Cellulose is a polysaccharide (a macromolecule composed of long chains of sugar molecules) made up of glucose molecules linked together with beta-1-4 bonds. As a structural part in many plants it is important for their mechanical properties. Wood and straw, rich in cellulose, are widely used on Earth for construction work, also as a composite material. It might be useful in a Martian settlement, as well. Lignin is another key element in plant structure, that serves, among other purposes, to give rigidity to woods.
Cellulose can be mass produced in grass farms, growing Miscanthus giganteus in greenhouses which operate with a CO2 enriched low pressure atmosphere growing on crushed regolith. This grass can then be mass produced via an automated lawn mowing system, producing large quantities of cellulose rich biomass. This atmosphere would be provided using raw martian atmosphere that has been filtered to remove dust and then compressed to produce the low pressure atmosphere. A quantum dot film may be placed on the top of the greenhouse to convert the incoming solar radiation to frequencies that are more useful for photosynthesis. The use of genetic modification, specifically in the PXY regulatory pathways has been shown to greatly increase biomass production speed.
Glucose can be liberated from cellulose through chemical processes or enzymatic cellulases. This then provides food for bioreactors to then produce either eatable food, medicine or industrial chemicals.
As a side product of the cellulose mass production process, grasses consume the CO2 enriched atmosphere producing O2. This atmosphere can then be processed, recycling the CO2 and using the remaining O2 and N2 as breathable air for a colony or using refined O2 for rockets.
The main industrial use of cellulose is for paper production. This could be done on Mars as well. Lignin is removed from the wood pulp on Earth and burned to power the paper making process. Other uses for lignin might be found on Mars as it contains significant amounts of energy.
Cotton, linen, and synthetic fibers such as Rayon are mostly cellulose. Clothing produced on Mars will likely be made from some form of cellulose fiber.