Carbohydrates

 

Carbohydrates

                                                                                                            Anup Bajracharya

Carbohydrates are organic compounds generally made up of carbon hydrogen and oxygen in the ratio 1:2:1, which could be represented by the general formula, Cn(H₂O)n.  It is better defined as polyhydroxy aldehydes or ketones and their derivatives or as substances that yield one of these compounds on hydrolysis. Examples: Glucose (C₆H₁₂O₆).

Classification of Carbohydrates

Carbohydrates are classified into three main groups—monosaccharides, oligosaccharides, and polysaccharides.

Monosaccharides (Simple Sugars)

Monosaccharides are the simplest form of carbohydrates and cannot be hydrolyzed into smaller sugar units. They are the basic building blocks of more complex carbohydrates.

Properties:

• Soluble in water
• Sweet in taste
• Crystalline in nature
• Reducing sugars (can reduce Fehling’s or Benedict’s reagent)
• Contain a free aldehyde (-CHO) or ketone (C=O) group

Classification based on number of carbon atoms:

• Trioses (3C): e.g., Glyceraldehyde, Dihydroxyacetone
• Tetroses (4C): e.g., Erythrose
• Pentoses (5C): e.g., Ribose (in RNA), Deoxyribose (in DNA), Xylose
• Hexoses (6C): e.g., Glucose, Fructose, Galactose, Mannose
• Heptoses (7C): e.g., Sedoheptulose

Classification based on functional groups:

Monosaccharides can be classified based on the presence of an aldehyde or ketone functional group.

•    Aldoses: Monosaccharides with an aldehyde group (-CHO) as their terminal functional group. Examples include glyceraldehyde and glucose.

•     Ketoses: Monosaccharides with a ketone group (C=O) within the carbon chain. Examples include dihydroxyacetone and fructose.

Examples and Functions:

• Glucose is the main energy source for cells.
• Fructose is found in fruits and honey.
• Ribose forms the sugar part of RNA, and deoxyribose is found in DNA.

Oligosaccharides (Short-chain Sugars)

Oligosaccharides are carbohydrates composed of 2 to 10 monosaccharide units joined by glycosidic bonds.

Properties:

• Slightly soluble in water
• Less sweet compared to monosaccharides
• May or may not be reducing sugars
• Can be broken down into monosaccharide units by hydrolysis

Subtypes:

a) Disaccharides (2 monosaccharides): Disaccharides consist of two monosaccharide units joined together by a glycosidic linkage formed during a condensation reaction.

•        Common disaccharides include:

• Sucrose (Glucose + Fructose) – Non-reducing sugar, found in cane sugar
• Lactose (Glucose + Galactose) – Reducing sugar, found in milk
• Maltose (Glucose + Glucose) – Reducing sugar, found in germinating grains

b) Trisaccharides (3 monosaccharides):

• Examples:
• Raffinose (Galactose + Glucose + Fructose) – Found in beans and sugar beet

c) Tetrasaccharides and higher:

• Rare, often occur in nature in trace amounts
• Example: Stachyose (found in legumes)

Polysaccharides (Complex Carbohydrates)

Polysaccharides are complex carbohydrates composed of long chains of more than 10 monosaccharide units. The prefix "poly-" refers to "many" or "multiple." When applied to carbohydrates, it indicates the presence of multiple monosaccharide units linked together to form a larger molecule.

Properties:

• Generally tasteless
• Insoluble or only slightly soluble in water
• Non-reducing in nature (no free aldehyde or ketone group)
• Do not form crystals
• Can be hydrolyzed into simpler sugars

Polysaccharides are further classified as

Homopolysaccharides (Homoglycans)- are complex carbohydrates that are made up of only one type of monosaccharide unit repeated many times. These sugars are linked together by glycosidic bonds and serve structural or storage functions depending on their arrangement.

Properties:

• Composed of a single type of sugar unit (e.g., only glucose or only fructose).
• Can be linear or branched in structure.
• May be digestible (like starch) or indigestible (like cellulose) in humans.
• Function as energy reserves or structural components.

Examples:

• Starch – A storage polysaccharide in plants made entirely of glucose units (amylose and amylopectin).
• Glycogen – The storage form of glucose in animals; highly branched, provide energy storage.
• Cellulose – A structural polysaccharide found in plant cell walls; made of β-glucose units, acts as dietary fiber in humans.
• Inulin – Found in plants like chicory and dahlia; made of fructose units.

Heteropolysaccharides (Heteroglycans)- are composed of two or more different kinds of monosaccharide units or their derivatives. These are often complex and are involved in structural or protective roles in organisms.

Properties:

• Contain mixed sugar units (e.g., glucose, galactose, uronic acids, amino sugars).
• Often unbranched or slightly branched.
• Found in extracellular matrix, connective tissue, and mucus.
• Typically not used for energy but for support and biological functions.

Examples:

• Hyaluronic acid – Composed of alternating units of glucuronic acid and N-acetylglucosamine; found in synovial fluid and eye vitreous humor.
• Chondroitin sulfate – Found in cartilage, tendons, and ligaments.
• Heparin – A natural anticoagulant found in mast cells and blood,prevents blood clotting
• Agar and agarose – Derived from red algae, used in microbiological media.

Functions of Carbohydrates

1. Source of energy: The main function of the carbohydrate is to give energy. 1 gm of carbohydrate provides 4 kcals of energy, which is essential for daily metabolic activities and physical exertion.
2. Storage form of energy: Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body. In addition to providing immediate energy, carbohydrates also act as a storage form of energy. In animals and humans, excess glucose is stored as glycogen in the liver and muscles, which can be rapidly mobilized to meet energy demands during fasting or vigorous activity.
3. Structural components: Carbohydrates also serve as important structural components in various organisms. In plants, the cell wall is made up of cellulose, bacterial cell wall is made up of peptidoglycan, a carbohydrate-protein complex, fungal cell wall composed of chitin. Certain carbohydrates contribute to the exoskeletons of insects.
4. Oxidation of fat : They are essential for the oxidation of fat. It is called fats burn in the flame of carbohydrates. Without adequate carbohydrates, fat breakdown results in the formation of ketone bodies, which may lead to a condition called ketosis.
5. Raw materials: Carbohydrates also find wide application as raw materials in various industries. For example, starch and cellulose are used in the production of paper, textiles, and adhesives, while sugars are fermented to produce alcoholic beverages in the brewing industry.
6. Immunity: Antibodies, the important component to fight against disease causing agents (antigens) are made up of carbohydrate, they are glycoproteins.
7. Functioning of brain: The brain and other parts of central nervous system are dependent on glucose for energy. Prolonged hypoglycemia may lead to irreversible brain damage.
8. Mucopolysaccharides in Connective Tissues: Carbohydrates form the bulk of mucopolysaccharides (glycosaminoglycans) such as hyaluronic acid, chondroitin sulfate, and heparin. These are important for lubricating joints (as in synovial fluid), providing elasticity to tissues and anticoagulation (heparin prevents blood clotting)
9. Formation of Nucleic Acids: Carbohydrates form the sugar backbone of nucleotides, which are the building blocks of DNA and RNA. DNA contains the sugar deoxyribose, while RNA contains ribose. Without these sugars, the structure of genetic material would not exist.
10.  Detoxification: Carbohydrates are involved in detoxifying drugs and toxins in the liver. The liver attaches glucose derivatives such as glucuronic acid to toxic substances (in a process called glucuronidation) to make them water-soluble and easier to excrete through urine or bile. Example: Bilirubin, a breakdown product of hemoglobin, is made water-soluble by conjugation with glucuronic acid before being excreted in bile.

            Reference

            Satyanarayana, U., & Chakrapani, U. (2017). Biochemistry (5th ed.). Elsevier Health Sciences.