Fungi comprise a hugely diverse kingdom, including mushrooms, molds, and yeasts. At the microscopic level, these organisms display unique cell structures that set them apart. A key difference is that fungal cells contain a tough cell wall exterior. Read on for an in-depth guide to fungal cell walls, their composition, and role in fungus biology.
The Fundamentals of Fungal Cell Structure
Fungi fall under the classification of eukaryotes, meaning their cells contain a defined nucleus and organelles. But beyond standard eukaryotic cell parts, fungal cells also have:
- A rigid cell wall exterior made of chitin and other polysaccharides
- Large vacuoles for food storage and waste
- Glycogen as an energy storage compound rather than starch
- Spores for reproduction rather than sperm and egg cells
These attributes distinguish fungi from plant and animal cells. The cell wall especially impacts the biology of fungi in many ways.
Fungal Cell Wall Composition
The cell walls of fungi contain complex carbohydrate polymers including:
- Chitin – This tough nitrogenous polysaccharide provides structure and rigidity. It is the same material found in arthropod exoskeletons.
- Glucans – Polysaccharides of glucose including cellulose, alpha-glucan, and beta-glucan. The latter has medicinal properties.
- Mannoproteins – Polysaccharides containing mannose involved in immunity and adhesion.
- Chitosan – A structural derivative of chitin.
In addition, fungal cell walls contain proteins and other biomolecules that interact with the surrounding environment. The exact composition varies between fungus phyla and species.
Key Functions of the Fungal Cell Wall
The defining cell wall of fungi serves multiple critical roles:
- Provides structural support and physical protection for the cell
- Maintains cell shape and integrity against internal turgor pressure
- Acts as a selective barrier regulating passage of molecules in and out of the cell
- Stores and provisions polysaccharides as an energy reserve
- Interacts with proteins in signaling pathways and sensory responses
- Its rigidity enables fungi to grow in multicellular filaments called hyphae
Modifications to the cell wall also enable fungi to form reproductive structures like spores.
How Fungal Cell Walls Differ from Plants and Bacteria
Fungal cell walls share some features with plants and bacteria, but have unique attributes:
- Contains chitin like bacterial cell walls rather than just cellulose
- Thicker than plant cell walls and lacks lignin
- Contains alpha- and beta-linked glucans not found in plants or bacteria
- Mannoproteins and mannose polymers are specific to fungal cell walls
While plants also have a protective cellulosic cell wall, the exact composition of fungi allows their cell walls to serve specialized functions.
Cell Wall-Targeting Antifungal Drugs and Resistance
The unique presence and properties of fungal cell walls are exploited in antifungal medications. Many treatments target the synthesis of fungal cell wall components:
| Drug Class | Mode of Action | Example Drugs |
|---|---|---|
| Echinocandins | Inhibit beta-glucan synthesis | Caspofungin, Micafungin |
| Polyenes | Bind ergosterols in cell membrane | Amphotericin B |
| Azoles | Block ergosterol biosynthesis | Fluconazole, Itraconazole |
However, mutations in the fungal cell wall can confer drug resistance. Candida species are developing resistance to echinocandins.
Understanding the fungal cell wall helps develop new therapeutic strategies against fungal infections. The wall provides an Achilles’ heel not shared by human cells.
The Cell Wall and Fungal Virulence
The cell wall directly impacts the infection potential of pathogenic fungi. Virulence factors related to the cell wall include:
- Adhesins – Cell wall proteins that attach to host cells to invade tissue.
- Hydrolytic enzymes – Secreted cell wall enzymes like proteases that break down tissues.
- Antigen masking – Cell wall beta-glucans block immune recognition.
- Chitin and melanin – Impede immune cells and prevent drug entry.
As a key interface between fungus and host, the cell wall plays an integral role in fungal disease.
Conclusion
From providing structure to aiding virulence, the distinct cell wall sets fungi apart from other organisms. The wall’s unique polysaccharides and properties enable fungi to thrive in diverse environments. Fungal cell wall dynamics continue shedding light on the pathology and treatment of infectious fungi. Though simple in structure, this defining fungal feature offers complexity we are just beginning to unravel.
