Plant cells have a cell wall, a rigid outer covering that protects the cell and provides shape and support. During cell division, a mixture of enzymes, proteins, and glucose molecules is transported via vesicles to the center of the cell. These vesicles continuously fuse and build a cell plate between the dividing cells. As the cell plate matures, new polysaccharides are added to it to form the cell walls of the daughter cells. The predominant polysaccharide in the cell wall is cellulose, made up of repeating glucose units. As a cell matures, its cell wall specializes according to its 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 give the strength and support by either partially thickening their primary cell wall (i.e., collenchyma) or depositing a secondary cell wall (i.e., sclerenchyma). 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. In each layer, the cellulose microfibrils are organized in different orientations.
Transportation through pits
All plant cell walls have small holes, or pits, that allow water transport, nutrients, and other molecules. In a pit, the middle lamella and primary cell wall merely form a thin membrane that separates adjacent cells. The secondary cell wall may be deposited around the pit but not within.
Rigidity against turgor pressure
Plant cells absorb water and nutrients and store them in the vacuole. As the vacuole expands, it pushes the plasma membrane against the cell wall. This turgor pressure supports the upright and rigid structure of plants. The cell wall prevents the cells from rupturing under this pressure.
Storage
In addition to providing structure and support, plant cell walls also store nutrients. Seeds, for example, may store sugars in the cell walls of cotyledon and endosperm tissues for use during early plant growth. The cell wall is the principal barrier and defense against pathogenic bacteria, viruses, and fungi.
Adapted from Section 4.3 Eukaryotic cells and 30.1 Plant biology Openstax biology 2e
