31.2
Plant Cell Wall
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…
The hardy, upright nature of plants comes from the cell wall, a rigid, semi-permeable structure enclosing the plant cell. The cell wall provides structural support and prevents the cell from bursting during water influx or turgor pressure from inside the cell.
Plant cell walls usually have three layers made up of proteins and polysaccharides.
The outermost layer or the middle lamella lies between two adjacent cells allowing the cells to adhere to one another.
The primary cell wall is the layer between the middle lamella and the plasma membrane.
It has cellulose microfibers embedded in a gel-like pectin matrix and hemicellulose. This thin and flexible structure allows the plant cell wall to expand during cell growth.
As a plant cell matures, a thick and rigid structure containing cellulose, hemicellulose, and hardening agents such as lignin, forms the secondary cell wall between the primary cell wall and plasma membrane, providing strength and rigidity.
View the full transcript and gain access to JoVE Core videos
Q1: What are the three layers of a plant cell wall?
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.
Q2: How does the primary cell wall differ from the secondary cell wall?
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.
Q3: What role does turgor pressure play in plant cell structure?
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.
Q4: How do pits in the cell wall facilitate transport between plant cells?
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.
Q5: Why do different plant cell types have different cell wall compositions?
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.
Q6: What is cellulose and why is it important in plant cell walls?
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.
Q7: How does the cell wall protect plants beyond providing structural support?
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.