A schematic illustration shows a three-dimensional representation of a plant cell's plasma membrane and cell wall. They are shown from the side, and are cut away to reveal distinct layers. Structures inside each layer are labeled. The plasma membrane is shown on the bottom. The primary cell wall is on top of the plasma membrane, and the top-most layer is the middle lamella.
View Full-Size Image Figure 3 : Plant plasma membrane and cell-wall structure
A plant cell wall is arranged in layers and contains cellulose microfibrils, hemicellulose, pectin, lignin, and soluble protein. These components are organized into three major layers: the primary cell wall, the middle lamella, and the secondary cell wall (not pictured). The cell wall surrounds the plasma membrane and provides the cell tensile strength and protection.
© 2008 Nature Publishing Group Sticklen, M. B. Plant genetic engineering for biofuel production: towards affordable cellulosic ethanol. Nature Reviews Genetics 9, 433-443 (2008). All rights reserved. View Terms of Use
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Besides the presence of chloroplasts, another major difference between plant and animal cells is the presence of a cell wall. The cell wall surrounds the plasma membrane of plant cells and provides tensile strength and protection against mechanical and osmotic stress. It also allows cells to develop turgor pressure, which is the pressure of the cell contents against the cell wall. Plant cells have high concentrations of molecules dissolved in their cytoplasm, which causes water to come into the cell under normal conditions and makes the cell's central vacuole swell and press against the cell wall. With a healthy supply of water, turgor pressure keeps a plant from wilting. In drought, a plant may wilt, but its cell walls help maintain the structural integrity of its stems, leaves, and other structures, despite a shrinking, less turgid vacuole.
Plant cell walls are primarily made of cellulose, which is the most abundant macromolecule on Earth. Cellulose fibers are long, linear polymers of hundreds of glucose molecules. These fibers aggregate into bundles of about 40, which are called microfibrils. Microfibrils are embedded in a hydrated network of other polysaccharides. The cell wall is assembled in place. Precursor components are synthesized inside the cell and then assembled by enzymes associated with the cell membrane (Figure 3).
