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Cholesterol: Structure, Types, Healthy Levels, Complications, Managements, Treatments, Functions, Practice Problems and FAQs

Cholesterol: Structure, Types, Healthy Levels, Complications, Managements, Treatments, Functions, Practice Problems and FAQs

We have a variety of food items which includes healthy and unhealthy foods. Most of you love to eat burgers and cheese loaded pizzas. At the same time you may not be interested in eating spinach, drumstick leaves, fresh carrots etc. In most cases the pizzas are loaded with meat toppings, cheese, pepperoni, olives, vegetables or onions and the sauce is sweet.

                                            Fig: Cheese loaded pizza and burgers

But have you ever wondered why our parents always ask us not to eat pizzas loaded with too much cheese, instead asking us to incorporate some vegetables and fruits in the diet. It is due to the fact that too much cheese is not good for our health. Most fast food items have high amounts of sodium, fats, and calories. All of these can raise our risk of a heart attack. When you hear about heart attack, the first word that comes into your mind is cholesterol, right? We all have quite often heard about cholesterol. But what exactly is cholesterol? Is it really harmful for our health? Come, let’s find out the answers to these questions in this article.

Table of contents

  • Lipids
  • Fatty acids
  • Classification of lipids
  • Sterol
  • Cholesterol
  • Types of cholesterol
  • Healthy cholesterol level
  • Complications of high cholesterol level
  • Management of high cholesterol level
  • Treatments for high cholesterol level
  • Functions of cholesterol
  • Practice Problems
  • FAQs


These are biomacromolecules made up of carbon, hydrogen and little oxygen. They are fatty acid esters of alcohols and related substances. These are not soluble in water but are easily soluble in organic solvent like ether, benzene, acetone, etc. They are not polymers. They are non-polar and hence insoluble in water.

                                    Fig: Food sources of lipids

Fatty acids

These are organic acids with a hydrocarbon chain ending in carboxylic groups (COOH). So they possess a carboxyl group and an R group. The R group can be methyl group (CH3), ethyl group (C2H5) or higher number of CH2 groups (1 - 19 carbon atoms). Examples include arachidonic acid.

                                Fig: Fatty acids

Types of fatty acids

Fatty acids are of two types as follows:

Saturated fatty acids

They do not have double bonds in their carbon chains. Examples include palmitic acid (C16H3202).

Unsaturated fatty acids

They have one or more double bonds in their carbon chains. They can take up additional hydrogen and hence are unsaturated. Examples include oleic acid (C18H34O2).

                                     Fig: Types of fatty acids


Each molecule of glycerol can react with three molecules of fatty acids. Depending on the number of fatty acids that are attached to the glycerol molecule three types of glycerides are formed as follows:

  • Monoglycerides
  • Diglycerides
  • Triglycerides

                                                    Fig: Triglyceride formation

Classification of lipids

Lipids are classified into three types as follows:

                                                          Fig: Classification of lipids

Simple lipids

These are formed of fatty acids and alcohols. In them the long, unbranched, hydrocarbon chain fatty acids are linked with an alcohol (usually, glycerol) via ester bond. Thus, simple lipids are fatty acid esters of glycerol. Examples include desi ghee.

                                  Fig: Desi ghee

Conjugated or compound lipids

These are also fatty acid esters of alcohols, but also contain an additional group like phosphate group (in phospholipids) or sugar (in glycolipids).

                                           Fig: Phospholipid

Derived lipids

These are not actual lipids, rather they are derived by hydrolysis of simple and conjugated lipids. These include steroids, sterols, prostaglandins, etc.


These are lipids of high molecular weight which possess four fused hydrocarbon rings and a long hydrocarbon side chain. Examples include cholesterol.

                                                                Fig: Cholesterol


It is the most common sterol. The word ‘Cholesterol’ is derived from the ancient Greek word ‘chole’ meaning ‘bile’, ‘stereos’ meaning solid and ‘-ol’ meaning ‘alcohol'. It is an organic compound. Sterols are a subgroup of steroids. Cholesterol is derived from acetic acid and is the most abundant steroid in animal tissues. It is found in meats, poultry, seafoods, eggs and dairy products.

Structure of cholesterol

It is represented as C27H45OH that means it is a 27 carbon compound. It possesses a hydrocarbon tail, a sterol nucleus in the centre made of four hydrocarbon rings (A, B, C and D), and a hydroxyl group. The centre sterol nucleus or ring is a feature of all steroid hormones. It is a tetracyclic compound with four fused rings.

                                                                     Fig: Cholesterol

Chemical and physical properties of cholesterol

The following table depicts the major physical and chemical properties of cholesterol:

                           Fig: Chemical and physical properties of cholesterol

Types of cholesterol

Cholesterol can be called good or bad cholesterol based on its function.

Good cholesterol

High density lipoprotein (HDL) is called the good cholesterol as high levels of HDL decreases the risk of heart failures and strokes. HDL carries excess cholesterol from the blood and carries it back to the liver, where the liver either converts it to bile salts or flushes it out of the body. Thereby, HDL decreases blood cholesterol levels.

                                         Fig: Good cholesterol

Common food sources of HDL

HDL is commonly found in food sources like olive oil (Olea europaea), flax seeds (Linum usitatissimum), chia seeds (Salvia hispanica), avocado (Persea americana), beans, legumes, high fibre fruits and omega-3 fatty acid rich fishes like mackerel (Scomber scombrus), sardine (Sardina pilchardus), salmon (Salmo salar) and tuna (Thunnini).

                            Fig: Common food sources of HDL

Bad cholesterol

Low density lipoproteins (LDL) is called bad cholesterol because it carries blood cholesterol to other tissues and not to the liver, thereby leading to the deposition of cholesterol in the tissues. If cholesterol gets deposited in the arteries supplying blood to heart muscles, it will lead to formation of a plaque. The plaque narrows the lumen of the artery thereby decreasing blood supply to the heart muscles and ultimately leading to heart attack or even cardiac arrest.

                                               Fig: Bad cholesterol

Common sources of LDL

LDL is easily found in food items like processed meats like ham, salami and bacon, processed foods like pastries and biscuits, deep fried fast foods, fat on meat, skin on chicken, ghee, etc.

                                                   Fig: Common sources of LDL

Healthy cholesterol level

The total amount of cholesterol in the body is calculated based on our dietary triglycerides, HDL and LDL. Triglycerides are the most common form of lipid that we consume on a daily basis. Ghee, butter, cooking oil, and cheese are the various common sources of triglycerides in our diet.

                                               Fig: Rich sources of triglycerides

The amount of cholesterol in blood is measured in mg per dL of blood. The normal range of blood cholesterol for a normal healthy adult is 80 - 180 mg/dL. Any amount in the range of 200 - 239 mg/dL is considered a borderline situation where a person must take immediate control over their cholesterol consumption. Any amount equal to or higher than 240 mg/dL is considered to be high blood cholesterol and requires medical attention.

Complications of high cholesterol level

High levels of triglycerides, low HDL and high LDL can increase the risk of cardiovascular diseases, thus it is important to keep a check on the amount of triglycerides we consume in our diet. Therefore, our parents ask us not to indulge in any food with a lot of butter or loaded with cheese. If a person is diagnosed with high blood cholesterol they must immediately seek medical attention to avoid cardiovascular complications. The common complications associated with high cholesterol are as follows:

Chest pain

Plaque formation in the coronary arteries supplying blood to heart muscles leads to decreased blood flow to the heart muscles thereby causing intense chest pain (angina).

                                       Fig: Angina

Heart attack

If a plaque formed in the arteries ruptures, it leads to clot formation that blocks the coronary artery. The tissue thereby receiving no blood supply starves and eventually dies. This condition is referred to as a heart attack.

                                                   Fig: Heart attack

Heart failure

If multiple tissues die, the heart will not beat at its regular rhythm and thereby it will not be able to pump blood efficiently. This inefficiency of heart in pumping blood is called heart failure.

                                                            Fig: Heart failure

Cardiac arrest

If a lot of tissues die, eventually the heart will stop beating and pumping blood. Blood flow to the brain and vital organs also will stop. The person now suffers a cardiac arrest and if not treated on time he/she dies.

                                              Fig: Cardiac arrest

Management of high cholesterol level

Once diagnosed with borderline or high blood cholesterol, the person suffering must immediately start changing his/her lifestyle to keep a check on it. Following are few lifestyle changes that can help one person in dealing with high cholesterol:

  • Increase the amount of healthy fats in the diet.
  • Lowering triglycerides in the diet.
  • Reducing or limiting alcohol intake.
  • Quit smoking.

                                      Fig: Practices need to be avoided

  • Regular exercises.
  • Practising any physical activities or sport.
  • Maintaining a healthy weight.
  • Reducing weight around the belly.

                                              Fig: Regular exercises

Treatment for high cholesterol level

Certain medications help in managing high cholesterol levels. It include the following:


Statins are produced by the yeast Monascus purpureus. They are used as blood-cholesterol lowering agents. It acts by competitively inhibiting the enzyme (3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase) responsible for synthesis of cholesterol. In this way statins increase HDL and decrease LDL.

                                                             Fig: Action of statins

Bile acid sequestrants

These help in decreasing levels of LDL. Examples include cholestyramine, colesevelam etc.

Cholesterol absorption inhibitors

These lower the blood cholesterol levels by inhibiting absorption of cholesterol from the alimentary canal (small intestine). Examples include fibrate, ezetimibe etc.


It helps in increasing HDL levels and lowers triglycerides. It is the vitamin B3.

                 Fig: Niacin or nicotinic acid

Functions of cholesterol

The following are the major functions of cholesterol:

  • It is a constituent of all biological membranes. It provides fluidity to the membrane and also prevents freezing of the membrane at low temperatures.

                                                             Fig: Lipid bilayer

  • It helps in building up new tissue and repairing damage to existing tissues.
  • Sex hormones like testosterone, estrogen and progesterone are derived from cholesterol.
  • Corticoid hormones like mineralocorticoid, glucocorticoid and sex corticoid are also derivatives of cholesterol.
  • In the presence of sunlight (UV-B rays) cholesterol is converted into Vitamin D or calciferol.

                           Fig: Calciferol

Practice Problems

Q1: Which of the following is a macromolecule?

a. Glucose
b. Proline
c. Haemoglobin
d. Glycerol

Solution: Macromolecules are biomolecules with a molecular weight of more than 10000 Da (10 KDa). Polysaccharides (starch, cellulose etc), proteins (haemoglobin, myoglobin, enzymes etc), nucleic acids (DNA, RNA) are macromolecules. Monosaccharides (glucose, fructose, ribose etc), oligosaccharides (sucrose, lactose, maltose etc), amino acids (glycine, proline, alanine etc) are biomicromolecules. Micromolecules have a molecular weight of <800 Da. Glycerol is an alcohol. Hence, the correct option is c.

                                             Fig: Structure of haemoglobin

Q2: Which of the following is a simple lipid?

a. Lecithin
b. Carotene
c. Vitamin A
d. Palmitic acid

Solution: Simple lipids are long chain fatty acid chains linked to glycerol with ester bonds. Palmitic acid (C16H32O2), stearic acid (C18H36O2), oleic acid (C18H34O2), linoleic acid (C18H32O2), are some examples of simple lipids. Lecithin is a phospholipid. Carotene and vitamin A are examples of chromolipids. Phospholipids and chromolipids are conjugated lipids. Hence, the correct option is d.

                                                           Fig: Palmitic acid

Q3: Which of the following is a saturated fatty acid?

a. Palmitic acid
b. Oleic acid
c. Arachidonic acid
d. Linoleic acid

Solution: Fatty acids that do not contain any double bonds are called saturated fatty acids. Examples include palmitic acid (C16H32O2) and stearic acid (C18H36O2). Fatty acids with one or more double bonds are called unsaturated fatty acids. Examples include oleic acid (1 double bond - C18H34O2), linoleic acid (2 double bonds - C18H32O2), linolenic acid (3 double bonds - C18H30O2), and arachidonic acid (4 double bonds - C20H32O2). Hence, the correct option is a.

                                                      Fig: Unsaturated fatty acids

Q4: If three fatty acids are linked with a glycerol, the product would be ________________.

a. Monoglyceride
b. Diglyceride
c. Triglyceride
d. Secondary structure

Solution: When fatty acids link with glycerol, a water molecule is released for the formation of an ester bond. This reaction is called a condensation reaction. If one fatty acid is linked to a glycerol, the product is called monoglyceride. If two fatty acids are linked with glycerol, the product is a diglyceride and if three fatty acids are linked, the product is called a triglyceride. Hence, the correct option is c.

                                                 Fig: Triglyceride formation


Q1: What are lipoproteins?
These are proteinaceous molecules that help in transport of fats in the body. Since lipids are insoluble in water and therefore cannot be transported by blood (as plasma contains mostly water) thereby lipoproteins are needed for their transport. Lipoproteins are of various types:

  • Chylomicrons: They help in transport of triglycerides.
  • Very low density lipoproteins (VLDL): They are a type of bad cholesterol and carry triglycerides and little cholesterol to body tissues.
  • Intermediate density lipoproteins (IDL): When VLDL gives up its cholesterol, IDL is formed. They are either removed by liver or converted to LDL.
  • Low density lipoproteins (LDL) - These carry cholesterol from blood to body tissues where the cholesterol accumulates. It is called bad cholesterol.
  • High density lipoproteins (HDL) - These carry cholesterol from blood to liver for metabolism or excretion, thereby decreasing blood cholesterol levels. It is called good cholesterol.

                                         Fig: Bad and good cholesterol

Q2: Why are lipids not strictly macromolecules?
The molecular weight of individual lipid molecules are far less than 800 Da. But for analysing the chemical composition of a cell when we rupture the cell and try to separate macromolecules from micromolecules, lipids molecules being hydrophobic form large vesicles and are not filtered along with micromolcules of the acid soluble pool. They rather remain with macromolecules in the acid insoluble pool. Therefore, even though they have low molecular weight but are found in the acid insoluble pool, lipids are not strictly macromolecules.

                        Fig: Lipid vesicle

Q3: What are the examples of essential and non-essential fatty acids?
Lipids can be categorised into two types based on whether they are synthesised in the body or not. If lipids are synthesised in the body, they are called non-essential lipids. Examples include palmitic acid (C16H32O2), stearic acid (C18H36O2), oleic acid (C18H34O2) and arachidonic acid (C20H32O2). Those lipids that are not synthesised in the body but are required for metabolism, normal growth and development and have to be taken in diet are called essential lipids. Examples include linoleic (C18H32O2) and linolenic acid (C18H30O2).

                                      Fig: Structures of fatty acids

Q4: What do you mean by MUFA and PUFA?
The full form of MUFA is monounsaturated fatty acid. It means the fatty acid has only one double bond. For example, oleic acid (C18H34O2) is a MUFA. The full form of PUFA is polyunsaturated fatty acid. These fatty acids have more than one double bond. For example, linoleic acid (C18H32O2), linolenic acid (C18H30O2), and arachidonic acid (C20H32O2) are all PUFA.

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