植物細胞壁

Plant cell walls consist of three layers: the middle lamella, the primary cell wall, and the secondary cell wall. The middle lamella lies between adjacent cells and allows them to adhere to one another. The primary cell wall, containing cellulose microfibers in a pectin matrix, is thin and flexible to permit cell growth. The secondary cell wall, deposited as cells mature, contains cellulose, hemicellulose, and hardening agents like lignin for strength and rigidity.

The primary cell wall is thin, flexible, and composed of cellulose microfibers embedded in a gel-like pectin matrix with hemicellulose, allowing the cell to expand during growth. The secondary cell wall is thick and rigid, containing cellulose, hemicellulose, and hardening agents such as lignin. It forms between the primary cell wall and plasma membrane as the plant cell matures, providing structural strength and rigidity rather than flexibility.

Turgor pressure develops when plant cells absorb water and store it in the vacuole, causing the vacuole to expand and push the plasma membrane against the cell wall. This pressure supports the upright and rigid structure of plants, maintaining their firmness. The cell wall prevents cells from rupturing under this internal pressure, allowing plants to maintain their characteristic hardy, upright nature.

Pits are small holes in plant cell walls that allow water, nutrients, and other molecules to transport between adjacent cells. In a pit, the middle lamella and primary cell wall form a thin membrane separating the cells. The secondary cell wall may be deposited around the pit but not within it, maintaining the opening for intercellular communication and nutrient exchange.

Cell wall composition specializes according to cell type and function. Parenchyma cells in leaves have only thin primary cell walls for flexibility. Collenchyma cells partially thicken their primary cell wall for moderate support, while sclerenchyma cells deposit a secondary cell wall for maximum strength. These varying compositions in plant tissues determine the specific structural and functional roles of different cells throughout the plant.

Cellulose is the predominant polysaccharide in plant cell walls, composed of repeating glucose units organized into microfibers. It provides structural integrity and forms the framework of both primary and secondary cell walls. Cellulose and pectic polysaccharides work together to create the rigid yet flexible matrix that supports plant cells and enables them to withstand mechanical stress and environmental pressures.

Beyond structural support, the cell wall serves as the principal barrier and defense against pathogenic bacteria, viruses, and fungi. Additionally, plant cell walls store nutrients; for example, seeds store sugars in the cell walls of cotyledon and endosperm tissues for use during early plant growth. This dual function makes the cell wall essential for both plant protection and resource management.