Classification of Carbohydrates Biochemistry
Carbohydrates are a group of organic compounds that occur in living tissues and foods in the form of sugars, cellulose, and starch. The general formula of carbohydrates is \[CH_{2}O_{n}\]. Just as in water, the ratio of oxygen and hydrogen is fixed in carbohydrates, which is 2:1. It generally breaks down to release energy in humans and animals. Today, we are going to learn about the classification of carbohydrates and their structures.
Carbohydrates Classification
Given below is the classification of carbohydrates in biochemistry.
Types of Carbohydrates – Simple Carbohydrates
The basic type of carbohydrates is simple carbohydrates that are found in natural sugars present in fruits, vegetables, milk, and honey. These carbohydrates are much simpler to study since they have a less complex structure.
Simple carbohydrates consist of only units of monosaccharides, which is why they are the smallest and simplest of all the other types of carbohydrates. Their smaller size plays a vital role in metabolism and digestion in the gastrointestinal tract.
Types of Carbohydrates – Complex Carbohydrates
Complex carbohydrates are an essential source of energy for our body. They give us the sustained fuel that our body needs for carrying out day-to-day activities, for working out, and for even taking rest. Complex carbohydrates often comprise different units of monosaccharides bound together and provide us with long-lasting energy. The complex carbohydrates are classified depending on their hydrolysis behavior. They are divided into three groups as follows.
Monosaccharides
Disaccharides
Polysaccharides
Monosaccharides
Monosaccharides are carbohydrates that cannot be hydrolyzed further for giving simpler units of either a polyhydroxy aldehyde or a ketone. If a monosaccharide consists of an aldehyde group, it is referred to as aldose and if it consists of a keto group then it is referred to as a ketose.
Disaccharides
After the process of hydrolysis, disaccharides tend to yield either two molecules of the same or the different monosaccharides.
Two units of monosaccharide are joined by an oxide linkage that is formed when there is the loss of water molecule, and this linkage is referred to as glycosidic linkage.
Sucrose is amongst the most common disaccharides that give both glucose and fructose on hydrolysis.
Maltose and lactose, often referred to as milk sugar, are also the two kinds of important disaccharides.
Maltose contains two α-D-glucose whereas lactose consists of two β-D-glucose that are connected through an oxide bond.
Polysaccharides
Polysaccharides consist of longer monosaccharide units that are joined together by glycosidic bonds.
Most of these polysaccharides act as storage for food, such as starch. Starch is known to be an important storage polysaccharide in plants.
Starch is a polymer of α glucose and has two components, that are amylose and amylopectin.
Cellulose is also an essential polysaccharide that is found mostly in plants.
It comprises β-D- glucose units that are joined by a glycosidic bond between the C1 of one glucose unit and the C4 of another glucose unit.
Structure of Carbohydrates
Carbohydrates have traditionally been characterized as compounds having the empirical formula \[Cn(H_{2}O)m\]. Glucose, fructose, and sucrose are popular sugars that suit this formula, however, currently, a carbohydrate is defined as a polyhydroxy aldehyde or polyhydroxy ketone with the traditional formula, a molecule closely similar to it, or oligomers or polymers of such molecules. Because they are water-soluble and difficult to crystallise, they need a different set of abilities to manipulate than traditional "natural products" like terpenes, steroids, and alkaloids.
A "monosaccharide" is a carbohydrate derivative with a single carbon chain; "disaccharide" and "trisaccharide" are compounds with two or three monosaccharide units linked together by acetal or ketal linkages. Larger aggregates with "a few" and "many" monosaccharide units are referred to as "oligosaccharide" and "polysaccharide," respectively. The divide between "few" and "many" appears to be drawn at roughly 10 units in current use.
By the middle of the nineteenth century, chemists in Europe, particularly in Germany, had discovered a variety of relatively pure carbohydrates such as sucrose, cotton cellulose, starch, glucose, fructose, mannose, and lactose. Emil Fischer produced phenylhydrazine for his University of Munich thesis in 1878. He also found in 1884 that carbohydrates produced crystalline phenylosazone when two phenyl hydrazines interacted with the aldehyde group and the carbon next to it.
Structure of Carbohydrates – Glucose
Glucose is amongst the most important monosaccharides. The two commonly used methods to prepare glucose are as follows.
From Sucrose: When sucrose is boiled with dilute acid in an alcohol solution, glucose and fructose are obtained.
From Starch: Glucose can also be obtained by the hydrolysis of starch and boiling it with dilute sulphuric acid, at a temperature of 393K, under high pressure.
Glucose is also known as dextrose and aldohexose and is plentiful on earth.
Structure of Carbohydrates – Fructose
Fructose is an essential ketohexose having a molecular formula \[C_{6}H_{12}O_{6}\]. It consists of a ketone functional group situated at carbon number 2 and contains six carbon atoms in the form of a straight chain. The ring member of fructose is analogous to the compound called Furan and is therefore termed furanose. The cyclic structure of fructose is as follows:
Conclusion
We are all surrounded Carbohydrates are a group of organic compounds that occur in living tissues and foods. The general formula of carbohydrates is \[CH_{2}O_{n}\]. Just as in water, the ratio of oxygen and hydrogen is fixed in carbohydrates, which is 2:1.
FAQs on Structure and Classification of Carbohydrates
1. What are carbohydrates and what is the primary basis for their classification?
Carbohydrates are a major class of biomolecules, chemically defined as polyhydroxy aldehydes or polyhydroxy ketones, or compounds that produce them on hydrolysis. They serve as a primary source of energy and structural components in living organisms. The primary classification is based on their behaviour upon hydrolysis:
- Monosaccharides: The simplest carbohydrates that cannot be hydrolysed into smaller units. Examples include glucose and fructose.
- Oligosaccharides: Carbohydrates that yield 2 to 10 monosaccharide units on hydrolysis. Disaccharides (like sucrose) are the most common.
- Polysaccharides: Complex carbohydrates that yield a large number of monosaccharide units on hydrolysis. Examples include starch and cellulose.
2. How are monosaccharides further classified? Give examples.
Monosaccharides are further classified based on two main criteria:
- Based on the number of carbon atoms: They are named as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), etc. For example, glucose is a hexose.
- Based on the functional group present:
- If a monosaccharide contains an aldehyde group (-CHO), it is known as an aldose. Example: Glucose.
- If it contains a ketone group (C=O), it is known as a ketose. Example: Fructose.
Combining these, glucose is an aldohexose, while fructose is a ketohexose.
3. What is the difference between the structures of glucose and fructose?
Although both glucose and fructose share the same molecular formula, C₆H₁₂O₆, they are structural isomers with key differences:
- Functional Group: Glucose is an aldose, containing an aldehyde group on the first carbon (C-1). Fructose is a ketose, with a ketone group on the second carbon (C-2).
- Ring Structure: In its cyclic form, glucose typically forms a six-membered ring called a pyranose structure. Fructose, on the other hand, forms a five-membered ring known as a furanose structure. This difference in structure is responsible for their different chemical and physical properties.
4. Explain the concept of reducing and non-reducing sugars with examples.
The distinction between reducing and non-reducing sugars is based on their ability to reduce Tollens' or Fehling's reagents.
- Reducing Sugars: These are carbohydrates that have a free aldehyde or ketone group, or exist in equilibrium with a form that has one. This allows them to act as reducing agents. In their cyclic forms, they possess a free hemiacetal or hemiketal group. Most monosaccharides (like glucose, fructose) and some disaccharides (like maltose, lactose) are reducing sugars.
- Non-Reducing Sugars: These sugars do not have a free hemiacetal or hemiketal group because their anomeric carbons are involved in a glycosidic bond. Therefore, they cannot act as reducing agents. The most common example is sucrose, where the anomeric carbons of both glucose and fructose are linked together.
5. What is a glycosidic linkage and how does it lead to the formation of disaccharides and polysaccharides?
A glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which can be another carbohydrate or a different molecule. This bond is formed through a condensation reaction when the hydroxyl group of one sugar's anomeric carbon reacts with a hydroxyl group of another molecule, releasing a water molecule. This linkage is crucial because it is the bond that connects monosaccharide units together to form larger structures:
- Disaccharides: Formed when two monosaccharides are joined by one glycosidic linkage (e.g., sucrose, lactose).
- Polysaccharides: Formed when many monosaccharides are linked by a chain of glycosidic linkages (e.g., starch, cellulose, glycogen).
The orientation (alpha or beta) of the glycosidic bond significantly affects the final structure and properties of the polysaccharide.
6. Compare the structures and functions of starch and cellulose.
Starch and cellulose are both polymers of glucose, but their structural differences lead to vastly different functions.
- Starch: It is the main energy storage polysaccharide in plants. It is a polymer of α-D-glucose. Starch has two components: amylose (a linear chain with α-1,4 glycosidic linkages) and amylopectin (a branched structure with α-1,4 and α-1,6 linkages). This branched, helical structure makes it compact and easily hydrolysed by enzymes for energy release.
- Cellulose: It is the primary structural component of plant cell walls. It is a polymer of β-D-glucose linked by β-1,4 glycosidic bonds. This β-linkage results in long, straight, unbranched chains that pack closely together via hydrogen bonds, forming strong, rigid fibres.
7. Why can humans digest starch but not cellulose, despite both being polymers of glucose?
The ability to digest starch but not cellulose is due to the high specificity of digestive enzymes. Human digestive systems produce enzymes, specifically amylase, which can recognise and break the α-1,4 glycosidic bonds present in starch. However, humans lack the enzyme cellulase, which is required to break down the β-1,4 glycosidic bonds found in cellulose. Because we cannot break cellulose down into its glucose monomers, it passes through our digestive tract undigested, acting as dietary fibre.
8. What are structural carbohydrates and why are they important in nature?
Structural carbohydrates are polysaccharides that provide rigidity and support to the cells and tissues of organisms. Unlike storage polysaccharides (like starch and glycogen), their primary role is not energy but to serve as a building material. The most prominent example is cellulose, which forms the strong, fibrous framework of plant cell walls. Another example is chitin, which forms the exoskeleton of arthropods (like insects and crustaceans) and the cell walls of fungi. Their importance lies in providing the essential structural integrity that allows plants to stand upright and protects many organisms.
