Plants, fungi, bacteria, archaea, and many algae all have cell walls, while animal cells do not. Each group builds its wall from different materials and for somewhat different purposes, but the underlying job is broadly the same: the wall wraps the outside of the cell, holds it in shape, and protects it from mechanical and environmental stress. The diversity of wall chemistry across these groups is striking, and it affects everything from how antibiotics work to how your gut digests a salad.
Plant Cell Walls
Plant cells are probably the most familiar example of walled cells. Every plant cell is encased in a rigid wall that determines its size and shape and that the cell must actively work against in order to grow.1PubMed Central. Revisiting the relationship between turgor pressure and plant cell growth The primary cell wall, present in young, growing cells, is built mainly from polysaccharides: cellulose, hemicellulose, and pectins. These components are not just thrown together randomly. Cellulose forms long, stiff microfibrils that act like reinforcing cables. Pectins fill the spaces between those cables, and research using high-resolution imaging techniques has shown that roughly a quarter to half of cellulose chains are in direct physical contact with pectin molecules.2PubMed. Pectin-cellulose interactions in the Arabidopsis primary cell wall from two-dimensional magic-angle-spinning solid-state nuclear magnetic resonance Strong interactions between pectin and cellulose have also been observed in the cell walls of carrots, tomatoes, and strawberries, pointing to these bonds as a widespread structural feature across plant species.3PubMed. Interactions between pectin and cellulose in primary plant cell walls
Many plant cells also develop a secondary wall once they stop growing. This thicker, stronger layer forms inside the primary wall and is found in specialized cells like the water-conducting tubes (tracheary elements) and structural fibers that give wood its rigidity. Secondary walls contain cellulose plus xylan, glucomannan, and lignin, a tough phenolic polymer that makes the wall extremely stiff and waterproof.4PubMed. Secondary cell wall biosynthesis Interestingly, the deposition of xylan and lignin into secondary walls proceeds on its own schedule and does not depend on cellulose being laid down first, suggesting that the cell orchestrates each component through separate regulatory pathways.5PubMed Central. Patterned Deposition of Xylan and Lignin is Independent from that of the Secondary Wall Cellulose of Arabidopsis Xylem Vessels
Fungal Cell Walls
Fungi have cell walls too, but their walls are built from an entirely different set of materials. Where plants rely on cellulose, fungi rely on chitin, the same polymer found in insect exoskeletons and crustacean shells. Alongside chitin, the fungal wall contains glucans (chains of glucose molecules) and glycoproteins.6PubMed Central. The Fungal Cell Wall: Candida, Cryptococcus, and Aspergillus Species This combination gives the wall mechanical strength and flexibility, and it also serves as the cell’s interface with its environment. In pathogenic fungi like Candida (the organism behind most yeast infections), the wall’s outer glycoprotein layer is what the human immune system first encounters, making it medically relevant as both a shield and a target.
Because the fungal wall is so chemically distinct from animal cells, it has become a major target for antifungal drugs. Echinocandins, one of the main classes of antifungal medications used in hospitals, work by blocking the enzyme that assembles β-(1,3)-D-glucan, a key structural glucan in the fungal wall. Without it, the wall weakens, osmotic pressure destabilizes the cell, and the fungus either stops growing or dies.7PubMed Central. Echinocandins – structure, mechanism of action and use in antifungal therapy The same logic applies to antibacterial drugs, as we will see below: if a drug can disrupt a wall that your own cells do not have, it can kill the pathogen without harming you.
Bacterial Cell Walls
Bacteria have cell walls made of peptidoglycan, a mesh-like polymer that is unique to bacteria and found in no other domain of life. Peptidoglycan is what gives bacteria their characteristic shapes, whether rod, sphere, or spiral. The thickness of the peptidoglycan layer is the classic dividing line between the two major categories of bacteria. Gram-positive bacteria have a thick peptidoglycan wall, while gram-negative bacteria have a thin one that is sandwiched between two membranes, with the outer membrane carrying lipopolysaccharide.8PubMed Central. The bacterial cell envelope This structural difference is why gram-positive and gram-negative infections often require different antibiotics. Penicillin, the first widely used antibiotic, works precisely by blocking peptidoglycan synthesis. It is far more effective against gram-positives, where the peptidoglycan layer is thick and exposed, than against gram-negatives, which hide their thin peptidoglycan behind the outer membrane.9PubMed Central. Agents Targeting the Bacterial Cell Wall as Tools to Combat Gram-Positive Pathogens
Bacteria That Shed Their Walls
Some bacteria can actually survive without their cell wall, at least temporarily. Under hostile conditions, including exposure to antibiotics that target the wall, certain bacteria shed their peptidoglycan layer and enter what is called the L-form state.10PubMed Central. Bacterial L-forms: Key Mechanisms of Drug Resistance, Disease Recurrence, and Immune Evasion These wall-less bacteria look nothing like their normal selves: they lose their defined shape, becoming blobby and irregular. They can still grow and divide, but they do it by a completely different mechanism, essentially blebbing off bits of membrane rather than splitting down the middle with the usual cell-division machinery.11PubMed Central. Cell wall-deficient, L-form bacteria in the 21st century: a personal perspective The catch is that they need a sheltered, osmotically stable environment to pull this off. Once the hostile conditions pass, L-form bacteria can rebuild their wall and revert to their normal shape. This ability has implications for chronic infections: bacteria that temporarily abandon their walls can evade wall-targeting antibiotics, then bounce back once treatment ends.
Archaeal Cell Walls
Archaea are often lumped with bacteria as “prokaryotes,” but their cell walls are strikingly different. Archaea do not make peptidoglycan. Instead, nearly all known archaea coat themselves in a surface layer, commonly called an S-layer, made of one or two types of protein that self-assemble into a repeating crystalline lattice on the cell surface.12PubMed Central. Archaeal S-Layers: Overview and Current State of the Art Beyond S-layers, different archaeal groups have evolved other envelope strategies. Some methane-producing archaea have a wall polymer called pseudopeptidoglycan (or pseudomurein), which superficially resembles bacterial peptidoglycan but is built from different chemical building blocks. Others have walls rich in polysaccharides or glycoproteins. The diversity is remarkable, and it means that antibiotics like penicillin, which target bacterial peptidoglycan, are completely useless against archaea.
Algal Cell Walls and the Diatom Exception
Algae are a sprawling and diverse group, and their cell walls reflect that diversity. Many green algae have walls similar to those of land plants, with cellulose as the primary structural component. Brown algae use a mix of cellulose and alginic acid. Red algae incorporate agar and carrageenan, polysaccharides familiar from the food industry. But the most unusual algal walls belong to diatoms, single-celled algae found in oceans, lakes, and rivers worldwide.
Diatoms build their cell walls from silica, essentially biological glass. Each species produces a wall with a unique, genetically encoded nanopattern, often spectacularly intricate under a microscope.13PubMed. Diatoms-from cell wall biogenesis to nanotechnology These silicified walls are the strongest known biological material relative to their density, and experiments have shown they serve a defensive function, making diatoms harder for small grazers to crush and eat.14PubMed Central. Silicified cell walls as a defensive trait in diatoms The precision of diatom wall architecture has also attracted interest from nanotechnologists, who see the structures as natural templates for building things like drug-delivery capsules and photonic devices.
Which Cells Lack Cell Walls
Animal cells are the most prominent example of cells that have no wall at all. Instead of a rigid exterior, animal cells use a flexible plasma membrane as their outer boundary. For structural support, they rely on an internal scaffold of protein filaments (the cytoskeleton) and, in tissues, an external network of proteins and carbohydrates called the extracellular matrix. The extracellular matrix contains collagens, laminins, tenascins, and proteoglycans, and it plays active roles in development and disease rather than just being passive scaffolding.15PubMed Central. Functional structure and composition of the extracellular matrix
Some single-celled organisms also lack walls but have come up with creative substitutes. Euglena, for example, has a structure called a pellicle just beneath its plasma membrane, made of interlocking protein strips with associated fibrils and internal membranes.16PubMed Central. THE ULTRASTRUCTURE OF THE PELLICLE COMPLEX OF EUGLENA GRACILIS This pellicle is flexible enough to let the cell change shape as it moves, but sturdy enough to maintain structural integrity. Amoebae take a different approach entirely, relying on their constantly shifting cytoskeleton to manage shape without any wall or pellicle at all.
Why Animal Cells Lost the Wall
The question of why animal cells never evolved a cell wall, or more accurately why their ancestors lost one, has an interesting evolutionary dimension. One hypothesis connects the loss to the acquisition of mitochondria. All bacteria use their cell wall and the space just inside it (the periplasm) for energy production, generating ion gradients across membranes to power the cell. Once an ancestral cell engulfed a bacterium that eventually became the mitochondrion, energy production moved inside the cell. The periplasm was no longer essential for metabolism, which meant the cell wall that bounded it could be discarded.17PubMed. A new aspect to the origin and evolution of eukaryotes Losing the wall opened up new possibilities, including the ability for naked cells to form direct physical connections with one another, a prerequisite for complex multicellularity as we see it in animals.
A complementary model proposes that the ancestral cell, still possessing a wall, began extruding membrane blebs beyond the wall’s surface to exchange materials with ectosymbiotic proto-mitochondria living nearby. Over evolutionary time, these blebs expanded and fused, eventually forming the continuous plasma membrane and internal membrane systems (like the endoplasmic reticulum) that characterize modern eukaryotic cells.18PubMed Central. An inside-out origin for the eukaryotic cell In either scenario, the cell wall went from being essential infrastructure to being something that got in the way of a more flexible, membrane-rich lifestyle.
How Plant Cell Walls Grow
A rigid wall presents an obvious problem: how does a cell encased in it manage to get bigger? Plant cells solve this with a mechanism called acid growth. The hormone auxin triggers proton pumps in the cell membrane to push hydrogen ions into the wall, lowering its pH. In this acidified environment, a family of proteins called expansins become active. Expansins do not cut or digest wall components. Instead, they disrupt the noncovalent bonds between cellulose microfibrils and the surrounding matrix polysaccharides, allowing the wall to yield to the internal turgor pressure pushing outward.19PubMed. Plant Cell Wall Loosening by Expansins The cell essentially stretches its wall from the inside, then fills in the thinned-out areas with new wall material.
This process is under tight hormonal control. Auxin promotes wall loosening and growth, while other signals can inhibit it, allowing the plant to direct growth toward light, around obstacles, or in response to gravity.20PubMed Central. The Role of Auxin in Cell Wall Expansion The discovery of expansins overturned older assumptions that wall growth was purely a matter of enzymatic degradation. Instead, it is a surprisingly delicate mechanical process: loosening bonds just enough to permit stretching without losing structural integrity.21PubMed. Assembly and enlargement of the primary cell wall in plants
Communicating Through the Wall
Having a wall between every pair of cells might seem like a recipe for isolation, but plant cells are far from disconnected. They are linked by tiny channels called plasmodesmata that punch through the wall and connect the cytoplasm of neighboring cells directly. Plasmodesmata are lined by the plasma membrane and threaded through the center by a thin tube of endoplasmic reticulum called the desmotubule. Most molecular traffic travels through the narrow cytoplasmic sleeve that sits between the desmotubule and the outer membrane of the channel.22PubMed Central. Communicating Across Cell Walls: Structure, Evolution, and Regulation of Plasmodesmatal Transport in Plants
Plasmodesmata allow the movement of sugars, signaling molecules, proteins, and even some RNA between cells, effectively turning a plant tissue into a connected network rather than a collection of walled-off individuals.23PubMed. Plasmodesmata and intercellular molecular traffic control Plants can regulate these channels, widening or narrowing them in response to developmental signals or threats. When a pathogen attacks, for instance, a plant may close plasmodesmata near the infection site to prevent the invader from spreading through the channel network. This dynamic regulation means the cell wall is not a sealed barrier but something closer to a selectively permeable fence.
Cell Walls as Battlegrounds
The cell wall is the first thing a pathogen encounters when it attacks a plant, and both sides have evolved sophisticated strategies around it. Pathogenic microorganisms deploy an arsenal of wall-degrading enzymes to break through. Endo-polygalacturonases chew through pectin, xylanases attack hemicellulose, and cellulases digest cellulose itself.24PubMed Central. Host Cell Wall Damage during Pathogen Infection: Mechanisms of Perception and Role in Plant-Pathogen Interactions Research on the wheat pathogen Fusarium graminearum showed that individually knocking out the genes for its major polygalacturonase or xylanase regulator had modest effects on virulence, but eliminating both at once significantly reduced the fungus’s ability to cause disease on both wheat and soybean. The enzymes work synergistically: xylanases loosen the hemicellulose matrix in ways that let polygalacturonases access and release more pectin fragments, amplifying the damage.25PubMed. Synergistic Effect of Different Plant Cell Wall-Degrading Enzymes Is Important for Virulence of Fusarium graminearum
Plants, for their part, do not passively accept being chewed through. When wall-degrading enzymes release fragments of cell wall polysaccharides, those fragments act as danger signals. These damage-associated molecular patterns are detected by receptors on neighboring cells, triggering immune responses that include strengthening the wall at the infection site, producing antimicrobial compounds, and sometimes deliberately killing infected cells to contain the spread.26PubMed Central. Damage-Associated Molecular Pattern-Triggered Immunity in Plants The wall is not just armor. It is an active participant in the immune system, a sensory organ of sorts that detects damage and broadcasts alarms.
Cell Walls and Medicine
The uniqueness of microbial cell walls has been a gift to medicine. Because human cells lack walls entirely, any drug that disrupts wall synthesis or integrity can theoretically kill the pathogen without poisoning the patient. Penicillin and its descendants (the beta-lactam antibiotics) exploit exactly this principle by interfering with the enzymes that cross-link peptidoglycan in bacteria.9PubMed Central. Agents Targeting the Bacterial Cell Wall as Tools to Combat Gram-Positive Pathogens The equivalent strategy in fungi involves the echinocandin drugs, which block glucan synthesis in the fungal wall.7PubMed Central. Echinocandins – structure, mechanism of action and use in antifungal therapy
Antibiotic resistance is now complicating this picture. As bacteria evolve ways to protect or modify their peptidoglycan, and as some retreat into the wall-less L-form state to dodge wall-targeting drugs altogether, researchers are looking for new vulnerabilities. One active area involves targeting the enzymes that transport wall precursors across the cell membrane rather than the final assembly steps. Another involves combining wall-targeting drugs with agents that disrupt the bacterial membrane, making it harder for the bacterium to survive even if it sheds its wall.
Cell Walls and Human Digestion
When you eat fruits, vegetables, and grains, you are eating plant cell walls. Your own digestive enzymes cannot break down cellulose, hemicellulose, or most pectins. This is what we call dietary fiber. Instead, the trillions of bacteria living in your large intestine handle the job. Symbiotic gut bacteria have evolved specialized enzymes to digest plant cell wall polysaccharides, fermenting them into short-chain fatty acids that your intestinal lining can absorb and use for energy.27Journal of Biological Chemistry. Recognition and Degradation of Plant Cell Wall Polysaccharides by Two Human Gut Symbionts These short-chain fatty acids, including butyrate, propionate, and acetate, do more than provide calories. Butyrate in particular is a major fuel source for the cells lining the colon and plays a role in reducing inflammation.
This means the plant cell wall, far from being nutritional dead weight, is a key input for maintaining a healthy gut microbiome. Diets low in fiber starve these bacterial communities, reducing the production of short-chain fatty acids and shifting the microbial population in ways that have been associated with increased inflammation and metabolic problems. From this angle, the cell walls of the plants you eat are not just something your body tolerates but something your gut ecosystem depends on.