The Ultimate Guide/Notes to Carbohydrates

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. 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.

Notes on Carbohydrates, their classification, functions, sources, and diseases caused by their deficiecny or abnormalities for BS Nursing, MDCAt, NEET, Paramedics, Pharm-A and Pharm-D examinations

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.

Classification of Carbohydrates | Monosaccharides, Disaccharides, Oligosaccharides, and polysaccharides.

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	Penatose	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 so that there are other carbons on both sides of it, it forms a Ketone group and the sugar is called a ketose. Similarly, Fructose and Ribulose are keto-hexoses (having the Ketone group on the sixth carbon).

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, meaning that its atoms are actually bonded together in a different order. 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 of these compounds,

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 the glycogen storage capacity of the liver and muscles 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

Through the HMP pathway.
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 (from which respective disaccharide is made up of)*
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 of disaccharides

1. Maltose (Also called fruit sugar)

Maltose is produced in our body by the action of two enzymes on starch i.e. salivary amylase and pancreatic amylase, present in saliva and pancreas respectively.
Upon hydrolysis of maltose produces two glucose molecules.
These two glucose molecules are joined with each other through 1, 4 glycosidic linkages. Here 1 indicates C#1 of the glucose-2 molecule, while 4 indicates C#4 of the glucose-1 molecule.
Maltose is a reducing sugar because the linkage is between C#1 (aldehyde group) of glucose-2 and C#4 of glucose-1, the C#1 (aldehyde group) of glucose one is left free which has reducing properties.

2. Lactose (also called milk sugar)

Lactose hydrolyzes in our intestine, in the presence of an enzyme called Lactase, and yields glucose and galactose (both are aldohexose).
Both of these aldohexose i.e. glucose and galactose are linked together through B-1,4 glycosidic linkage.
Similar to maltose lactose is also a reducing sugar.
Through the process of fermentation, bacteria convert lactose to lactic acid.

3. Sucrose (also known as cane sugar)

It is commonly known as table sugar.
Sucrose also hydrolyzes in our intestine, in the presence of an enzyme called Sucrase (invertase) and yields glucose (an aldohexose) and fructose (a ketohexose).
Both of these monosaccharides i.e. glucose and fructose are linked together through Alpha-1,2 glycosidic linkage (C#1 of glucose is joined with C#2 of fructose).
Sucrose is a nonreducing disaccharide. It is because the aldehyde group of glucose at C#1 and the keto group of fructose at C#2 are engaged in making glycosidic bonds, so there are no active groups left free.

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 as well as plant sources.

The animal's source is glycogen.

Plant Sources are starch, dextrins, dextrans, and cellulose.

All of these contain glucose.

Examples of Homo-Polysaccharides, their structures, examples. Sources, and functions

1. Glycogen

It is also known as animal starch.
It is present mainly in the liver and muscles, where it acts as a storage polysaccharide for glucose.
It has a branched-like structure, like a tree, and is similar to that of amylopectin of starch. But each branch of glycogen contains less than 12 glucose units, whereas amylopectin contains more than 12 glucose residues.
It has two glycosidic linkages i.e.

i. Alpha-1,4 glycosidic linkage forms a straight chain of glucose in glycogen and

ii. Alpha-1,6 glycosidic linkage joins the two straight chains with each other forming a branch.

These linkages can be broken down by two different enzymes i.e

i. Phosphorylase, which breaks Alpha-1,4 linkage and

ii. The debranching enzyme breaks the Alpha-1,6 linkage.

The main function of liver glycogen is to maintain the normal blood glucose level during fasting.
While on the other side, muscle glycogen has no role in maintaining the blood glucose level, instead it only provides energy to the muscles.

2. Starch

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

i. Amylose: It is a straight chain Homo-Polysaccharide.

ii. Amylopectin: It has a tree-like branched structure, but unlike glycogen, each branch in amylopectin has more than 12 glucose residues.

Starch hydrolyzes into maltose, maltotriose, and dextrins by the action of the amylase enzyme.

3. Dextrins

These are intermediate hydrolytic products of starch.
They have a sweet taste.
Examples of dextrins are Amylodextrins, Erythrodextrins, and Achrodextrins.

4. Dextrans

These are highly viscous polysaccharides.
They are 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 and it is the main dietary fiber other dietary fibers include Lignin and Pectin.
These fibers can absorb water 10 to 15 times its own weight, drawing water into the intestinal lumen, thus increasing bowel motility.
Therefore the main function of cellulose is to stimulate intestinal peristalsis thus helping in preventing constipation (softens stools), and hemorrhoids.
It also decreases the absorption of toxic compounds e.g. carcinogens, hence it is preventive against colon cancer.
Another function of cellulose is that it can bind trace elements, such as zinc (Zn), and can decrease the absorption of fat-soluble vitamins, hence excess dietary fiber supplements are not recommended.
Fibers also help in lowering blood cholesterol.
Sources of cellulose (fibers) include whole-grain cereals and bread, fruits, vegetables, and legumes.

Carbohydrates | Classification, Functions, Importance and uses of Carbohydrates.