Carbohydrates- Definition, Structure, Types, Examples, Functions

Carbohydrates are organic molecules of carbon, hydrogen, and oxygen atoms in a fixed ratio. The general formula for carbohydrates is (CH2O)n, where n is the number of carbon atoms. Carbohydrates are one of the four major classes of biomolecules, along with proteins, lipids, and nucleic acids.

Carbohydrates are also known as saccharides, which means "sugars" in Greek. The name reflects their sweet taste and role as the primary energy source for living organisms. Carbohydrates can be classified into three main types based on their size and complexity: monosaccharides, oligosaccharides, and polysaccharides.

Monosaccharides are the simplest and smallest carbohydrates with only one sugar unit. They have the general formula CnH2nOn, where n can range from 3 to 7. Examples of monosaccharides include glucose, fructose, and ribose.

Oligosaccharides are carbohydrates that consist of two to ten monosaccharide units linked by glycosidic bonds. They have the general formula Cn(H2O)n-1, where n is the number of monosaccharide units. Examples of oligosaccharides include sucrose, lactose, and raffinose.

Polysaccharides are carbohydrates with more than ten monosaccharide units linked by glycosidic bonds. They have no general formula, as they can vary in their structure and composition. Examples of polysaccharides include starch, glycogen, cellulose, and chitin.

Carbohydrates have diverse functions in living organisms. They serve as energy sources, energy storage molecules, structural components, signaling molecules, and precursors for other biomolecules. Carbohydrates also play important roles in metabolism, cell recognition, immunity, and gene expression.

Carbohydrates are organic molecules of carbon, hydrogen, and oxygen atoms. The general empirical formula for carbohydrates is (CH2O)n, where n is the number of carbon atoms. Carbohydrates can be classified into three main types based on their structure: monosaccharides, oligosaccharides, and polysaccharides.

Monosaccharides

Monosaccharides are the simplest and smallest carbohydrates. They are also called simple sugars because they cannot be further hydrolyzed into smaller units. Monosaccharides have the general formula CnH2nOn, where n can range from 3 to 7. The most common monosaccharides have 5 or 6 carbon atoms called pentoses and hexoses.

Monosaccharides can be further classified based on the functional group they contain. They are called aldoses if they have an aldehyde group at one end of the carbon chain. They are called ketoses if they have a ketone group in the middle of the carbon chain. For example, glucose and galactose are aldohexoses, while fructose is a ketohexose.

Monosaccharides can exist in different forms depending on how the carbon atoms are arranged and how the hydroxyl groups (-OH) are attached. These forms are called isomers and have different physical and chemical properties. For example, glucose and galactose have the same molecular formula (C6H12O6) but other structures. They are called structural isomers.

Another type of isomerism in monosaccharides is stereoisomerism. This occurs when the spatial arrangement of the atoms around a chiral carbon atom (a carbon atom with four different groups attached to it) differs. For example, glucose and mannose have the same structure but differ in the configuration of the hydroxyl group at carbon 2. They are called diastereoisomers.

A special case of stereoisomerism is enantiomerism. This occurs when two molecules are mirror images of each other and cannot be superimposed. For example, D-glucose and L-glucose are enantiomers. They have opposite configurations at all chiral carbon atoms. Most naturally occurring monosaccharides are D-isomers.

Monosaccharides can also exist in different forms depending on how they cyclize or form ring structures. This happens when a hydroxyl group on one carbon atom reacts with an aldehyde or ketone group on another carbon atom, forming a hemiacetal or hemiketal bond. The resulting ring can have five or six members and is called a furanose or pyranose ring, respectively.

The ring structure of monosaccharides can also exhibit isomerism depending on how the hydroxyl groups are oriented above or below the plane of the ring. These forms are called anomers, designated as alpha (α) or beta (β). For example, α-D-glucose and β-D-glucose are anomers of glucose.

Oligosaccharides

Oligosaccharides are carbohydrates that consist of two to ten monosaccharide units linked by glycosidic bonds. A glycosidic bond is a covalent bond that forms between one monosaccharides anomeric carbon atom and another monosaccharides hydroxyl group, releasing a water molecule.

Oligosaccharides can be classified based on the number of monosaccharide units they contain. For example, disaccharides include two monosaccharide units; trisaccharides have three monosaccharide units, and so on. Some common examples of oligosaccharides are sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose), raffinose (galactose + glucose + fructose), and stachyose (galactose + galactose + glucose + fructose).

Oligosaccharides can also be classified based on the type of glycosidic bond they contain. For example, α-glycosidic bonds link the α-anomers of monosaccharides, while β-glycosidic bonds link the β-anomers of monosaccharides. Some oligosaccharides can have both types of glycosidic bonds in their structure.

Polysaccharides

Polysaccharides are carbohydrates with more than ten monosaccharide units linked by glycosidic bonds. They can have hundreds or thousands of sugar units in their structure. Polysaccharides can be classified into two main types based on their composition: homopolysaccharides and heteropolysaccharides.

Homopolysaccharides are polysaccharides that contain only one type of monosaccharide unit in their structure. For example, starch, glycogen, cellulose, and chitin are homopolysaccharides composed of glucose units with different styles and degrees of branching.

Heteropolysaccharides are polysaccharides that contain more than one type of monosaccharide unit in their structure. For example, hyaluronic acid, chondroitin sulfate, heparin, and peptidoglycan are heteropolysaccharides composed of various combinations of sugars such as glucuronic acid, N-acetylglucosamine, N-acetylgalactosamine, iduronic acid, etc.

Polysaccharides can also be classified based on their function: storage or structural polysaccharides. Storage polysaccharides serve as energy reserves for living organisms. For example, starch is the main storage polysaccharide in plants, and glycogen is the main storage polysaccharide in animals.

Structural polysaccharides serve as support or protection for living organisms. For example, cellulose is the main structural polysaccharide in plant cell walls, and chitin is the main structural polysaccharide in fungal cell walls and arthropod exoskeletons.

Carbohydrates have various physical and chemical properties that depend on their structure and type. Some of the common properties of carbohydrates are:

• Stereoisomerism: Carbohydrates can exist in different forms with the same molecular formula but other spatial arrangements of atoms. These forms are called stereoisomers and can have various biological activities and properties. For example, glucose and galactose are stereoisomers that differ only in the orientation of a hydroxyl group on one carbon atom, but they have different tastes and metabolic pathways.

• Optical activity: Many carbohydrates are optically active, meaning they can rotate the plane of polarized light. The direction and degree of rotation depend on the structure and concentration of the carbohydrate. Optical activity can be used to identify and quantify carbohydrates in solutions. For example, glucose rotates polarized light to the right, while fructose rotates it to the left.

• Reduction and oxidation: Carbohydrates can act as reducing or oxidizing agents depending on their structure and conditions. A reducing carbohydrate can donate electrons to another molecule, while an oxidizing carbohydrate can accept electrons from another molecule. For example, glucose can be oxidized to gluconic acid by Benedict`s reagent, which is a test for reducing sugars. On the other hand, glucose can be reduced to sorbitol by sodium borohydride, which is a sugar alcohol.

• Glycosidic bonds: Carbohydrates can form covalent bonds with other molecules by losing a water molecule. These bonds are called glycosidic bonds, and they link monosaccharides to form disaccharides and polysaccharides. Glycosidic bonds can also link carbohydrates to other biomolecules, such as proteins and lipids, forming glycoconjugates. For example, lactose is a disaccharide formed by a glycosidic bond between glucose and galactose. Glycoproteins are proteins that have carbohydrates attached to them by glycosidic bonds.

• Solubility: Carbohydrates are generally soluble in water because they have many hydroxyl groups that can form hydrogen bonds with water molecules. However, some carbohydrates are less soluble than others, depending on their size, shape, and degree of branching. For example, starch is less soluble than glucose because it has a more complex structure. Cellulose is insoluble in water because it has a linear and tightly packed design that prevents water molecules from penetrating it.

Single sugars (monosaccharides) and polymers, oligosaccharides, and polysaccharides are examples of simple carbohydrates.

Monosaccharides are a type of polysaccharide.

• The most basic group of carbohydrates, also known as simple sugars because they cannot be further hydrolyzed. Colorless, crystalline solids that are soluble in water but not in a non-polar solvent.

• These are compounds that possess a free aldehyde or ketone group.

Cn(H2O)n or CnH2nOn is the general formula. They are classified based on the number of carbon atoms and the functional group present.

• Thus, monosaccharides with 3, 4, 5, 6, 7,... carbons are known as trioses, tetroses, pentoses, hexoses, heptoses, and so on, as well as aldoses or ketoses depending on whether they contain an aldehyde or ketone group.

Glucose, Fructose, Erythrulose, and Ribulose are some examples

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Oligosaccharides

• Oligosaccharides are compound sugars that yield 2 to 10 molecules of the same or different monosaccharides on hydrolysis.

• The monosaccharide units are joined by glycosidic linkage.

• Based on the number of monosaccharide units, it is further classified as a disaccharide, trisaccharide, tetrasaccharide, etc.

• Oligosaccharides yielding 2 molecules of monosaccharides on hydrolysis are known as disaccharides, and the ones producing 3 or 4 monosaccharides are known as trisaccharides and tetrasaccharides, respectively, and so on.

• The general formula of disaccharides is Cn(H2O)n-1 and that of trisaccharides is Cn(H2O)n-2 and so on.

• Examples: Disaccharides include sucrose, lactose, maltose, etc. Trisaccharides are Raffinose and Rabinose.

Polysaccharides

• They are also called "glycans."

• Polysaccharides contain over 10 monosaccharide units and can be hundreds of long sugar units.

• They yield more than 10 molecules of monosaccharides on hydrolysis.

• Polysaccharides differ in the identity of their recurring monosaccharide units, the length of their chains, the types of bond linking units, and the degree of branching.

• They are primarily concerned with two important functions, i.e., Structural functions and energy storage.

• They are further classified depending on the type of molecules produced due to hydrolysis.

• They may be homopolysaccharidese containing monosaccharides of the same type, or heteropolysaccharides, i.e., monosaccharides of different types.

• Examples of Homopolysaccharides are starch, glycogen, cellulose, and pectin. Heteropolysaccharides are Hyaluronic acid, Chondroitin.

Carbohydrates are widely distributed molecules in plant and animal tissues. They perform various biological functions that are essential for life. Some of their major parts include:

• Energy source and storage: Carbohydrates are the most abundant dietary source of energy (4 kcal/gram) for all living beings. They are broken down by cellular respiration to produce ATP, the universal energy currency of cells. Glucose is the most common monosaccharide that fuels cellular activities. Carbohydrates can also be stored as glycogen in animals and starch in plants for later use. Stored carbohydrates act as an energy reserve and prevent extra proteins from being used as energy sources.

• Structural and protective components: Carbohydrates form the structural framework of many biological molecules, such as nucleic acids (DNA and RNA), glycoproteins, glycolipids, and peptidoglycans. These molecules are involved in genetic information storage, cell recognition, cell signaling, and immune responses. Carbohydrates also form the main component of the cell wall in plants (cellulose), bacteria (peptidoglycan), and fungi (chitin). The cell wall provides mechanical strength, rigidity, and protection to the cells.

• Metabolic intermediates: Carbohydrates serve as precursors for synthesizing other biomolecules, such as amino acids, fatty acids, nucleotides, and coenzymes. For example, glucose can be converted to ribose-5-phosphate, which makes nucleotides. Carbohydrates can also be derived from other biomolecules, such as glycerol and amino acids, through gluconeogenesis.

• Regulation of nerve tissue and brain function: Carbohydrates play a vital role in the functioning of the nervous system and the brain. The brain relies on glucose as its primary energy source and consumes about 120 g of glucose per day. Glucose is transported across the blood-brain barrier by special glucose transporters. A constant supply of glucose is essential for maintaining normal brain activity, cognition, memory, and mood. Carbohydrates also modulate the activity of neurotransmitters, such as serotonin and dopamine, which affect mood and behavior.

• Fiber and digestion: Carbohydrates rich in fiber content helps prevent constipation and promote bowel health. Fiber is a carbohydrate that cannot be digested by human enzymes but can be fermented by gut bacteria. Fiber adds bulk to the stool and stimulates peristalsis, the movement of the intestinal muscles. Fiber also helps to lower cholesterol levels, regulate blood sugar levels, and prevent colon cancer.

Carbohydrates are indispensable molecules that perform various functions in living organisms. They are involved in energy production and storage, structural support and protection, metabolic intermediation, nerve tissue and brain regulation, and fiber and digestion. A balanced intake of carbohydrates is important for maintaining good health and well-being.

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