4.12
Plant Cell Wall
A plant cell has a cell wall, a rigid structure located outside the plasma membrane that surrounds the cell. The cell wall helps support and protect the cell, maintains its shape, and prevents excessive expansion of the cell when water enters.
Plant cell walls can have up to three layers—the middle lamella, the primary cell wall, and the secondary cell wall.
Each layer contains polysaccharides such as pectin, hemicellulose, and cellulose, as well as structural proteins.
The outermost layer of the cell wall is shared between adjacent cells, which is why it is called the “middle” lamella. This layer is rich in pectin, which helps cells stick together and form plant tissues.
The primary cell wall forms between the middle lamella and the plasma membrane.
It is a thin, flexible layer that forms as the plant divides and grows.
The primary cell wall is a layer of cellulose microfibrils embedded in a gel-like matrix of pectin and hemicellulose.
After a plant cell matures and stops growing, it may build a secondary cell wall between the primary cell wall and the plasma membrane.
This layer is thick and rigid. It contains cellulose and hemicellulose, along with lignin, a strong polymer that hardens the wall.
Both wood and grasses build secondary cell walls that include lignin. But their cell wall compositions differ to suit their structural needs.
Wood, unlike grass, has thick secondary cell wall layers that give trees strength and rigidity.
Even within the same plant, different cell types build specialized secondary walls. For example, vessel elements and tracheids—cells that transport water—develop thickened walls that help them withstand the pressure exerted by water movement.
Plant cell walls are not impenetrable barriers. Small channels called plasmodesmata connect the cytoplasm of neighboring cells. These channels allow cells to exchange water, nutrients, and other vital molecules.
The plant cell wall gives plant cells shape, support, and protection. As a cell matures, its cell wall specializes according to the cell type. For example, the parenchyma cells of leaves possess only a thin, primary cell wall.
Collenchyma and sclerenchyma cells, on the other hand, mainly occur in the outer layers of a plant's stems and leaves. These cells provide the plant with strength and support by either partially thickening their primary cell wall (i.e., collenchyma), or depositing a secondary cell wall (i.e., sclerenchyma). Altogether, the varying cell wall compositions determine the function of specific cells and tissues.
Some plants, such as trees and grasses, deposit a secondary cell wall around mature cells. Secondary cell walls typically contain three distinct layers: the secondary wall layer 1 (S1) to the outside, the secondary wall layer 2 (S2) in the middle, and the innermost secondary wall layer 3 (S3). In each layer, the cellulose microfibrils are organized in different orientations. The S2 layer may make up to 75% of the cell wall.
Regardless of composition, all plant cell walls have small holes, or pits, that allow for the transport of water, nutrients, and other molecules. In a pit, the middle lamella and primary cell wall merely form a thin membrane that separates adjacent cells. Plasmodesmata span the resulting channel and connect the cytoplasm of neighboring cells. The secondary cell wall may be deposited around the pit but not within.
When plants absorb water and nutrients, plant cells store it in the vacuole. As the vacuole expands, it pushes the plasma membrane against the cell wall. This so-called turgor pressure supports the upright and rigid structure of plants. The cell wall, however, prevents the cells from rupturing under this pressure.
In addition to providing structure and support, plant cell walls may also provide plants with nutrient storage. Seeds, for example, may store sugars in the cell walls of cotyledon and endosperm tissues for use during early plant growth. The cell wall also acts as the principal barrier and defense against pathogenic bacteria, viruses, and fungi. Plant cell walls are dynamic structures rather than rigid and unchanging barriers.
A plant cell has a cell wall, a rigid structure located outside the plasma membrane that surrounds the cell. The cell wall helps support and protect the cell, maintains its shape, and prevents excessive expansion of the cell when water enters.
Plant cell walls can have up to three layers—the middle lamella, the primary cell wall, and the secondary cell wall.
Each layer contains polysaccharides such as pectin, hemicellulose, and cellulose, as well as structural proteins.
The outermost layer of the cell wall is shared between adjacent cells, which is why it is called the “middle” lamella. This layer is rich in pectin, which helps cells stick together and form plant tissues.
The primary cell wall forms between the middle lamella and the plasma membrane.
It is a thin, flexible layer that forms as the plant divides and grows.
The primary cell wall is a layer of cellulose microfibrils embedded in a gel-like matrix of pectin and hemicellulose.
After a plant cell matures and stops growing, it may build a secondary cell wall between the primary cell wall and the plasma membrane.
This layer is thick and rigid. It contains cellulose and hemicellulose, along with lignin, a strong polymer that hardens the wall.
Both wood and grasses build secondary cell walls that include lignin. But their cell wall compositions differ to suit their structural needs.
Wood, unlike grass, has thick secondary cell wall layers that give trees strength and rigidity.
Even within the same plant, different cell types build specialized secondary walls. For example, vessel elements and tracheids—cells that transport water—develop thickened walls that help them withstand the pressure exerted by water movement.
Plant cell walls are not impenetrable barriers. Small channels called plasmodesmata connect the cytoplasm of neighboring cells. These channels allow cells to exchange water, nutrients, and other vital molecules.
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4.12 : Plant Cell Wall
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