Carbohydrates | Classification, Functions, Importance and uses

The Ultimate Guide/Notes to Carbohydrates

A Comprehensive Study Resource for Health Sciences & Biochemistry

Reader's Guide to This Post

🔵 Blue Box: Scientific Phenomenon

🟢 Green Box: Core Concepts

🟡 Yellow Line: Quick Study Tips

🔴 Pink Box: Memory Mnemonics

Table of Contents

1. Introduction to Carbohydrates
2. Definition & Essentials
3. Important Points to Know
4. Classification Overview
5. Monosaccharides (Details & Table)
6. Glucose, Isomers & Optical Activity
7. Examples & Functions of Monosaccharides
8. Conversion of Glucose
9. Galactose Functions
10. Disaccharides & Medical Importance
11. Oligosaccharides
12. Polysaccharides (Homo-polysaccharides)
13. Glycogen (Animal Starch)
14. Starch, Dextrins & Dextrans
15. Cellulose & Dietary Fibers

Carbohydrates are an essential part of a healthy diet and play a key role in the functioning of our bodies. Whether you're a healthcare professional, a biochemistry student, or preparing for a college-level exam, understanding the basics of carbohydrates is important. This comprehensive guide covers everything you need to know about carbohydrates, including their sources, functions, and classification into monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Classification of carbohydrates based on the number of sugar units.

The article is designed to be easy to understand, self-explanatory, and tailored for students and self-learners preparing for exams such as MBBS, MBDS, BS Nursing, BS Biochemistry, BS Nutrition, NMDCAT, and Army Medical College entry tests, among others. Get ready to unlock the mysteries of carbohydrates and deepen your knowledge with this informative guide.

What are Carbohydrates/ Saccharides/ Sugars?

Carbohydrates simply mean “Hydrates of Carbon” (Where Hydrates is for Water).

Definition of Carbohydrates:
Poly-hydric or Poly-hydroxy alcohols with potentially active carbonyl groups (Aldehyde/Keto) are called Carbohydrates.

A detailed note of Carbs and their classification
From the Biochemistry Library of H.E.S (Health, Education, and Skills)

Important points to know regarding Carbohydrates:

• They are made up of a minimum of three Carbon atoms.
• They have potentially active carbonyl groups i.e Aldehyde Group or a Keto Group.
• If Carbonyl groups are not involved in bond formation i.e. free, they have reducing properties.
• Sugars with at least one free Aldehyde or Keto group, in their structure, are called reducing sugars.
• While, Sugars with no free Aldehyde or Keto group, are involved in bond formation and are called non-reducing sugars.
• The Aldehyde group is always present at Carbon-1 and the Keto group is always at Carbon-2.
• Two or more sugar residues are joined to each other by a bond known as Glycosidic Bond or Glycosidic Linkage.

Diagram generated via ConceptViz

Classification of Carbohydrates

Although carbohydrates are divided in a number of ways, however, their division into the following four categories is among the most common. These are monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Their detail is given below:

1. A. Monosaccharide – (mono=one, Saccharum=sugar)

The following points are important to know about monosaccharides:

• They are classified according to the Carbon atoms present in them.
• They are the simplest sugars, the most common of which is Glucose (C6H12O6).
• They can’t be further hydrolyzed (broken down).
• They typically contain three to seven carbon atoms and have names ending in “ose”.
• They may be aldose/Aldo-sugar due to the presence of the “Aldehyde group” or ketose/keto-sugar due to the presence of the “Keto group”.
• All monosaccharides are reducing sugars.

Examples of Monosaccharide - Aldoses, and Ketoses

Serial No	Number of Carbon atoms	Generic Name	Aldoses	Ketoses
1	3	Triose	Glycerose/Glyceraldehyde	Dihydroxy-Acetone
2	4	Tetrose	Erythrose	Erythrulose
3	5	Pentose	Ribose	Ribulose
4	6	Hexose	Glucose, Galactose, Manose	Fructose

Note: Glucose and Ribose are aldohexoses (having an Aldehyde group on the sixth carbon). If the carbonyl C is internal to the chain, it forms a Ketone group and the sugar is called a ketose. Similarly, Fructose and Ribulose are keto-hexoses.

Common examples of Aldoses and Ketoses based on carbon chain length
(Trioses, Tetroses, Pentoses, and Hexoses)
Diagram generated via ConceptViz

Glucose and its isomers

Glucose, Galactose, and fructose are isomers of each other, which means that they have the same chemical formula of C6H12O6, but they differ in the organization of their atoms, making them isomers of one another.

All of these isomers are inter-convertible. Fructose is a structural isomer of glucose and galactose. Glucose and galactose are stereoisomers of each other, meaning that their atoms are bonded together in the same order, but they have different 3D organizations of atoms.

Optical Activity of monosaccharides

Monosaccharides are optically active, which means that if polarized light is passed through a solution, the plane of light will be rotated to the left (Levorotatory or L-form) or to the right (Dextrorotatory or D-form).

Thus, L-glucose and D-glucose are optical isomers of each other. Our body can only use right-handed sugar, left-handed sugar is indigestible.

Examples and Functions of important monosaccharides

1. Glyceraldehyde: It is an aldo-triose. Used as reference sugar, because all sugars are derived from it and all the “D” and “L” sugars are referred to it.

2. Dihydroxy-acetone: It is a keto-triose. It is produced and utilized in Glycolysis (breakdown of glucose) to produce ATP (energy). It is also produced from Glycerol through tri-glyceride degradation.

3. Erythrose: It is an aldo-tetrose. It is also produced by Glucose breakdown through Hexose Monophosphate Pathway (HMP), and not through Glycolysis.

4. Ribose: It is an aldo-pentose. It is most commonly distributed in nature. It is also produced through HMP Shunt. Ribose and its reduced form, known as deoxyribose, are important components of RNA and DNA respectively.

5. Ribulose, Xylose, and Xylulose: These are keto-pentoses. They are produced through an HMP shunt. They can be converted to Glucose in vivo.

6. Glucose (Grape Sugar): It is an aldohexose. Due to its dextrorotatory property, it is also known as Dextrose. It is the main sugar in the human body. Sucrose (also known as table sugar), is an isomer of Glucose and is the most common source of Glucose. It is the first line of nutrition for the production of energy (ATP) through Glycolysis and Citric Acid Cycle. To produce energy, Glucose first enters the cell. This entry is facilitated by insulin (however certain cells, such as erythrocytes, hepatocytes, brain cells, and intestinal mucosal cells don’t need insulin).

Conversion of Glucose into Glycogen, lipids, non-essential amino acids, Galactose, lactose, alcohols, and sugar acids, NADPH2:

• Excess Glucose is converted to Glycogen, which is stored in the liver and muscles.
• When glycogen storage capacity is full, glucose is then converted into lipids and stored in adipose tissues as fats.
• Glucose helps in the synthesis of non-essential amino acids by providing a Carbon skeleton.
• Glucose forms Galactose and Lactose (milk sugar) in the mammary gland.
• Glucose also synthesizes Alcohol (e.g. Sorbitol) when gets Reduced.

While upon oxidation, Glucose forms sugar acids e.g.

• Gluconic acid (Aldonic acid) is produced by the oxidation of the Aldehyde group at C#1.
• Glucuronic acid (Uronic acid) is produced by the oxidation of the Carboxyl group at C#6.
• Glucaric acid (Saccharic acid) is produced by the oxidation of both C#1 and C#6.

Glucose forms NADPH2 by following two methods:

1. Through the HMP pathway.
2. Through Glycolysis.

7. Galactose

It is aldohexose. It is present in milk in the form of Lactose (also known as milk sugar). It readily gets converted into glucose in vivo. It is required for the development of brain tissues i.e. Cerebrosides and Gangliosides.

B. Disaccharides (Di=two, saccharum=sugar)

As the name indicates, the disaccharide is a class of carbohydrates that produces two monosaccharide residues upon hydrolysis.

Examples of disaccharides

Disaccharides	Monosaccharides (Components)
1. Maltose (Fruit Sugar)	Glucose + Glucose
2. Lactose (Milk Sugar)	Glucose + Galactose
3. Sucrose (Cane Sugar)	Glucose + Fructose

Medically important disaccharides | structures, functions, and examples

1. Maltose (Fruit sugar): Produced in our body by the action of salivary amylase and pancreatic amylase on starch. Hydrolysis produces two glucose molecules joined through 1, 4 glycosidic linkages. Maltose is a reducing sugar because the aldehyde group of glucose one is left free.

2. Lactose (Milk sugar): Hydrolyzes in our intestine in the presence of Lactase and yields glucose and galactose. Linked through B-1,4 glycosidic linkage. It is also a reducing sugar. Bacteria convert lactose to lactic acid through fermentation.

3. Sucrose (Cane sugar): Commonly known as table sugar. Hydrolyzes in the presence of Sucrase (invertase) and yields glucose and fructose. Linked through Alpha-1,2 glycosidic linkage. Sucrose is a nonreducing disaccharide because active groups are engaged in making glycosidic bonds.

C. Oligosaccharides (Oligo=few, Saccharum=sugar)

Oligosaccharide is a class of carbohydrates that produces three to ten (3-10) monosaccharide residues upon hydrolysis. They are not physiologically too much important.

D. Polysaccharides (Poly=many, Saccharum=sugar)

Polysaccharide is a class of carbohydrates that produces more than ten (10) monosaccharide residues upon hydrolysis. They form structural elements of the cells. They are all non-reducing carbohydrates. Polysaccharides are of the following two types:

a. Homopolysaccharides (Hm-Ps)

Homo polysaccharides upon hydrolysis yield only one type of monosaccharide unit. They can be obtained from animal sources (Glycogen) as well as plant sources (Starch, dextrins, dextrans, and cellulose). All of these contain glucose.

1. Glycogen

It is also known as animal starch. It is present mainly in the liver and muscles. It has a branched tree-like structure, similar to amylopectin, but branches contain less than 12 glucose units.

Two glycosidic linkages:
i. Alpha-1,4 glycosidic linkage forms a straight chain.
ii. Alpha-1,6 glycosidic linkage joins the straight chains forming a branch.

Enzymes: Phosphorylase (breaks Alpha-1,4) and the debranching enzyme (breaks Alpha-1,6).

Main function of liver glycogen is to maintain blood glucose during fasting. Muscle glycogen only provides energy to the muscles.

2. Starch

It is the main form of ingested carbohydrates. It is of two types:

• Amylose: A straight chain Homo-Polysaccharide.
• Amylopectin: Branched tree-like structure; each branch has more than 12 glucose residues.

Starch hydrolyzes into maltose, maltotriose, and dextrins by amylase.

3. Dextrins & 4. Dextrans

Dextrins: Intermediate hydrolytic products of starch with a sweet taste (Amylodextrins, Erythrodextrins, Achrodextrins).

Dextrans: Highly viscous polysaccharides used as Plasma expanders in the treatment of shock.

5. Cellulose

Humans don't have a Cellulase enzyme hence they cannot digest cellulose.

Cellulose is mainly present in plants as the main dietary fiber (along with Lignin and Pectin). These fibers absorb water 10 to 15 times their weight, increasing bowel motility.

• Functions: Stimulates intestinal peristalsis, prevents constipation (softens stools), and hemorrhoids.
• Detoxification: Decreases absorption of toxic compounds like carcinogens, preventing colon cancer.
• Trace elements: Can bind zinc (Zn) and decrease absorption of fat-soluble vitamins (excess fiber not recommended).
• Cholesterol: Helps in lowering blood cholesterol.

Sources: Whole-grain cereals and bread, fruits, vegetables, and legumes.

© 2026 Educationized Science Series | H.E.S Biochemistry Library