What Carbohydrates Are Not Soluble in Water?

December 14, 2025 •

5 min read

Polysaccharides, long chains of monosaccharide units, are generally not soluble in water, a characteristic that contrasts sharply with simple sugars like glucose and fructose. This insolubility is primarily due to the large size and strong internal hydrogen bonding of the molecule, which prevents water molecules from breaking apart the complex structure and forming a solution. This property plays a crucial role in both food structure and biological function.

Quick Summary

This article explains why complex carbohydrates, or polysaccharides like starch and cellulose, are not soluble in water. It covers the structural reasons for their insolubility, their different types, and their important roles in food science and human biology.

Key Points

Polysaccharides are insoluble: Complex carbohydrates like starch, cellulose, and glycogen are generally not soluble in water due to their large size and high molecular weight.
Internal hydrogen bonds: Strong hydrogen bonds within the polysaccharide chains create rigid, compact structures that water molecules cannot easily break apart to achieve dissolution.
Solubility depends on structure: The degree of branching and molecular weight significantly influence solubility, with linear polysaccharides like cellulose being less soluble than branched ones like glycogen.
Starch and gelatinization: Starch is insoluble in cold water but forms a gel when heated, a process called gelatinization, due to its amylose and amylopectin components.
Cellulose is insoluble fiber: Cellulose provides structural support in plants and passes through the human digestive system undigested, acting as essential insoluble dietary fiber.
Glycogen storage: As an animal's energy reserve, glycogen's limited solubility allows it to be stored compactly in the liver and muscles without affecting cellular water balance.
Chitin's protective role: Chitin's insolubility is key to its role as a rigid, structural component of exoskeletons and cell walls.

The Chemical Reasons for Carbohydrate Insolubility

Solubility in water is a fundamental property of carbohydrates that largely depends on their size and molecular structure. Small carbohydrates, known as simple sugars (monosaccharides and disaccharides), have numerous hydroxyl (-OH) groups that readily form hydrogen bonds with water molecules. This interaction allows them to dissolve easily. However, complex carbohydrates, or polysaccharides, are made up of hundreds or even thousands of these sugar units bonded together. The sheer size of these macromolecules, combined with their intricate internal hydrogen bonding, makes them largely or entirely insoluble.

Size and Molecular Structure

The high molecular weight and dense structure of polysaccharides are the primary culprits for their insolubility. The long polymer chains are held together by strong internal forces, including hydrogen bonds between adjacent sugar units. For a substance to dissolve, the solvent (water) must be able to break these intermolecular bonds and surround the individual molecules. For large polysaccharides like cellulose, the crystalline structure formed by tight hydrogen bonding is too strong for water to penetrate effectively. This leads to the carbohydrate being insoluble.

Branching and Molecular Weight

The degree of branching in a polysaccharide also affects its solubility. More highly branched polysaccharides, such as glycogen, are more soluble than linear ones like cellulose, because the branching disrupts the formation of a rigid crystalline structure. However, even branched polysaccharides can have limited solubility due to their massive molecular size. Molecular weight is also a key factor, with solubility decreasing as the molecular weight increases.

Key Types of Insoluble Carbohydrates

Many important biological and dietary carbohydrates fall into the insoluble category. Their unique properties allow them to serve specific functions, from providing structural support in plants to aiding human digestion.

Starch

Starch is a plant's energy storage carbohydrate and is a significant part of the human diet. It is composed of two types of polysaccharides: amylose and amylopectin. While starch is insoluble in cold water, it absorbs water and swells when heated, forming a paste in a process called gelatinization. Amylose is a linear polymer that is only sparingly soluble in hot water, while the highly branched amylopectin component is largely responsible for the gelling property. The inability of starch granules to dissolve in cold water is due to their semicrystalline structure.

Cellulose

Cellulose is the most abundant organic molecule on Earth and is the primary structural component of plant cell walls. It is a linear polymer of D-glucose units linked by β(1→4)-glycosidic bonds. The tight, parallel alignment of these linear chains is stabilized by numerous hydrogen bonds, forming microfibrils with high tensile strength. This crystalline, rigid structure makes cellulose completely insoluble in water and indigestible by humans. It passes through the digestive system as insoluble dietary fiber, promoting healthy bowel function.

Glycogen

Referred to as "animal starch," glycogen is the primary energy reserve in animals, stored mainly in the liver and muscles. It has a similar structure to amylopectin but is even more highly branched, which allows for faster mobilization of glucose. However, in its large, complex form, glycogen is considered insoluble in water, despite its branched nature. The intricate internal hydrogen bonding within the large molecule leaves few polar groups available to interact with water.

Chitin

Chitin is a structural polysaccharide found in the exoskeletons of insects, arthropods, and the cell walls of fungi. It is composed of N-acetyl-D-glucosamine units and is known for its incredible strength and rigidity. Like cellulose, its tightly packed, crystalline structure is held together by strong hydrogen bonds, making it virtually insoluble in water and most other common solvents. Its derivative, chitosan, has higher solubility in acidic environments due to deacetylation.

Comparison of Key Carbohydrates and Their Solubility


Feature	Starch	Cellulose	Glycogen	Chitin
Classification	Storage (plants)	Structural (plants)	Storage (animals)	Structural (fungi, arthropods)
Solubility in Cold Water	Insoluble	Insoluble	Insoluble	Insoluble
Effect of Hot Water	Absorbs water to form a gel/paste	No change	Minimal change (limited solubility)	No change
Chemical Linkages	$\alpha$-1,4 and $\alpha$-1,6 glycosidic bonds	$\beta$-1,4 glycosidic bonds	$\alpha$-1,4 and $\alpha$-1,6 glycosidic bonds	$\beta$-1,4 glycosidic bonds
Molecular Structure	Helical amylose and highly branched amylopectin	Linear, extended rod-like chains	Highly branched chains	Linear, similar to cellulose but with nitrogen
Human Digestibility	Digestible by amylase	Indigestible; acts as fiber	Digestible	Indigestible by humans

Functions and Dietary Significance

The insolubility of these complex carbohydrates is directly linked to their biological functions.

Starch serves as a dense, compact energy store that is not osmotically active within plant cells. This allows plants to store large amounts of glucose without affecting the cell's water balance. For humans, cooking starch helps to disrupt the crystalline structure, making it more digestible and accessible as an energy source.
Cellulose, as insoluble dietary fiber, is crucial for digestive health. It adds bulk to the stool, which aids in promoting regular bowel movements and helps prevent constipation. Its presence in the digestive tract also supports a healthy gut microbiome.
Glycogen's insolubility is also an advantage for energy storage in animals. Like starch, it can be stored compactly in liver and muscle cells without causing adverse osmotic effects. Its branched structure allows for quick glucose release when energy is needed, which is vital for high-energy demands during activity.
Chitin, found in the hard exoskeletons of insects and crustaceans, provides rigid, protective support that is resistant to breakdown. This structural integrity is possible precisely because of its strong, insoluble molecular arrangement.

Conclusion

In summary, while simple sugars are highly soluble in water, complex carbohydrates like polysaccharides are generally not. This key difference is a result of their size, intricate branching, and strong internal hydrogen bonding, which create dense, compact, and often crystalline structures that water cannot easily penetrate. The insolubility of these macromolecules is not a flaw but a feature, allowing them to fulfill vital roles as structural components in plants and animals (cellulose, chitin) and as efficient, non-osmotic energy storage molecules (starch, glycogen). These properties are fundamental to their function in both biological systems and the food we consume.

Explore more about polysaccharides and their functions in living organisms on Wikipedia.

Frequently Asked Questions

The main reason is their complex, large molecular structure, which is held together by strong internal hydrogen bonds. Water molecules cannot easily break these bonds and surround the large polymer, preventing it from dissolving.

Not all complex carbohydrates are completely insoluble. Some, like starches, can be made partially soluble or form gels by heating. The degree of solubility also depends on factors like branching and molecular weight.

Cellulose is made of long, linear glucose chains with strong hydrogen bonding that form a tight, crystalline structure. The human digestive system lacks the enzymes necessary to break these bonds, causing it to pass through as indigestible, insoluble fiber.

Cooking, especially with heat and water, causes starch granules to swell and gelatinize. This process disrupts the internal structure and makes the starch more accessible for digestion, although it still doesn't fully dissolve in the same way as sugar.

Insoluble fiber, such as cellulose, does not dissolve in water and adds bulk to stool to aid in bowel regularity. Soluble fiber, found in oats and beans, dissolves in water to form a gel-like substance and can help lower cholesterol and blood glucose levels.

No, glycogen's branched structure is the key to its function as a quick energy source. Enzymes can rapidly cleave glucose units from the numerous branches, making energy available on demand despite its overall insolubility in its large form.

Chitin is insoluble due to its strong, crystalline structure reinforced by hydrogen bonds. In nature, it provides strong, rigid support for the exoskeletons of insects and crustaceans and is a component of fungal cell walls.