What is a second function for carbohydrates?

December 4, 2025 •

3 min read

Beyond their well-known role as a primary energy source, carbohydrates perform several other critical functions in living organisms. The most prominent secondary role is their use as essential building blocks for structural components and important macromolecules like DNA and RNA. This structural capacity is fundamental to the architecture and function of cells in plants, fungi, and animals.

Quick Summary

Carbohydrates function as vital structural components, forming cell walls in plants and exoskeletons in insects. They also construct macromolecules such as DNA and RNA, and create cell surface markers for immune recognition.

Key Points

Structural Component: Beyond energy, a primary function of carbohydrates is building structural components, such as cellulose in plant cell walls and chitin in fungal cell walls and insect exoskeletons.
Macromolecule Synthesis: Carbohydrates are essential building blocks for synthesizing key macromolecules, including the ribose and deoxyribose sugars that form the backbone of RNA and DNA.
Cell Recognition: On the surface of animal cells, carbohydrate chains form the glycocalyx, a molecular signature that enables cell-to-cell recognition, which is critical for immune system function.
Protective Layer: The glycocalyx acts as a protective barrier, shielding the cell membrane from mechanical and chemical damage.
Prebiotic Fiber: Undigestible carbohydrate fiber provides bulk for stool and nourishes beneficial gut bacteria, promoting overall digestive health.
Protein Sparing: Adequate carbohydrate intake ensures the body doesn't need to break down its own proteins for energy, thus 'sparing' protein for crucial functions like tissue repair.

Carbohydrates as Structural Building Blocks

While carbohydrates are most famous for powering cellular processes with glucose, their role as structural building blocks is equally vital for life. This function is evident across different kingdoms of life, from rigid plant structures to the protective coats of animal cells.

Cellulose: The Plant's Structural Backbone

In plants, cellulose is a complex carbohydrate that provides structural support and rigidity to cell walls. Composed of long, unbranched chains of glucose monomers linked by β 1-4 glycosidic bonds, cellulose is the most abundant organic polymer on Earth. This strong, linear arrangement of glucose molecules forms a framework that allows plants to stand upright against gravity. Humans cannot digest cellulose, but it is an important component of dietary fiber, promoting digestive health.

Chitin: The Armor of Arthropods and Fungi

Arthropods, including insects and crustaceans, utilize a polysaccharide called chitin to form their hard, protective exoskeletons. Chitin is a modified sugar polymer, comprised of repeating units of N-acetyl-β-d-glucosamine. This same nitrogen-containing carbohydrate also provides the structural integrity for the cell walls of fungi.

The Glycocalyx: A Protective Cell Coat

In animal cells, carbohydrates form a protective, gel-like outer layer called the glycocalyx. This layer is made of short, branched carbohydrate chains that are covalently linked to proteins (glycoproteins) and lipids (glycolipids) embedded in the cell membrane. The glycocalyx acts as a cellular shield, protecting the membrane from mechanical stress and pathogens. In bacteria, the glycocalyx, in the form of a capsule, offers protection and helps in biofilm formation.

Building Essential Macromolecules

Carbohydrates are also precursors for the synthesis of other critical macromolecules. Glucose is converted into smaller sugar molecules, such as ribose and deoxyribose, which are the fundamental components of RNA and DNA, respectively. This means that without carbohydrates, the genetic material that governs all cellular functions could not be built.

Carbohydrates in Cellular Communication

A lesser-known but equally significant function is the role of carbohydrates in cell-to-cell communication and recognition, which is crucial for immune function. The carbohydrate chains of the glycocalyx act as unique cellular markers or "ID badges". These surface-level carbohydrates allow the immune system to distinguish between the body's own cells and foreign invaders, such as bacteria. This ability is what prevents the immune system from attacking healthy tissue and enables it to mount a targeted response against pathogens. For example, different blood types are determined by specific carbohydrate markers on the surface of red blood cells.

A Comparison of Carbohydrate Functions


Feature	Primary Function: Energy Provision	Secondary Function: Structural Support	Secondary Function: Recognition & Communication
Carbohydrate Type	Simple sugars (glucose, fructose), starches (amylose, amylopectin)	Polysaccharides (cellulose, chitin)	Glycoconjugates (glycoproteins, glycolipids)
Location	Stored in liver/muscle (glycogen), circulates in bloodstream	Plant cell walls, fungal cell walls, insect exoskeletons	Extracellular surface of animal cell membranes
Mechanism	Breakdown of glucose via cellular respiration to produce ATP	Monomers linked into long, rigid or protective chains	Unique carbohydrate chains act as surface markers
Biological Role	Fuel for all cellular activity, especially brain and muscles	Provides rigidity, strength, and protection	Facilitates immune response, cell adhesion, and blood typing

Conclusion

While energy provision is the most immediate and well-understood function of carbohydrates, their role in structural support and cellular communication is equally fundamental to biology. These complex sugars construct everything from the robust cell walls of plants to the protective exoskeletons of insects. In animal cells, they form the glycocalyx, an essential component for recognizing 'self' versus 'non-self,' a cornerstone of our immune system. Without this second, less-celebrated function, the diverse and complex structures of life could not exist. The structural and communicative roles of carbohydrates highlight their irreplaceable importance far beyond just fueling our bodies.

The Complexity of Carbohydrate Roles

Carbohydrates' functional diversity stems from their varying structures. Simple sugars like glucose are easily metabolized for energy, but when thousands of these monosaccharides polymerize, they form complex structures like cellulose and chitin with vastly different properties. Furthermore, their ability to combine with other macromolecules like proteins and lipids expands their functionality, allowing them to act as identification markers and communicators on the cell surface. This versatility demonstrates why carbohydrates are an indispensable part of life.

Visit the NCBI Bookshelf to read more about the diverse functions of carbohydrates in the body.

Frequently Asked Questions

The most important second function is providing structural support. Carbohydrates, in the form of cellulose, are the main component of plant cell walls, and chitin builds the exoskeletons of arthropods and fungal cell walls.

Carbohydrate chains on the surface of animal cells form a layer called the glycocalyx. These chains act as markers that allow the immune system to recognize healthy body cells and identify foreign invaders like bacteria.

No, the function depends on the carbohydrate type. Simple carbohydrates like glucose are primarily for energy, while complex, non-digestible polysaccharides like cellulose and chitin are mainly for structural purposes.

Yes, carbohydrates are directly involved in the synthesis of nucleic acids. The sugar components, ribose and deoxyribose, which are derived from glucose, are essential building blocks of RNA and DNA.

Dietary fiber, which is a non-digestible carbohydrate, plays a vital role in digestion by promoting regular bowel movements. Soluble fiber also helps regulate cholesterol and blood sugar levels, while insoluble fiber adds bulk to stool.

Beyond fueling the body, adequate carbohydrate intake 'spares' protein from being used for energy. This allows the body to use protein for its more critical functions, such as building and repairing tissues.

Cellulose, found in the cell walls of plants, and chitin, which makes up the exoskeletons of insects and cell walls of fungi, are two prime examples of carbohydrates with a structural function.