Why can the body digest starch but not cellulose?

December 28, 2025 •

4 min read

A compelling biological fact is that while both starch and cellulose are polysaccharides made of glucose, the human body can digest starch but not cellulose due to a critical enzymatic and structural difference in their molecular bonds. This fundamental distinction dictates what serves as a key energy source versus what functions as indigestible dietary fiber in our digestive system.

Quick Summary

Humans can break down starch's alpha-glycosidic bonds using the enzyme amylase, but we lack the necessary cellulase enzyme to hydrolyze cellulose's beta-glycosidic bonds, making it indigestible fiber.

Key Points

Enzyme Specificity: Humans possess the enzyme amylase, which specifically breaks the alpha-glycosidic bonds in starch, but lack the cellulase enzyme needed for cellulose's beta-glycosidic bonds.
Bond Type is Key: Starch is formed with alpha-glycosidic bonds, while cellulose uses beta-glycosidic bonds, and this subtle molecular difference prevents human enzymes from acting on cellulose.
Structural Contrast: Starch has a coiled, helical structure that is easily accessible to enzymes, whereas cellulose's linear, tightly packed structure resists enzymatic breakdown.
Fiber, Not Fuel: Because cellulose is indigestible, it passes through the human digestive tract as dietary fiber, which is essential for gut health but provides no calories.
Not all Animals are Equal: Unlike humans, herbivores like cows and sheep have symbiotic gut bacteria that produce cellulase, allowing them to digest cellulose efficiently.
Two Forms of Glucose: Both starch and cellulose are polymers of glucose, but the orientation of the glucose units (alpha vs. beta) is what causes the difference in their final structure and function.

The Fundamental Differences in Molecular Bonds

At the heart of the matter lies a seemingly small, yet significant, structural disparity between starch and cellulose. Both are homopolymers, meaning they are long chains composed solely of glucose monosaccharides. However, the way these glucose units are linked is profoundly different, determining whether the molecule is a fuel source or a structural component.

Starch's Alpha Bonds: A Digestible Structure

In starch, the glucose units are connected by alpha-1,4-glycosidic bonds, and in some branched areas, alpha-1,6-glycosidic bonds. This arrangement creates a coiled, helical structure that is relatively open and accessible. This open shape allows human digestive enzymes to easily bind to and cleave the bonds, breaking the large polysaccharide into smaller, digestible glucose molecules. Starch serves as the primary energy storage molecule in plants, found in foods like potatoes, rice, and corn.

Cellulose's Beta Bonds: A Rigid, Indigestible Fiber

Conversely, cellulose consists of glucose units joined by beta-1,4-glycosidic bonds. This linkage causes the glucose molecules to be oriented in an alternating pattern, leading to a straight, linear chain. These parallel chains then form strong hydrogen bonds with one another, creating robust microfibrils that give plants their structural rigidity. This tight, rigid structure is impenetrable to the enzymes produced by the human body.

The Enzyme Factor: Amylase vs. Cellulase

Enzymes are highly specific biological catalysts, often described using a "lock-and-key" model where a specific enzyme (the key) fits a particular substrate (the lock). This specificity is the primary reason for the difference in starch and cellulose digestion.

The Human Digestive System and Amylase

Enzyme Name: Amylase, including salivary amylase (in the mouth) and pancreatic amylase (in the small intestine).
Action: Amylase is perfectly shaped to recognize and break the alpha-1,4-glycosidic bonds found in starch.
Process: Digestion begins in the mouth, where salivary amylase starts breaking down starch into smaller sugar units. This process continues in the small intestine, where pancreatic amylase completes the hydrolysis, turning starch into absorbable glucose.

The Missing Enzyme: Cellulase

Enzyme Name: Cellulase.
Action: Cellulase is the specific enzyme required to break the beta-1,4-glycosidic bonds of cellulose.
Absence in Humans: The human body does not produce cellulase. This is why cellulose passes through our digestive tract largely unchanged.

The Indigestible Nature of Cellulose: More Than Just a Lack of Enzymes

While the missing cellulase enzyme is the main reason, the structural properties of cellulose further reinforce its indigestibility. The tightly packed microfibrils formed by the linear chains and hydrogen bonds create a dense material that is not easily accessed by any potential enzymes, even if a cellulase were present. For this reason, animals like cows and termites, which can break down cellulose, rely on symbiotic gut bacteria that produce the necessary enzymes and have specialized, multi-chambered digestive systems to provide enough time for the process to occur.

The Health Benefits of Indigestible Fiber

Despite being indigestible, cellulose is not without its benefits. It is a form of dietary fiber, which is crucial for human digestive health.

Adds Bulk: Cellulose adds bulk to stool, which helps with regular bowel movements and prevents constipation.
Gut Microbiome: It provides food for beneficial bacteria in the large intestine, promoting a healthy gut microbiome.
Nutrient Absorption: By slowing down the absorption of sugars, it can help regulate blood sugar levels.
Colon Health: A high-fiber diet is associated with a reduced risk of certain colon diseases.

Comparison of Starch and Cellulose Digestion


Feature	Starch	Cellulose
Primary Bond Type	Alpha-1,4-glycosidic bonds	Beta-1,4-glycosidic bonds
Molecular Structure	Helical, coiled	Linear, straight chains
Key Enzyme	Amylase (produced by humans)	Cellulase (not produced by humans)
Human Digestibility	Easily digested; provides energy	Indigestible; passes through as fiber
Function in Nature	Energy storage in plants	Structural support in plant cell walls
Role in Human Diet	Source of carbohydrates/energy	Source of dietary fiber/roughage

Conclusion: The Final Verdict on Starch and Cellulose

The core reason why can the body digest starch but not cellulose is the difference in their chemical bonds and the corresponding enzymatic capabilities of the human body. Starch's alpha-glycosidic bonds and helical structure make it a perfect substrate for the human enzyme amylase, providing accessible energy. In stark contrast, cellulose's beta-glycosidic bonds and rigid, linear structure are completely resistant to human enzymes, requiring specialized gut bacteria found in herbivores for breakdown. While it doesn't provide calories, cellulose serves a vital role as dietary fiber, promoting a healthy and efficient digestive system.

For further reading on the biological differences between these carbohydrates, explore additional resources on cellular biology and nutrition. Lesson Summary

Frequently Asked Questions

Starch is digested by the enzyme amylase, which is produced in both the salivary glands and the pancreas. It breaks down the alpha-glycosidic bonds in starch into smaller, absorbable sugar units.

The human body does not produce the enzyme cellulase because our evolutionary path led to a diet that didn't require breaking down vast amounts of plant cell walls for energy, relying instead on more easily digestible carbohydrates and proteins.

When humans eat cellulose, it passes through the digestive tract largely undigested. It functions as dietary fiber, adding bulk to our stool and aiding in bowel movements.

Although we cannot digest cellulose for energy, it is healthy because it acts as dietary fiber. This roughage promotes digestive regularity, supports a healthy gut microbiome, and aids in blood sugar regulation.

No vertebrate animal, including cows and other ruminants, produces its own cellulase. Instead, they house symbiotic bacteria in their specialized gut compartments that produce the necessary enzymes to break down cellulose for them.

Alpha and beta glycosidic bonds refer to the orientation of the linkage between glucose units. In alpha bonds (starch), the bond is in one orientation, creating a coiled structure. In beta bonds (cellulose), the orientation alternates, creating a straight, rigid structure.

Yes, both starch and cellulose are polysaccharides composed entirely of glucose monomers. The difference lies in the type of glycosidic linkage connecting the glucose units, not the basic building block.