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Cell Walls-Primary and Secondary

With very few exceptions, plant cells always form cell walls. The cell walls contains layers of cellulose fibers interspersed within a hemicellulose packing. Together, these substances yield a very strong but pliable supportive layer for plant cells. Adjacent cell walls are cemented together by botanic "glues" or pectins in a layer called the middle lamella.

The cell wall forms outside the plasma membrane initially as a thin primary cell wall. Thereafter, the primary cell wall may thicken or a more durable secondary cell wall can form between the primary cell wall and plasma membrane.

Plant cells remain in contact with one another via plasmodesmata, membrane lined channels that extend through cell walls to link adjacent cells. Plasmodesmata facilitate intercellular communication and transport.

Now, roll over the most central plasmodesmata in the diagram.

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Variations in Cell Wall Thickness

Note variations in cell wall thickness in this 1000x view of some typical plant cells. Thin primary walls of parenchyma cell types can be contrasted to the thicker secondary walls of sclerenchyma cell types. These sclerenchyma are fibers. Note the plasmodesmata extending through cell walls. The middle lamellae are visible as dark lines between adjacent cells. Take note of layering in the secondary cell walls to the right, an indicator walls are thickened via addition of successive layers.

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Lignin and the Secondary Wall

These xylem or wood cells in the image to the right possess thick secondary cell walls impregnated with a substance called lignin. Lignin is a highly cross-linked molecule that accumulates as a supportive substance in secondary walls. However, as lignin accumulates, it also effectively waterproofs cells, blocking diffusion of water and dissolved nutrients into the cells. This is one reason most cells with lignified cell walls are dead at maturity. Some retain enough plasmodesmata as life-lines with adjacent cells to remain alive.

Lignified secondary cell walls not only create transport problems, but also prevent mitosis. For this reason, cells of meristematic tissues, where mitotic cell divisions produce new tissues, only possess primary cell walls.

Now, roll your mouse around the secondary wall of the largest cell.