Wikipedia Deep Dive

Cell wall

11 min read

The Invisible Armor That Made Penicillin Possible

Here's a curious fact about the antibiotic that changed medicine: penicillin doesn't actually kill bacteria directly. Instead, it sabotages a construction project. Bacteria are constantly rebuilding their cell walls—the rigid outer layer that keeps them from exploding—and penicillin jams the machinery. Without new wall material, the bacteria literally burst from their own internal pressure.

To understand why this works, we need to understand what cell walls are and why some cells have them while others don't.

Pressure Vessels and Wicker Baskets

Every cell faces a fundamental problem: water wants to rush in. This happens because of osmosis—water naturally flows toward areas with higher concentrations of dissolved molecules, and the inside of a cell is packed with proteins, sugars, and salts. Without some way to resist this inward flow, cells would swell and rupture like an overfilled water balloon.

Animal cells solve this problem chemically, by carefully controlling what's dissolved inside them. But plants, fungi, and bacteria took a different path. They built walls.

A cell wall is a structural layer that wraps around the outside of a cell, just beyond the cell membrane. Think of the membrane as a soap bubble—flexible and fragile—and the wall as a rigid cage surrounding it. The biologist John Howland offered a useful analogy: imagine a wicker basket with a balloon inflated inside it. The balloon pushes outward, the basket pushes back, and the result is a structure far stronger than either component alone.

This is why plants wilt when they don't get enough water. The cell walls haven't gotten weaker—they're just as sturdy as ever. But without enough water to inflate the cells inside them, there's nothing pushing back against the walls, and the whole structure goes floppy. Give the plant water again and the cells reinflate, snapping the stems back to attention.

Why Bacteria Need Walls and You Don't

Your cells don't have walls. Neither do the cells of any other animal. This isn't a limitation—it's a feature.

Walls make cells rigid, which is great if you're a tree that needs to stand upright without a skeleton. But animals move. We bend, we stretch, we flow through narrow spaces. Our cells need to be flexible. They squeeze through capillaries, crawl toward wounds, and reshape themselves constantly. A rigid wall would make all of this impossible.

Animals also evolved internal skeletons—first made of cartilage, then bone—which provide structural support without requiring every individual cell to be reinforced. A tree has no skeleton; it's held up entirely by millions of walled cells stacked together. An oak tree is essentially a tower of tiny pressure vessels.

Bacteria, by contrast, absolutely need their walls. A typical bacterium experiences internal pressure several times greater than atmospheric pressure—comparable to the pressure inside a car tire. Without a wall to contain this pressure, the bacterium would simply pop.

This is exactly what happens when penicillin goes to work.

The Chemistry of Different Walls

Not all cell walls are built the same way. In fact, cell walls evolved independently multiple times across the tree of life, and different groups use completely different materials.

Plants build their walls primarily from cellulose—long chains of sugar molecules linked together into tough fibers. If you've ever eaten celery, you've chewed through plant cell walls. Cellulose is the most abundant organic compound on Earth, and it's what makes wood hard, cotton fluffy, and paper possible. Plant walls also contain hemicelluloses (shorter, branched sugar chains that link the cellulose fibers together) and pectin (a gel-like substance that glues neighboring cells together). Pectin is what makes jam set—when you boil fruit with sugar and acid, you're extracting pectin from the cell walls and causing it to form a gel.

Fungi, despite often being grouped with plants in everyday thinking, build their walls from something completely different: chitin. This is the same material that makes up insect exoskeletons and crab shells. When you eat a mushroom, you're eating fungal cell walls—chitin is tough, which is why mushrooms have that distinctive chewy texture.

Bacteria use yet another material: peptidoglycan, a mesh-like polymer made of sugar chains cross-linked by short amino acid bridges. This is the target that penicillin attacks. The antibiotic mimics one of the building blocks of peptidoglycan, tricking the bacterial enzymes that assemble the wall into incorporating a faulty piece. The result is a wall full of weak spots, and when the bacterium tries to divide, it falls apart.

Here's the crucial point: human cells have no peptidoglycan. We don't have cell walls at all. This means penicillin can destroy bacteria while leaving our cells completely untouched. It's not that antibiotics are universally poisonous and we just take a small enough dose—penicillin is genuinely harmless to human cells because it targets a structure we simply don't possess.

A Wall Made of Glass

Perhaps the strangest cell walls belong to diatoms, a group of single-celled algae that build their walls from silica—essentially, glass.

Diatoms are everywhere. A single liter of seawater might contain millions of them. They're responsible for roughly twenty percent of all photosynthesis on Earth, producing more oxygen than all the rainforests combined. And every single one is encased in an intricate glass shell.

These shells, called frustules, are astonishingly beautiful. Under a microscope, they reveal geometric patterns of holes, ridges, and chambers that look like alien architecture. Victorian scientists were so captivated by them that arranging diatom shells into decorative patterns became a popular hobby—a kind of microscopic flower arranging.

Why glass? Building a silica wall takes only about eight percent of the energy required to build an organic wall of the same size. This energy savings might explain why diatoms grow so quickly—they're not spending their resources on expensive construction materials.

Layers Upon Layers

Plant cell walls aren't simple single-layer structures. Most plant cells actually have two distinct walls, built at different stages of the cell's life.

The primary wall forms first, while the cell is still growing. It's thin and flexible, allowing the cell to expand. Primary walls are held together partly by hydrogen bonds—the same weak attractions that make water molecules stick to each other—and plant cells can actually loosen these bonds to allow growth. They do this by releasing proteins called expansins and acidifying the wall, which weakens the connections between cellulose fibers. The cell inflates with water, the loosened wall stretches, and the cell grows larger.

Once a cell reaches its final size, it may build a secondary wall inside the primary one. Secondary walls are thicker and more rigid. They're often reinforced with lignin, a complex polymer that penetrates the spaces between the cellulose fibers, pushing out water and hardening the structure. Lignin is what makes wood woody—it's the difference between a flexible green stem and a rigid wooden branch.

Some cells take this even further. The conducting tubes in plant stems—the pipes that carry water from roots to leaves—have walls so thick and lignified that the cell inside eventually dies. The dead cell becomes a hollow tube, and water flows through the empty channel. When you look at a piece of wood, you're mostly looking at dead cells—the thick secondary walls remain even after everything else is gone.

Between the Walls

Plant cells don't exist in isolation. They're glued together by a shared layer called the middle lamella, a pectin-rich substance that forms the boundary between adjacent cells. When you bite into a ripe peach and it turns soft and mushy, that's because enzymes have broken down the middle lamella, allowing the cells to slide apart instead of rupturing.

But being stuck together creates a problem: how do plant cells communicate? They can't send signals through the wall itself—it's too dense.

The solution is plasmodesmata, tiny channels that punch through the walls between neighboring cells. Each plasmodesma is a microscopic tube lined with cell membrane, allowing the cytoplasm of adjacent cells to connect directly. Through these channels, cells can share small molecules, signals, and even some proteins. Plant cells aren't isolated boxes—they're connected into a continuous network, with the walls providing structure while the plasmodesmata maintain communication.

Storage Vaults

Cell walls aren't just structural—they can also serve as storage. Some plants pack their cell walls with extra carbohydrates, essentially using the wall as a pantry.

This is particularly important in seeds. The endosperm—the part of a grain that surrounds the embryo—often has walls loaded with digestible polysaccharides. When the seed germinates, enzymes break down these wall components into simple sugars, feeding the growing seedling until it can photosynthesize on its own.

This is also why whole grains are nutritious in ways that refined grains aren't. When you mill wheat into white flour, you remove the outer layers of the grain—including the cell walls of the bran, which are rich in fiber. The fiber in whole grain foods is largely cell wall material.

The History of a Discovery

Robert Hooke first observed cell walls in 1665, though he didn't call them that. Looking at thin slices of cork under his microscope, he saw a honeycomb of tiny compartments. He named them "cells" because they reminded him of the small rooms—called cellulae—where monks lived in monasteries.

What Hooke was actually seeing were the empty walls of dead cork cells. Cork is made from the bark of cork oak trees, and by the time it's harvested, the cells inside have long since died, leaving only their thick, waxy walls behind. Hooke was looking at architecture without inhabitants.

For nearly three centuries after Hooke's observation, cell walls were largely ignored by scientists. They were treated as dead waste products, interesting mainly to industries that used them—papermakers, textile manufacturers, woodworkers. The living contents of cells seemed far more exciting than the "dead excretion products" surrounding them.

It wasn't until the twentieth century that scientists began to appreciate cell walls as dynamic, carefully regulated structures rather than passive husks. The discovery that antibiotics could target cell wall synthesis helped cement this shift in thinking. The wall wasn't dead scaffolding—it was a vital, constantly maintained system, and disrupting it could kill the cell inside.

Evolution's Repeated Invention

Cell walls evolved independently many times. Plants, fungi, and bacteria all have walls, but they didn't inherit them from a common wall-bearing ancestor. Instead, each group invented walls separately, using whatever materials were available.

This is called convergent evolution—when unrelated organisms develop similar solutions to similar problems. Flight evolved separately in birds, bats, and insects. Eyes evolved independently dozens of times. And cell walls evolved multiple times because the problem they solve—resisting osmotic pressure—is universal.

The evidence for independent evolution comes from the walls themselves. If plants and fungi had inherited their walls from a common ancestor, you'd expect their walls to be chemically similar. Instead, they're completely different—cellulose versus chitin, built by different enzymes through different biochemical pathways.

Interestingly, plants may have gotten their wall-building machinery from the bacteria that became their chloroplasts. When ancient cells engulfed photosynthetic cyanobacteria in the symbiotic event that gave rise to all plant life, some bacterial genes transferred into the host cell's genome. The cellulose synthase enzyme in plants appears to have descended from a cyanobacterial ancestor, suggesting that plants inherited their wall-building ability as part of the package deal that came with photosynthesis.

Why This Matters Beyond Penicillin

Understanding cell walls has implications far beyond antibiotics. Plant cell walls are the reason we can build houses from wood, wear clothes made of cotton, and write on paper. They're the source of dietary fiber, which feeds the beneficial bacteria in our intestines. They're the reason vegetables have texture and fruits have structure.

Cell walls also represent one of the great untapped resources in renewable energy. All that cellulose locked up in plant matter could theoretically be broken down into sugars and fermented into biofuels. The challenge is that plant walls evolved specifically to resist being broken down—they're too good at their job. Billions of dollars in research have gone into finding efficient ways to breach these microscopic fortresses and liberate the energy inside.

And in medicine, cell walls remain important targets. Beyond penicillin and its relatives, researchers continue to search for new antibiotics that attack bacterial walls through different mechanisms. As antibiotic resistance spreads, each new angle of attack becomes increasingly valuable.

The cell wall is a structure we rarely think about, precisely because it's so fundamental. Like the walls of a building, it fades into the background—until something goes wrong. When Alexander Fleming noticed that mold juice was dissolving bacteria, he was watching cell walls fail. Every antibiotic that's saved a life since then has been exploiting the same vulnerability: the wall that keeps bacteria alive can also be used to kill them.