The cell wall gives cells shape, enables plant growth, prevents bursting from water pressure, keeps out water and pathogens, stores carbohydrates and sends signals to cells. The flexible cell wall surrounds plant cell membranes.
Plant cell walls perform many functions. Their main task is to support proper plant growth. This is accomplished by the cell wall creating a skeleton-like frame that enables plants to grow vertically and develop a rigid stem. Cell walls vary considerably in thickness and organization, which accounts for the wide range of plant shapes and sizes on the planet. They consist of two layers ‰ÛÓ a primary cell wall, which supports the cell as it matures, and a rigid secondary cell wall that appears after the primary wall stops growing. The primary cell wall is thinner and more flexible than the secondary wall. Internally, the primary and secondary walls have a similar physical composition. Over the course of a plant's life, they perform complementary functions to keep the plant healthy and vibrant.
Primary Cell Walls
Primary cell walls are comprised mostly of a complex carbohydrate called cellulose. Cellulose is a complex sugar that provides cells with shape and protection against outside harm including bacteria and dehydration. This cell wall also contains a group of polysaccharides, which breaks down into pectins and cross-linking glycans. Pectins, or pectic saccharides, congeal into a gel-like substance as they bind with neighboring polymers. Pectic saccharides provide cell walls with immunity and protection against environmental harms. They also facilitate cell recognition and enable plant cells to combine with each other. Cross-linking glycans bond with cellulose molecules by forming hydrogen bonds. As they form a network of bonds, cross-linking glycans give strength to the cellulose, which builds cell rigidity and structure. Primary cell walls also contain small amounts of protein, which produces enzymes that help cells grow, break down and change. These enzymes are responsible for common plant behavior, such as changing leaf color in autumn or when under stress.
Secondary Cell Walls
Secondary cell walls, which form inside primary cell walls as plants grow, have a similar composition to primary cell walls. However, they contain additional substances that aid in various plant functions. One such substance is lignin, which is a group of hard polymers. Lignin gives cell walls their rigid shape. It also provides cell walls with an extra layer of defense against bacteria and fungi. Lipids in the secondary cell wall, such as wax and cutin, keep cells from absorbing too much water.
Surrounding the cell wall is a structure called the middle lamella, which is comprised mostly of pectins. The middle lamella acts as a binding agent that connects plant cells to their neighbors. This special glue-like substance has tiny passageways called plasmodesmata, which are essentially channels for inter-cellular communication. Plasmodesmata cross between the inner and outer cell walls, and they also branch out into a network that connects to other cells. Through these passageways, the middle lamella lets cells share vital nutrients and minerals. Collectively, the components of the cell wall work together to give the plant strength and immunity and regulate growth and development. Cell walls are constantly growing and changing to meet the plant's needs as it progresses through its life cycle.