cellulose

**Structure:** Linear homopolymer of D-glucose joined by β(1→4) glycosidic bonds. Alternate glucose units are flipped, producing straight chains that hydrogen-bond into rigid microfibrils, which give plant cell walls their tensile strength. Contrast with starch (amylose: α-1,4 linear; amylopectin: α-1,4 with α-1,6 branches) and glycogen (α-1,4 with α-1,6 branches about every 8–10 residues).

**Digestion:** Human salivary and pancreatic α-amylase cleave only α-1,4 bonds, not β-1,4. Humans have no cellulase, so cellulose passes through the small intestine undigested. Ruminants (cows) and termites rely on symbiotic microbes that produce cellulase. In humans, colonic bacteria ferment a small portion of fiber to short-chain fatty acids (butyrate, propionate, acetate). Butyrate is the preferred fuel of colonocytes.

**Clinical significance:** - Insoluble fiber (cellulose, lignin, hemicellulose) increases fecal bulk, speeds colonic transit, and helps prevent constipation and diverticulosis. - Low-fiber diets are associated with constipation, diverticular disease, hemorrhoids, and possibly colorectal cancer. Fiber's protective role in colon cancer is suggested but not definitive. - Soluble fiber (pectin, β-glucan) differs in that it lowers LDL by binding bile acids and blunts postprandial glucose. - Cellulose-derived compounds are used pharmacologically as excipients, and as bulk-forming laxatives (methylcellulose). - Oxidized cellulose is used as a topical hemostatic agent (e.g., Surgicel) in surgery. Retained material can mimic an abscess or mass on imaging (gossypiboma/textiloma context). - Cellulose is a common component of plant cell walls and appears in histology/pathology as a birefringent artifact. Plant material, including cellulose, can be seen as a contaminant in tissue sections.

**High-yield pearls for boards:** - Humans cannot digest β-1,4 linkages. Lactose is also a β-1,4 linkage (galactose-β-1,4-glucose), but it is cleaved by lactase, which is specific for that bond. - Cellulose is a nonstarch polysaccharide and the most abundant organic polymer on Earth. - It is a structural polysaccharide, whereas starch and glycogen are storage polysaccharides.