and skin in animals and on fruits, leaves and stems in plants. For example: bee wax, cerumen (ear wax).

Compound or Conjugated Lipids

They are esters of fatty acids and alcohol with additional groups such as sugar, proteins or phosphorus. They are usually found in cell membranes. On the basis of additional groups present, conjugated lipids are of three types as follows:

• Phospholipids
• Glycolipids
• Sphingolipids

Phospholipids

They are formed by the addition of a phosphate group in a triglyceride. Basically, phospholipids have three components:

• Glycerol backbone
• Fatty acids
• Phosphate group

They are the major lipids found in the cell membrane. Examples include lecithin.

Glycolipids

They are formed by the addition of a carbohydrate group in a triglyceride. They have three components:

• Glycerol backbone
• Fatty acids
• Carbohydrate group

They are mainly found on the cell membrane surface and help in recognising other cells. They are present on the surface of neurons such as the myelin sheath (mainly composed of a glycolipid galactocerebroside).

Sphingolipids

These are conjugated lipids which contain an organic aliphatic amino alcohol known as sphingosine or a substance structurally similar to it.

Derived Lipids

Lipids derived from simple or conjugated lipids are called derived lipids. Their structure is complex. For example: Cholesterol - It is tetracyclic in nature.

Functions of Lipids

Lipids are important for the optimum functioning of living organisms. They play various roles as follows.

• They store energy for the long term. For example: triglycerides. 
• They retain the body heat and prevent it from escaping. Thus, they provide insulation. 
• They are the main component of cell membranes such as phospholipids. 
• Some of them are chemokines and act as chemical messengers. 
• They protect against physical shock. For example, lipids in cell membranes of fish help them to survive in frigid conditions. 
• They protect against water loss.

Practice Problems of Lipids

Question 1. Select the incorrect statement about fats.

a. All lipids are esters of fatty acids and glycerol.
b. Fats are solids at room temperature whereas oils are liquids at room temperature.
c. The melting point of fats depends upon the chain length of the constituent fatty acid.
d. They float on water as their specific gravity is less than 1.

Answer: Lipids have a lot of diversity on the basis of different constituent units. Simple lipids are fatty acid esters of alcohol and the alcohol in the bulk of lipids is glycerol. Long chain alcohol is found in waxes, whereas glycerol and fatty acids are found in fats and oils. Hence, option a is correct.

Question 2. Read the given assertion and reason and select the correct option.

Assertion: Fats, wax and oils are lipids.
Reason: Lipids are polymers of fatty acids.

a. Both assertion and reason are correct, reason is the correct explanation of assertion.
b. Both assertion and reason are correct but reason is not the correct explanation of assertion.
c. Assertion is correct, Reason is incorrect.
d. Both assertion and reason are incorrect.

Answer: Lipids are fatty acid polymers. Esters of glycerol and three fatty acids make up oils and fats. A wax is a long-chain alcohol esterified with a fatty acid. Thus, lipids include oils, fats, and waxes.Hence, option a is correct.

Question 3. Which bond links glycerol and fatty acids in simple lipids?

Answer: Fatty acids and glycerol are linked together by ester bonds. Ester bond is formed between the OH of carboxylic group of fatty acids and alcohol group of glycerol. 

Question 4. Compare simple lipids, conjugated lipids and derived lipids.

Answer: The comparative study between simple lipids, conjugated lipids and derived lipids can be summarised as -

FAQs of Lipids

Question 1. Where are lipids found in the human body?

Answer: Lipids are the primary contents of the cell membranes of all the cells in our bodies. Lipids are also stored in the adipose tissues in the human body and are present beneath the skin as an insulating layer. The myelin sheath surrounding the nerve cells also contain lipid.

Question 2. Is cholesterol a lipid?

Answer: Cholesterol is a derived lipid and is a derivative of sterol.

Question 3. Which fatty acids are better for health and why?

Answer: Saturated fatty acids have only a single covalent bond between carbon atoms. They have higher melting points and thus, they are solid at room temperature. While, unsaturated fatty acids have two covalent bonds between carbon atoms. They have lower melting points and are liquid at room temperature. Due to this reason, unsaturated fatty acids will not form deposition in the blood vessels. Thus, unsaturated fatty acids are better.

Question 4.  What are the common sources of lipids?

Answer: The common sources of lipids for our body are fatty foods such as fatty fishes, eggs, dairy products such as cheese, seeds and nuts, etc.

Question 5. What happens when your body does not receive sufficient lipids?

Answer: Lack of sufficient lipids in the body can weaken the immune system and affect the absorption of fat soluble vitamins in the body. This reflects in the form of weakened immunity against infections, vitamin deficiency symptoms such as skin rashes, etc.

Related Concepts to Lipids 

• Carbohydrates
• Disaccharides Oligosaccharide Polysaccharides
• Dynamic state of body constituents
• Amino acids

NCERT Class 11 Biology Chapters