Cell wall microstructure. Illustration of the molecules and materials found in the cell wall of plant cells. The main tubes (green) are cellulose microfibrils. The dark green attachments are lengths of non-cellulosic polysaccharides. Cross-links between the cellulose microfibrils are formed by glycoproteins (orange) and pectin molecules (red, acid; yellow, neutral). Calcium ions (one at top) form bridges between the pectin molecules. In combination, these materials form an extracellular matrix that makes the plant cell wall strong and rigid. For this artwork with labels, see C023/8833.