Plant cell wall. Coloured scanning electron micrograph (SEM) of cellulose microfibrils (strands) in a plant cell wall. Unlike animal cells, plants cells are enclosed in a protective rigid cell wall. The main constituent of the wall is cellulose, a complex polysaccharide that forms microfibrils. Two other polysaccharides, pectin and glycan, cross-link with the cellulose to increase the strength of the cell wall. Cellulose microfibrils measure between 5 nanometres and 15 nanometres in diameter.