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How do some plant-eating animals obtain nutrients from cellulose?

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A biochemical flowchart shows the metabolic pathways for carbohydrate utilization, including glycolysis, the pentose phosphate pathway, and the tricarboxylic acid (TCA) cycle. The diagram traces the conversion of glucose, fructose, and glycerol through various intermediates, leading to energy production, carbon dioxide release, and biosynthesis of amino acids and porphyrins. Key steps and branches are labeled, with colored boxes highlighting biosynthetic outputs. — Carbohydrate Pathways for the utilization of carbohydrates.

Most animals, including herbivores, do not produce the enzymes necessary to break down cellulose, the primary structural component of plant cell walls. Instead, they rely on symbiotic microorganisms—such as bacteria and protozoans—residing in their digestive tracts to ferment and digest this complex carbohydrate.1234

Ruminants

Ruminants (e.g., cattle, sheep) utilize a specialized, multichambered stomach. Ingested plant material is stored in the rumen, where it is softened and fermented by microorganisms. Animals regurgitate this material as cud to chew it again, increasing the surface area for further microbial breakdown. The microorganisms ferment cellulose into short-chain fatty acids, which the host absorbs as a primary energy source.12

Hindgut Fermenters

Other herbivores, such as horses, rabbits, and elephants, are hindgut fermenters. In these animals, undigested food residues undergo bacterial fermentation in the cecum, a pouch located at the end of the small intestine, where fatty acids are produced and absorbed.2

Insects

Termites also depend on symbiotic microorganisms in their hindgut to digest cellulose, converting it into glucose and acetic acid.45