4.15:
Plant Cell Wall
A plant cell has a cell wall, a rigid structure located outside the plasma membrane, which encloses the cell.
Plant cell walls can have up to three different layers—the middle lamella, primary cell wall, and secondary cell wall. Each layer incorporates polysaccharides—such as pectin, hemicellulose, and cellulose—as well as proteins.
The middle lamella is the outermost layer of the cell wall. This layer lies between two adjacent cells, hence the name “middle” lamella. The middle lamella consists of pectin which allows the cells to adhere to one another and form plant tissues.
The primary cell wall forms between the middle lamella and the plasma membrane. As young plant cells divide and grow, they secrete a layer of cellulose microfibers that are embedded in a gel-like matrix consisting of pectin and hemicellulose. This thin and flexible structure allows the plant cell wall to expand during cell growth.
After a plant cell matures and stops growing, it may deposit a secondary cell wall between the primary cell wall and plasma membrane. This thick and rigid secondary cell wall contains cellulose, hemicellulose, and hardening agents—such as lignin—that provide plant cells with protection and support.
Wood and grasses, both develop secondary cell walls that incorporate lignin. However, their cell wall compositions differ to suit their structural needs. Woody tissue cells, unlike grass cells, mostly consist of secondary cell wall layers that provide strength and rigidity.
Even within the same plant, different cell types—such as the vessel elements and tracheids that are responsible for water transport in vascular plants—develop specialized secondary cell walls.
Plant cell walls are not, however, impenetrable barriers. Small holes directly connect the cytoplasm of neighboring cells. These connections permit the exchange of water, nutrients, and other vital molecules.
By and large, the plant cell wall is responsible for the hardy, upright nature of plants such as grasses, shrubs, and trees. Depending upon the species or cell type, the composition of the cell wall is specific to the cell’s structure and function within the plant.
4.15:
Plant Cell Wall
The plant cell wall gives plant cells shape, support, and protection. As a cell matures, its cell wall specializes according to the cell type. For example, the parenchyma cells of leaves possess only a thin, primary cell wall.
Collenchyma and sclerenchyma cells, on the other hand, mainly occur in the outer layers of a plant's stems and leaves. These cells provide the plant with strength and support by either partially thickening their primary cell wall (i.e., collenchyma), or depositing a secondary cell wall (i.e., sclerenchyma). Altogether, the varying cell wall compositions determine the function of specific cells and tissues.
Some plants, such as trees and grasses, deposit a secondary cell wall around mature cells. Secondary cell walls typically contain three distinct layers: the secondary wall layer 1 (S1) to the outside, the secondary wall layer 2 (S2) in the middle, and the innermost secondary wall layer 3 (S3). In each layer, the cellulose microfibrils are organized in different orientations. The S2 layer may make up to 75% of the cell wall.
Regardless of composition, all plant cell walls have small holes, or pits, that allow for the transport of water, nutrients, and other molecules. In a pit, the middle lamella and primary cell wall merely form a thin membrane that separates adjacent cells. Plasmodesmata span the resulting channel and connect the cytoplasm of neighboring cells. The secondary cell wall may be deposited around the pit but not within.
When plants absorb water and nutrients, plant cells store it in the vacuole. As the vacuole expands, it pushes the plasma membrane against the cell wall. This so-called turgor pressure supports the upright and rigid structure of plants. The cell wall, however, prevents the cells from rupturing under this pressure.
In addition to providing structure and support, plant cell walls may also provide plants with nutrient storage. Seeds, for example, may store sugars in the cell walls of cotyledon and endosperm tissues for use during early plant growth. The cell wall also acts as the principal barrier and defense against pathogenic bacteria, viruses, and fungi. Plant cell walls are dynamic structures rather than rigid and unchanging barriers.
Suggested Reading
Lampugnani, Edwin R., Ghazanfar Abbas Khan, Marc Somssich, and Staffan Persson. 2018. “Building a Plant Cell Wall at a Glance.” Journal of Cell Science 131 (2): jcs207373. [Source]
Bacete, Laura, Hugo Mélida, Eva Miedes, and Antonio Molina. 2018. “Plant Cell Wall-Mediated Immunity: Cell Wall Changes Trigger Disease Resistance Responses.” The Plant Journal 93 (4): 614–36. [Source]