Cellulose is an organic compound essential to plants. Cellulose is a non-branched polysaccharide, meaning the compound is a linear, chemically bound chain of sugar molecules, more exactly beta-glucose, and a constitutional form of glucose. The chemical sum formula is (C6 H10 O5 )n where n stands for the number of glucose molecules contained in the cellulose chain. Simply put, cellulose is just a long chain of sugars. What makes the structure so special is that each glucose molecule is upside down in relation to its direct neighbor, which is due to the complex structure of beta-glucose.
Cellulose is also chemically interesting, as most organic solvents are not able to solvate the cellulose. Exceptions are strong solvents and acids and high temperatures, which not facilitates the creation of a solution, but also might break down cellulose to glucose under certain conditions.
About a third of the mass of every plant is cellulose, as it essential to every plant cell. It constitutes the strong cell wall in plants. Every year, plants produce around 100 billion tons of cellulose, making it the most common organic compound on earth. But in plants, cellulose is not simply arranged in long chains. In plants, microfibers form through parallel aligned chains of Cellulose bound by hydrogen bridges formed by the hydroxyl groups of glucose. Around 80 aligned chains form a microfiber. These microfibers form superimposed storeys of cellulose, with other fibers running transversely in multiple layers. Through this layout, the cell wall of plants is very rigid, allowing the plant to gain form and structure. More importantly, the cell wall is still partially permeable for nutrients. As the structure of the fibers can be very diffuse, the plant cell has some elasticity for varying levels of plant tugor pressure. The fibers are further cross-linked by hemicellulose and are in suspended in the gel-like pectin. The primary cell wall directly protects the cell’s plasma membrane. Outside the primary cell wall there is just a thin layer of pectin, wax and cutin to provide some padding, called the middle lamella. In some plants a secondary cell wall forms in the plants adult stages as an extra protection – this wall again is mostly made up of cellulose. A very thin cellulose tertiary layer is sometimes formed as well.
From a biochemical standpoint, there are not too many enzymes which facilitate the hydrolysis or breakdown of cellulose. This is an important positive evolutionary factor for plants, as this fact inhibits many organisms from feasting on plants. In fact, humans are not able break down cellulose in their digestive system. They are still important however as dietary fibers cleanse the digestive tract thoroughly. Some animals, for example cows, can digest cellulose and use the contained glucose through microbes and bacteria in their stomach and digestive system. Organisms who can break down cellulose are an important factor to the global ecosystem, as they pass back minerals and other substances to the soil.
Sources include: Campbell/Reece, “Biology”, Spektrum Verlag 2003