セルロースとペクチン多糖類

Cellulose is a linear polysaccharide composed of glucose molecules linked by β (1→4) glycosidic bonds, making it the most abundant organic compound on Earth. It comprises over 30% of plant matter and serves as the primary structural component of plant cell walls, providing rigidity and resistance to tensile forces that protect and support plant cells.

Around sixteen cellulose molecules connect in parallel through hydrogen bonds, forming rod-like microfibrils that impart rigidity to the cell wall. Branched polysaccharides like hemicellulose cross-link these microfibrils into a structural network that allows expansion during cell growth while maintaining mechanical strength.

Pectic polysaccharides are heteropolysaccharides with a galacturonic acid backbone and various sugar side chains. They are deposited in the middle lamella and cell walls, where they regulate hydration, facilitate cell-cell adhesion, influence wall porosity, and control ion transport. Homogalacturonan, the most prominent pectin type, accounts for approximately 60% of total pectin in cell walls.

Calcium ions cross-link pectin polysaccharides, which contain negatively charged galacturonic acid units, into a semirigid gel matrix. This gel cements the walls of adjacent cells together, providing structural cohesion and stability essential for maintaining plant cell integrity and organization throughout the plant.

Primary cell walls are thin structures present in all young plant cells, composed of cellulose microfibrils cross-linked by hemicellulose and suspended in a pectin matrix. Secondary cell walls form in mature cells between the primary wall and plasma membrane, containing hardening polymers like lignin that provide enhanced protection and support for water movement.

The middle lamella is a gel-like matrix containing polysaccharides and proteins that surrounds cellulose fibers in the primary cell wall. Pectic polysaccharides are deposited here during cell growth, creating a cement-like substance that bonds adjacent cells together and maintains structural organization throughout the plant.

As plant cells mature, their cell walls specialize according to cell type and function. Parenchyma cells in leaves have only thin primary walls, while collenchyma and sclerenchyma cells in stems and leaves thicken their primary walls or deposit secondary walls containing lignin. Trees and grasses deposit secondary walls with three distinct layers of differently oriented cellulose microfibrils for enhanced strength and support.