An introduction to Starch and Cellulose
Polysaccharides are the most abundantly available in nature among carbohydrates and perform a variety of functions, such as energy storage or as components of plant cell walls. Polysaccharides are very large polymers made up of tens to thousands of monosaccharides, linked by glycosidic linkages. Common polysaccharides: Starch, glycogen, and cellulose.
Heteropolymers can include, in addition to monosaccharides, sugars, amino sugars, or non-carbohydrate substances. Heteropolymers are common in nature and are non reducing carbohydrates (with no sweet taste).
Starch
Starch is a glucose polymer in which all repeat units are directed in one direction and connected by alpha bonds. Starch is edible and can be eaten safely by humans as we have the enzymes which can break it down into glucose.
It is a grain, with the main source for starch being potatoes, wheat, corn, and rice. These carbohydrates are broken down into glucose and used as a source of energy and a metabolism aid when reaching our bodies.
Starch is less crystalline and is weaker than cellulose. Despite the fact that raw starch is not soluble in cold water, it can be dissolved in warm water and can be used in different ways.
It's most commonly used as food but has other uses as well. It can be used as glue, for tightening up clothes and weaving linen, for thickening sauces and for paper treatment.
Cellulose
Cellulose is a glucose polymer whose units can be rotated around the axis of a backbone of glucose unit polymer chains, and are connected by beta links. It is the most natural organic compound and is a fundamental component of plant cells.
While humans can easily consume any type of cellulose, such as the outer shells of corn, the only animals that can digest cellulose are termites and cud-chewing goats, deer, or buffalo because they have some enzymes that can break down cellulose into glucose.
It has many commercial uses and is the main component of the paper and the fibre used to make clothes such as linen and cotton. Cellophane and rayon are also made from cellulose through dissolving the pulp by viscose, causing it to degrade and then dissolve into cellulose xanthate in caustic soda.
Structure of Starch
Starch is made from a chain of α-glucose monomers. The glycosidic bonds are present in its linkage which is formed through the reaction of condensation. Water is released in this reaction. Mainly starch is made up of sugar glucose. Glucose is a molecule made up of carbon (C), hydrogen (H), and oxygen (O) whose basic chemical formula of C6H12O6. An α- glucose monomer structure is drawn below.
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Structure of Cellulose
The successive unit of glucose monomer present in the structure of starch is rotated 180 o around the polymer backbone chain’s axis and cellulose structure is obtained. At high temperatures, cellulose can be broken down into glucose (C6H12O6) by treating with concentrated minerals acids. (C6H10O5)n is the general formula used for cellulose.
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Structural Difference Between Starch and Cellulose
Starch consists of two ingredients-amylose and amylopectin. Amylose is a long linear chain of -D-(+)-glucose units joined by glycosidic association C1-C4 (along-link).
Amylopectin is a branched-chain polymer consisting of ‐D-glucose units in which the chain is formed by glycosidic connection C1-C4 and glycosidic connection C1-C6 branches.
Cellulose, on the other hand, is a straight-chain polysaccharide with β - D-glucose units joined by glycosidic linkage C1-C4 (β-link).
Starch vs Cellulose
There are some parameters to differentiate between starch and cellulose. The difference between starch and cellulose are given below on the basis of parameters.
Starch and cellulose are made from the same monomer i.e. glucose hence they are very similar polymers. As their monomer is the same so they also have the same glucose-based repeat units. Cellulose is comparatively much stronger than starch. Hence cellulose is used for making fibres, clothes, papers, rope etc. while starch is practically not that useful in making things.
FAQs on Difference Between Starch and Cellulose
1. What are the key stereochemical and structural differences between starch and cellulose that are important for the JEE Advanced 2026 syllabus?
The primary difference lies in the glycosidic linkages between their glucose monomers. Starch is composed of α-glucose units, while cellulose is composed of β-glucose units. This leads to two distinct structures:
- Starch: Features α-1,4 glycosidic bonds (and α-1,6 bonds in amylopectin), causing the polymer chain to form a helical or coiled structure. This shape is ideal for compact energy storage.
- Cellulose: Features β-1,4 glycosidic bonds, which result in long, straight, and unbranched chains. These linear chains can pack closely together, forming strong fibres.
2. From a biochemical perspective, why can humans digest starch but not cellulose?
Human digestive systems produce enzymes, specifically α-amylase, which are stereospecific. The active site of α-amylase is shaped to recognise and hydrolyse the α-1,4 glycosidic bonds present in starch, breaking it down into glucose. However, humans lack the enzyme cellulase, which is required to break the β-1,4 glycosidic bonds of cellulose. Therefore, cellulose passes through the human digestive tract undigested, functioning as dietary fibre.
3. Why is cellulose a significantly stronger structural polymer than starch?
Cellulose's superior strength is a direct result of its linear structure formed by β-1,4 glycosidic linkages. This arrangement allows parallel cellulose chains to align perfectly, facilitating extensive intermolecular and intramolecular hydrogen bonding between the hydroxyl groups on adjacent chains. This network of hydrogen bonds creates a rigid, crystalline, and fibrous structure with high tensile strength, making it ideal for plant cell walls. Starch's helical structure prevents such effective intermolecular bonding, making it less strong and more suitable for energy storage.
4. For competitive exams like JEE, how should one differentiate between starch, cellulose, and glycogen?
While all three are polysaccharides of glucose, their key differences for JEE aspirants are:
- Starch: The energy storage polysaccharide in plants. It is a mixture of linear amylose (α-1,4 links) and branched amylopectin (α-1,4 and α-1,6 links).
- Cellulose: The structural polysaccharide in plants. It is a linear polymer of β-glucose with only β-1,4 links, providing rigidity.
- Glycogen: The energy storage polysaccharide in animals, often called 'animal starch'. Its structure is similar to amylopectin but is more highly branched, allowing for quicker glucose release to meet animal metabolic demands.
5. How do the distinct functional roles of starch and cellulose in plants relate directly to their molecular structures?
The structure of each polysaccharide is perfectly adapted for its function. Starch, with its helical and branched structure, is compact and can be easily hydrolysed by enzymes to release glucose for energy. It serves as a plant's energy reserve. In contrast, cellulose's long, straight chains and extensive hydrogen bonding create strong microfibrils that provide the structural framework and rigidity for plant cell walls, offering physical protection and support.
6. What is the final product upon complete acid-catalysed hydrolysis of both amylopectin and cellulose?
Upon complete acid-catalysed hydrolysis, all glycosidic bonds in both polysaccharides are broken. Since both amylopectin (a component of starch) and cellulose are polymers made exclusively of glucose units, the sole final product for both is D-glucose. This is a key concept testing the fundamental building blocks of these polymers, irrespective of their different linkages or branching.
7. How can you chemically distinguish between a sample of starch and cellulose in a laboratory?
The most common and effective method is the iodine test. When a solution of iodine-potassium iodide (I₂/KI) is added to a sample:
- Starch: The amylose component of starch has a helical structure that traps iodine molecules, forming a characteristic deep blue-black complex.
- Cellulose: Due to its linear, non-helical structure, cellulose cannot trap the iodine molecules in the same way. Therefore, it does not produce a colour change, and the solution remains a yellowish-brown.
