<P> In the primary (growing) plant cell wall, the major carbohydrates are cellulose, hemicellulose and pectin . The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose - hemicellulose network, which is embedded in the pectin matrix . The most common hemicellulose in the primary cell wall is xyloglucan . In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by glucuronarabinoxylan, another type of hemicellulose . Primary cell walls characteristically extend (grow) by a mechanism called acid growth, which involves turgor - driven movement of the strong cellulose microfibrils within the weaker hemicellulose / pectin matrix, catalyzed by expansin proteins . The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle . </P> <P> Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability . The major polymers that make up wood (largely secondary cell walls) include: </P> <Ul> <Li> cellulose, 35 - 50% </Li> <Li> xylan, 20 - 35%, a type of hemicellulose </Li> <Li> lignin, 10 - 25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall . </Li> </Ul> <Li> cellulose, 35 - 50% </Li>

Synthesis of cell wall material takes place in