31.4:
Cellulose and Pectic Polysaccharides
New plant cells have a primary cell wall and middle lamella, comprising cellulose fibers embedded in a gel-like matrix containing polysaccharides and proteins.
Cellulose is a long, linear chain of glucose. Around sixteen cellulose molecules are connected in parallel by hydrogen bonds, forming rod-like microfibrils that impart rigidity and resist tensile forces.
Branched polysaccharides, such as hemicellulose, cross-link microfibrils into a structural network that allows expansion during cell growth.
The cross-linked cellulose layers are suspended in a pectin matrix.
Pectins are polysaccharides containing negatively charged galacturonic acid units surrounded by cations and water molecules. Calcium ions crosslink pectins into a semirigid gel that cements the walls of adjacent cells.
As a plant cell matures, a secondary cell wall can form between the primary cell wall and the plasma membrane. It contains hardening polymers such as lignin that provide the protection and support required for water movement.
31.4:
Cellulose and Pectic Polysaccharides
Every plant cell has a cell wall that protects the cell, provides structural support, and gives the cell shape. Cellulose, the main structural component of the plant cell wall, makes up over 30% of plant matter. It is the most abundant organic compound on earth. Cellulose is an unbranched polysaccharide composed of linear chains of glucose molecules linked by β (1→4) glycosidic bonds.
As a cell matures, its cell wall specializes according to its 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 strength and support by partially thickening their primary cell wall or depositing a secondary cell wall. 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.
The cell wall contains other polysaccharides, such as pectin. Pectic polysaccharides are heteropolysaccharides that predominantly have a galacturonic acid sugar backbone with acid sugar and neutral sugar side chains. During cell growth and expansion, pectic polysaccharides are deposited in the middle lamella, followed by the primary and secondary cell walls. They are crucial in wall hydration and cell-cell adhesion. They also influence wall porosity, regulate ion transport, and help in the morphogenesis of plants by impacting the enzymes in the cell wall and their water-holding capacity.
Homogalacturonan, the most prominent type of pectin, accounts for around 60% of total pectin in cell walls.
Adapted from openstax biology 2e
Suggested Reading
- Caffall, K. H., & Mohnen, D. (2009). The structure, function, and biosynthesis of plant cell wall pectic polysaccharides. Carbohydrate research, 344(14), 1879-1900.
- Broxterman, S. E., & Schols, H. A. (2018). Interactions between pectin and cellulose in primary plant cell walls. Carbohydrate polymers, 192, 263-272.