Cell Growth
In nutrient-rich conditions, plants synthesize many proteins and lipids to divide and grow quickly. Under nutrient-low conditions, they stop this synthesis and elongate their bodies to digest unnecessary organelles. ~ Japanese botanist Kiminori Toyooka
The cell walls of plants are comprised of cellulose fibers reinforcing a matrix of hemicellulose, along with lignin or pectin in one or more layers, with the relative volume and orientation of the cellulose fibers varying in each layer.
As a cell grows, the primary cell wall layer is secreted first, comprising cellulose fibers in a matrix of hemicellulose and pectin. Hemicellulose binds to the surface of the cellulose microfibrils, while pectin cross-links the hemicellulose of adjacent microfibrils.
During cell growth, enzymes keep the cell wall pliable. Organelles within a cell proliferate and move about to meet processing demands.
Once growth is complete, cell wall stiffness and strength typically increase. The number and size of organelles decrease.
In some plant materials, notably wood and palms, additional secondary layers are deposited, comprising cellulose fibers in a matrix of hemicellulose and lignin. Cellulose fibers are typically oriented at different angles in each secondary layer.
Layers may differ in thickness. Plants expertly architect cell walls at the molecular level on up for specific application.
Substituting lignin for pectin in secondary layers strengthens and stiffens the cell wall. In mature cells, such as xylem, the protoplast dies, leaving the cell walls to provide mechanical support, while the lumen (empty space) allows water and liquefied nutrient transport. With age, the middle cell wall layers and primary cell wall may become lignified.
