Introduction
Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, generally following the formula Cn(H2O)n. They include a diverse range of molecules from simple sugars to complex polymers.Carbohydrates are classified into monosaccharides, disaccharides, polysaccharides, and glycoconjugates.
Functions of carbohydrates:
1. Primary source of energy
2. Structural components
3. Storage molecules
4. Cell recognition
5. Components of DNA and RNA
Types of Carbohydrates:
A. Monosaccharides
Monosaccharides are the simplest units of carbohydrates, often referred to as simple sugars. They are the basic building blocks from which all more complex carbohydrates—such as disaccharides and polysaccharides—are formed. Monosaccharides cannot be hydrolyzed further into smaller carbohydrate units, making them the foundational molecules in carbohydrate chemistry and biological metabolism.
Monosaccharides typically follow the formula CₙH₂ₙOₙ, and they contain 3 to 7 carbon atoms. They are sweet in taste, crystalline in nature, and highly soluble in water due to the presence of multiple hydroxyl (–OH) groups.
Classification of Monosaccharides
Monosaccharides are classified based on:
(a) Number of Carbon Atoms (Carbon Chain Length)
This classification gives rise to several groups:
1. Trioses (3 carbon atoms)
Simplest monosaccharides which play important roles in respiration.
Examples:
Glyceraldehyde (an intermediate of glycolysis)
Dihydroxyacetone (DHA)
2. Tetroses (4 carbon atoms)
Less common in nature.
Examples:
Erythrose (involved in the pentose phosphate pathway)
3. Pentoses (5 carbon atoms)
Very significant biologically.
Major examples:
Ribose – component of RNA, ATP, NAD⁺
Deoxyribose – component of DNA (lacks one oxygen at carbon 2)
Xylose, Arabinose – found in plant gums
4. Hexoses (6 carbon atoms)
It is most abundant in nature and primary source of metabolic energy.
Examples:
Glucose – universal energy currency
Fructose – fruit sugar
Galactose – part of lactose (milk sugar)
5. Heptoses (7 carbon atoms)
Rare but biologically important in bacteria.
Example: Sedoheptulose-7-phosphate (PPP pathway)
(b) Functional Group Present (Aldoses vs. Ketoses)
Monosaccharides have either:
1. Aldoses
Contain an aldehyde group (-CHO) at the terminal carbon.
Example structures:
Glucose (Aldohexose)
Galactose (Aldohexose)
Ribose (Aldopentose)
2. Ketoses
Contain a ketone group (>C=O) at the second carbon.
Example structures:
Fructose (Ketohexose)
Dihydroxyacetone (Ketotriose)
Key difference:
Aldoses → have reducing property due to free aldehyde group
Ketoses → can also show reducing behavior after tautomerization
2. Structural Features of Monosaccharides
(a) Linear and Cyclic Forms
Monosaccharides exist in two forms:
Open-chain (linear) form containing the carbonyl group
Cyclic form formed via intramolecular reaction between carbonyl and hydroxyl groups
Hexoses favor cyclic forms:
Pyranose (6-membered ring)
Furanose (5-membered ring)
Example:
Glucose → mostly in pyranose form
Fructose → mostly in furanose form
3. Biochemical Importance of Major Monosaccharides
1. Glucose
Glucose is the body’s main fuel, supplying energy to every cell. It sits at the center of key metabolic pathways like glycolysis, the TCA cycle, and oxidative phosphorylation, and it is the sugar that circulates in our blood to keep vital functions running
2. Fructose
It is found in fruits and honey and metabolized mainly in the liver.
3. Ribose
Ribose is essential component of RNA, ATP, FAD, NAD⁺. It is critical for metabolism and genetic information.
4. Deoxyribose
It is the sugar of DNA backbone. It lacks oxygen at C-2 → increases DNA stability.
B. Disaccharides
Formed by glycosidic bonds between two monosaccharides.
Examples:
Sucrose (Glucose + Fructose)
Lactose (Glucose + Galactose)
Maltose (Glucose + Glucose)
4. Polysaccharides
Polysaccharides are large, complex carbohydrates formed by linking many monosaccharide (simple sugar) units through glycosidic bonds. Because they are long chains—sometimes made up of thousands of sugar molecules—they are not sweet, generally insoluble in water, and serve as major storage and structural components in living organisms.
Polysaccharides can be straight-chained or highly branched, depending on how the sugar units are linked. Their diverse structures allow them to perform a wide range of biological functions.
Types of Polysaccharides
Polysaccharides can be broadly divided into storage, structural, and heteropolysaccharides.
1. Storage Polysaccharides
These polysaccharides store energy that can be released when needed by the organism.
a. Starch (Plant storage polysaccharide)
Found in seeds, roots, and tubers (e.g., potato, rice, wheat). Made up of two components:
Amylose (20–30%): unbranched, helical chain of glucose
Amylopectin (70–80%): highly branched chain of glucose
Starch serves as the main energy reserve in plants. When humans consume starchy foods, enzymes break starch down into glucose for energy.
b. Glycogen (Animal Storage Polysaccharide)
Glycogen is often called animal starch because it serves the same purpose in animals that starch does in plants. Its structure is similar to amylopectin, but it is far more highly branched, with new branches appearing every 8–12 glucose units.
Because of this dense branching, the body can break down glycogen very quickly, making it an ideal emergency fuel. Animals store glycogen mainly in the liver, where it helps regulate blood sugar levels, and in the skeletal muscles, where it provides instant energy during movement and exercise.
2. Structural Polysaccharides
Structural polysaccharides help build and support cells and tissues, giving organisms strength and protection.
a. Cellulose (Plant Structural Polysaccharide)
Cellulose is the most abundant organic molecule on Earth. It is made of glucose molecules linked by β(1→4) bonds, which allow the chains to form long, straight fibers. These fibers give rigidity and strength to plant cell walls.
Humans cannot digest cellulose because we do not have the enzyme cellulase, so it passes through our digestive system as dietary fiber, helping in bowel movement and gut health.
b. Chitin (Animal and Fungal Structural Polysaccharide)
Chitin is the second most abundant polysaccharide after cellulose. It consists of N-acetylglucosamine units joined by β(1→4) bonds, forming a structure that is tough, flexible, and chemically resistant.
Chitin forms the exoskeleton of arthropods such as insects, spiders, and crustaceans, and it also makes up the cell walls of fungi. It provides excellent protection and mechanical support, much like cellulose does in plants.
Heteropolysaccharides are made from two or more different monosaccharides, giving them specialized roles in the body, especially in connective tissues, lubrication, cushioning, and cell communication.
Hyaluronic acid is composed of repeating units of glucuronic acid and N-acetylglucosamine. It is found in places where smooth movement and hydration are essential, such as: synovial fluid of joints, where it acts as a natural lubricant, Vitreous humor of the eye, Skin and connective tissues, where it helps retain moisture. Its ability to hold large amounts of water gives tissues their cushioning, elasticity, and hydration.
Chondroitin sulfate is made up of glucuronic acid and sulfated N-acetylgalactosamine. It is a major component of cartilage, tendons, and ligaments, where it provides elasticity, strength, and shock absorption.
Because of its structural role in cartilage, chondroitin sulfate is commonly used in joint-health supplements to support mobility and reduce wear in aging joints.
.5. Glycoconjugates
Carbohydrates do not exist only as simple sugars or long polysaccharides. In many cases, they attach themselves to proteins or lipids, forming complex biomolecules known as glycoconjugates. These combinations are essential for proper cell functioning, communication, immunity, and maintaining the structural integrity of tissues.
Glycoconjugates are found mainly on the cell membrane, where they act like “ID tags,” helping cells recognize each other and respond to signals in their environment. They play a crucial role in processes like cell recognition, hormone reception, immune response, fertilization, and even pathogen entry.
There are three major types of glycoconjugates:
Glycoproteins are proteins that carry short chains of carbohydrates attached to them. You can imagine the sugar portions as small “decorations’’ added to the protein, yet these decorations are vital for the protein’s function. Glycoproteins are present in the cell membrane and in many important molecules such as antibodies, hormones, and receptors. They help immune cells identify harmful agents, allow hormones to bind to their target cells, and assist cells in recognizing each other. In fact, even blood groups such as A, B, and O are determined by specific glycoproteins on red blood cells. This makes glycoproteins key players in cell communication and identification.
Glycolipids, on the other hand, are lipids that have carbohydrate chains attached to them. These carbohydrate “flags’’ extend outward from the cell membrane and help cells interact with their environment. Glycolipids are especially abundant in the brain and nervous tissue. They help maintain the stability of the cell membrane, protect the cell surface, and contribute to vital recognition processes, including the determination of blood group antigens. By serving as identity markers, glycolipids ensure cells can properly recognize and respond to one another.
Proteoglycans differ from glycoproteins and glycolipids because they contain long, brush-like carbohydrate chains attached to a protein core, making them mostly carbohydrate by weight. These sugar chains attract water and form gel-like structures within tissues. Proteoglycans are found in cartilage, tendons, ligaments, skin, and the extracellular matrix that surrounds cells. Their ability to hold water allows them to provide cushioning, flexibility, and shock absorption—especially in joints. They also support tissue repair, cell adhesion, and hydration, maintaining the structural integrity of various organs and tissues.
In summary, when carbohydrates combine with proteins or lipids, they form powerful biological molecules that support communication, protection, and structural stability in the body. Glycoproteins act as communication signals, glycolipids function like identity tags on the cell surface, and proteoglycans help keep tissues strong, hydrated, and resilient. Their combined roles are essential for healthy cell function and overall biological balance.
FAQs
1. What is the difference between aldoses and ketoses?
Aldoses have an aldehyde group, whereas ketoses have a ketone group.
2. Why can’t humans digest cellulose?
Humans lack the enzyme cellulase needed to break β1→4 linkages.
3. What makes glycogen a better storage molecule than starch?
Glycogen is more branched, allowing rapid release of glucose.
4. Are all disaccharides reducing sugars?
No. Sucrose is a non-reducing sugar.
5. What are glycoconjugates?
Complex molecules containing carbohydrates covalently linked to proteins or lipids.
MCQs (with Answer Key)
1. The most abundant carbohydrate in nature is:
A. Starch
B. Glycogen
C. Cellulose
D. Chitin
Answer: C
2. Lactose is composed of:
A. Glucose + Glucose
B. Glucose + Fructose
C. Glucose + Galactose
D. Fructose + Galactose
Answer: C
3. Glycosidic linkage in sucrose is:
A. α1→4
B. β1→4
C. α1→β2
D. α1→6
Answer: C
4. Chitin contains:
A. Glucose
B. N-acetylglucosamine
C. Fructose
D. Ribose
Answer: B
5. Storage polysaccharide in animals is:
A. Starch
B. Glycogen
C. Cellulose
D. Chitin
Answer: B
Worksheet
Short Answer Questions
1. Define monosaccharides and give two examples.
2. Explain the difference between amylose and amylopectin.
3. What are glycoproteins? Mention their role in immunity.
4. Why is glycogen highly branched?
5. Describe chitin and give its biological significance.
Long Answer Questions
I. Describe the structure, formation, and biological role of disaccharides.
II. Compare and contrast starch, cellulose, and glycogen.
III. Explain glycoconjugates with suitable examples and functions.
IV. Discuss the classification and structural properties of monosaccharides.
References
1. Lehninger Principles of Biochemistry – Nelson & Cox
2. Harper’s Illustrated Biochemistry – Victor W. Rodwell
3. Voet & Voet – Biochemistry
4. Campbell & Reece – Biology
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