The cell wall is the first structure observed by humans under a microscope. When Robert Hook observed cork slices under his self-made optical microscope in 1665, he saw neatly arranged rooms like a honeycomb. These were actually the xylem cells of plants that had already died. The walls of rooms are the plant cell walls.
They are one of the significant features that distinguish plant cells from animal cells. They are a rigid structure surrounding plant plasma membrane whose main components are polysaccharides: cellulose, hemicellulose, and pectin. Woody plants also have lignin to increase hardness and resist compression.
Primary cell wall, Middle Lamella, secondary cell wall
The earlier synthesized components are farther from the plasma membrane. The earliest form layer is pectin-rich middle lamella that is shared by adjacent cells. When pectin binds to water, it becomes as sticky as glue, so adjacent cells are glued together. The primary cell wall is the second region to be created. Young and some mature plant cells have very thin and flexible primary cell walls. Its shape changes when cell growth. The most notable feature is that cellulose is arranged in a specific direction and regulates cell growth orientation. If plant cell is a cylinder, cellulose is like the rope that binds them. Young plant cell is hard to widen, but it is easy to elongate. The primary cell wall slowly expands to accommodate the gradually growing cells. Polysaccharides is their main component: cellulose (15%~30%), hemicellulose (20%~30%), and pectin (30%~40%).
When some cells mature (such as tissues transporting water and nutrients, or providing support), they develop a secondary cell wall between primary cell wall and plasma membrane to add extra strength and prevent cell from enlarging. Its main components are more cellulose and aromatic hydrocarbon derivatives like lignin. They are divided into multiple layers. The cellulose in each layer is arranged in a different direction to hinder cell growth. Very thick secondary cell walls store considerable solar energy and carbon fixed by plant. Most of mass is concentrated here. Plants on Earth synthesize 100 billion tons of cellulose and 20 billion tons of lignin annually.
Components and structure of the cell wall
Cellulose
No matter what kind of cell wall, cellulose is the core component and structure. Hundreds to thousands of glucose are linked together by β-1,4-glycosidic bonds to form a long, unbranched, straight chain. Each chain contains many hydroxyl groups that are key to hydrogen bond formation. About a dozen cellulose chains are connected through these hydrogen bonds and arranged in parallel to form microfibrils whose length is about several micrometers and tensile strength is comparable to steel. They are arranged so orderly and tightly that they form crystalline regions. Multiple cellulose microfibrils aggregate together to form thicker macrofibrils.
Hemicellulose and pectin
Hemicellulose is a polysaccharide polymerized from various monosaccharides (xylose, glucose, mannose, etc.). They are a type of linear polysaccharide, but the main chain is shorter than cellulose and has some little side chains. The gaps between cellulose microfibrils are filled by them, and adjacent microfibrils are linked by them via hydrogen bonds. The mechanical strength and rigidity of cell wall are enhanced by them.
Pectin is a polysaccharide composed of galacturonic acid and other monosaccharides. They are negatively charged and branched, which is similar to the glycosaminoglycan in animal. Therefore, water is absorbed by them to form hydrated gels. Pectin is abundant in middle lamella and primary cell wall. The main function is to promote cell adhesion and fill gaps between microfibrils. Microfibrils are embedded in the hydrated gel and interact with pectin through hydrogen bonds. This not only makes cell wall resilient but also resists external pressure.
Lignin is aromatic compound
Lignin replaces pectin to fill the gaps between celluloses in secondary cell walls where pectin is almost absent. Benzene ring is located at the core. Their side chains linked covalently to create a three-dimensional network that greatly enhances strength and toughness. Therefore, in macroscopic world, grass is soft and wood is very hard to resist compression and bending. Even the entire tree can be supported by them..
The stable aromatic π-system makes lignin resistant to chemical and biological degradation, including pathogens and extreme climates. This is why wood takes years or even decades to degrade completely in nature.
In woody plants, lignin accounts for 18%-35% of the wood mass. The proportion in softwoods (conifers) reaches to 25-35%. There is 18% to 30% lignin in hardwoods (flowering woody plants). They are harder because the cell structure is more complex, and not solely dependent on lignin. The survival strategy in herbaceous plants is rapid growth and reproduction, rather than investing in long-term substance like lignin. This strategy determines the softness of stem. That is, herbaceous plants also contain lignin, but the amount is significantly lower. The average content is about 10-15%. Bamboo is an exception. Although they categorize as herbs, they live for decades. Therefore, they have stems as hard as wood (15-25%).