Cellulose could provide an abundant and cheap form of glucose for humans. why is cellulose not a source of nutrients for humans? humans, and most vertebrates, lack the enzyme cellulase. human gut bacteria break down cellulose in the large intestine, after nutrient absorption occurs. animals use starch and glycogen as an energy source, whereas plants use cellulose for energy. humans cannot absorb the hydrolyzed form of cellulose. vertebrate enzymes hydrolyze (α 1–4) glucose linkages, but not glucose in the β configuration.