Introduction to Biomolecules - Classification of Carbohydrates

Biomolecules are the organic molecules present in living organisms.
Carbohydrates are one of the important classes of biomolecules.
They are composed of carbon, hydrogen, and oxygen atoms.
Carbohydrates are classified based on their chemical structure and properties.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates.
They cannot be broken down into smaller units by hydrolysis.
Examples: glucose, fructose, and galactose.
They have a general formula of (CH2O)n, where n is the number of carbon atoms.

Disaccharides

Disaccharides are formed by the condensation reaction between two monosaccharides.
They can be broken down into monosaccharides by hydrolysis.
Examples: sucrose, lactose, and maltose.
Sucrose is formed by the condensation reaction between glucose and fructose.

Polysaccharides

Polysaccharides are complex carbohydrates made up of many monosaccharide units.
They are insoluble in water and serve as storage and structural molecules in organisms.
Examples: starch, cellulose, and glycogen.
Starch is the storage form of glucose in plants, while glycogen is the storage form in animals.

Functions of Carbohydrates

Carbohydrates serve as a source of energy.
Glucose is the primary source of energy for cellular respiration.
They provide structural support to plants and animals.
Carbohydrates are also involved in cell recognition and signaling.

Structural Isomerism in Carbohydrates

Structural isomers have the same molecular formula but different structural arrangements.
Monosaccharides exhibit structural isomerism due to the arrangement of hydroxyl groups.
Example: glucose and fructose are structural isomers.

Stereoisomerism in Carbohydrates

Stereoisomers have the same molecular formula and structural arrangements but differ in spatial arrangement.
Monosaccharides exhibit stereoisomerism as a result of asymmetric carbon atoms.
Example: glucose exists in two stereoisomeric forms - D-glucose and L-glucose.

D and L Configuration

D and L configuration are used to distinguish between stereoisomers of monosaccharides.
D configuration refers to the arrangement of hydroxyl group on the right side of the asymmetric carbon.
L configuration refers to the arrangement of hydroxyl group on the left side of the asymmetric carbon.

Haworth Projection Formula

The Haworth projection formula is used to represent cyclic compounds, such as monosaccharides.
In this projection, the carbon chain is represented as a hexagon, and the substituents are shown as vertical lines.
The oxygen atom is usually represented at the top of the projection.

Examples of Haworth Projections

D-Glucose: The hydroxyl group on the right side of the asymmetric carbon is shown as a vertical line towards the bottom of the projection.
D-Fructose: The hydroxyl group on the right side of the asymmetric carbon is shown as a vertical line towards the top of the projection.

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Linkage in Disaccharides

Disaccharides are formed by the condensation reaction between two monosaccharides.
The linkage between the monosaccharide units can be either α or β.
In α linkage, the hydroxyl group on the anomeric carbon is below the plane of the ring.
In β linkage, the hydroxyl group on the anomeric carbon is above the plane of the ring.
Example: Sucrose has an α,β-glycosidic linkage between glucose and fructose.

Sucrose

Sucrose is a disaccharide composed of glucose and fructose units.
It is commonly known as table sugar.
Sucrose is obtained from sugar cane or sugar beet.
It is a non-reducing sugar because both glucose and fructose are bonded through glycosidic linkage.
It is hydrolyzed by the enzyme sucrase in the small intestine.

Lactose

Lactose is a disaccharide composed of glucose and galactose units.
It is commonly found in milk and dairy products.
Lactose intolerance occurs when the body lacks the enzyme lactase, which is required to break down lactose.
Lactose can be hydrolyzed into its component monosaccharides by lactase.

Maltose

Maltose is a disaccharide composed of two glucose units.
It is formed during the digestion of starch by the enzyme amylase.
Maltose is present in malted grains, such as barley.
It is a reducing sugar because it has a free anomeric carbon that can react with Benedict’s reagent.

Starch

Starch is a polysaccharide composed of glucose units.
It is the main storage carbohydrate in plants.
Starch is present in the form of granules in plant cells.
It can be broken down into glucose by the enzyme amylase.
Amylose and amylopectin are the two components of starch.

Cellulose

Cellulose is a polysaccharide composed of glucose units.
It is the major structural component of plant cell walls.
Cellulose molecules are linked together by β-1,4-glycosidic bonds.
Humans lack the enzyme to hydrolyze cellulose, so it cannot be digested.
Cellulose provides dietary fiber and helps in the movement of food in the digestive system.

Glycogen

Glycogen is a polysaccharide composed of glucose units.
It is the storage form of glucose in animals.
Glycogen is mainly stored in the liver and muscles.
It is a highly branched molecule that allows for rapid release of glucose when needed.
Glycogen can be hydrolyzed by the enzyme glycogen phosphorylase.

Functions of Polysaccharides

Starch and glycogen serve as energy storage molecules in plants and animals.
Cellulose provides structural support to plant cells.
Polysaccharides such as chitin are found in the exoskeleton of arthropods.
Some polysaccharides, like hyaluronic acid, have lubricating properties in joints.
Polysaccharides also play a role in cell-cell recognition and immune responses.

Biologically Important Monosaccharides

Glucose: It is the primary source of energy for cellular respiration.
Fructose: It is found in fruits and is converted to glucose in the liver.
Galactose: It is present in milk and is converted to glucose in the liver.
Ribose and deoxyribose: They are components of nucleotides in DNA and RNA.
Glucosamine: It is an amino sugar used in the synthesis of chitin and glycosaminoglycans.

Summary

Carbohydrates are classified into monosaccharides, disaccharides, and polysaccharides.
Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed.
Disaccharides are formed by condensation reactions and can be hydrolyzed into monosaccharides.
Polysaccharides are complex carbohydrates with various functions in organisms.
Understanding the structure and function of carbohydrates is essential in studying biomolecules.

Properties of Monosaccharides

Monosaccharides are sweet-tasting, crystalline solids.
They are soluble in water and can form solutions.
Monosaccharides are reducing sugars due to the presence of a free anomeric carbon.
They can undergo oxidation reactions.
Monosaccharides can form cyclic structures in solution.

Mutarotation of Monosaccharides

Mutarotation refers to the interconversion between different stereoisomeric forms of a sugar.
In solution, monosaccharides exist in an equilibrium between the open-chain and cyclic forms.
Mutarotation occurs as the α and β anomers of a sugar interconvert through the open-chain form.
This process is catalyzed by the surrounding solvent, such as water.

Functional Groups in Carbohydrates

Carbohydrates contain multiple functional groups, including hydroxyl (-OH) groups.
These hydroxyl groups make carbohydrates polar and capable of forming hydrogen bonds.
The carbonyl group (C=O) in monosaccharides is responsible for their reactivity.
Carbohydrates can react with other molecules, such as proteins, lipids, and nucleic acids.

Glycosidic Bond Formation

Glycosidic bonds are formed by the condensation reaction between the hydroxyl groups of two monosaccharides.
The reaction involves the elimination of a water molecule.
The glycosidic bond can be α or β depending on the relative orientation of the hydroxyl groups involved.

Importance of Carbohydrates in Metabolism

Carbohydrates are the primary source of energy for cells.
Glucose is the main fuel molecule for cellular respiration.
Carbohydrates are metabolized to produce ATP, which is used in various cellular processes.
Excess glucose is stored as glycogen in animals and as starch in plants.
Carbohydrates also play a role in the biosynthesis of other biomolecules, such as lipids and proteins.

Carbohydrates in Food

Carbohydrates are a major component of the human diet.
They are commonly found in grains, fruits, vegetables, and dairy products.
Complex carbohydrates, such as starch and fiber, provide sustained energy and regulate digestion.
Simple carbohydrates, such as sugars, provide quick energy but should be consumed in moderation.

Importance of Fiber in the Diet

Fiber is a type of carbohydrate that cannot be digested by humans.
It adds bulk to the diet and promotes healthy digestion.
Fiber helps prevent constipation and aids in weight management.
Foods rich in fiber include whole grains, fruits, vegetables, and legumes.
Increasing fiber intake is beneficial for overall health.

Common Carbohydrate Tests

Benedict’s Test: It is used to detect the presence of reducing sugars.
Iodine Test: It is used to detect the presence of starch. A blue-black color indicates the presence of starch.
Molisch’s Test: It is used to detect the presence of carbohydrates. A purple or red color indicates a positive test.
Fehling’s Test: It is similar to Benedict’s test and is used to detect reducing sugars.

Carbohydrate Derivatives

Carbohydrates can be chemically modified to produce derivatives with different properties.
Derivatives, such as sugar alcohols (e.g., sorbitol) and amino sugars (e.g., glucosamine), have various applications in pharmaceuticals, food, and cosmetics.
Derivatives of carbohydrates can also be used as building blocks for the synthesis of complex molecules.

Conclusion

Carbohydrates are essential biomolecules with diverse functions in living organisms.
They are classified into monosaccharides, disaccharides, and polysaccharides based on their structure and properties.
Monosaccharides are the building blocks of carbohydrates and can exist as cyclic structures.
Disaccharides are formed by the condensation reaction between two monosaccharides.
Polysaccharides serve as storage and structural molecules in organisms.
Understanding carbohydrates is important for studying metabolism, nutrition, and biochemistry.