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Plant cell walls consist of three specific types of layers, including the primary wall, secondary wall, and the middle lamella. The first layer formed during cell division, the middle lamella, is the outer wall of the cell that holds two adjacent cells together. Rich in pectin, this is the cementing layer of the cell, sometimes called the pectin layer.
The cell walls of plants provide strength and protection, keeping the cells from bursting or rupturing. They also protect against insects and parasites, and keep cells hydrated. Some cell walls are relatively thick, providing structural support for the plant, while others are thinner in order to promote cell-to-cell transfer and communication.
Plant growth and development involves enlargement of the cells, changes in the cell wall composition, and cell division. Cytokinesis, the primary reproductive procedure of the plant, is the process in which one cell divides itself into two. During this process, a partition called a cell plate is formed to separate the two new daughter cells from each other. The middle lamella is formed from the cell plate as a continuous, opaque layer made of pectic compounds and proteins. It is thin, gelatinous, and pliant.
The primary wall, flexible and thin, is formed after the middle lamella while the cell is still growing. Following the creation of the primary wall, the secondary wall is formed. This is the most rigid of the three plant cell layers, and is formed within the primary wall only after the cell is completely grown. In some plants, the secondary wall contains lignin, which acts as a waterproofing agent. Though every plant cell has a middle lamella and a primary wall, not all plant cells will have secondary walls.
Pectin is produced commercially as a powder used in cooking and baking as a gelling agent. In plant cells, pectin has a similar role. The middle lamella's primary function is to act as an interface to keep adjacent cells together, so it's made up primarily of pectins. The amount of pectin in plant cells will vary widely from plant to plant.
The middle lamella is the part of the cell wall that touches the surrounding cells, so it could perhaps best be described as the glue that holds the plant together. It's the most flexible and opaque of the three layers, and the first formed when cells divide.
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