The panda's gut is the standard ursid arrangement: a simple stomach, a short intestine and no caecum worth the name. Ruminants and other committed herbivores process fibrous plants in large fermentation chambers where microbes have hours or days to break down cellulose. Food passes through a panda in a matter of hours, so much of the bamboo emerges recognisably intact.
The animal's genome carries the same shortfall. Pandas lack the genes for cellulose-digesting enzymes, and they carry an inactivated copy of T1R1, one of the two genes required for the umami taste receptor, which is consistent with a lineage that stopped seeking out meat. Gut bacteria contribute some cellulose breakdown, but far less than in a cow or a sheep.
The compensation is volume and time. Adults spend a large share of each day feeding and pass through many kilograms of bamboo, selecting the most nutritious parts, shoots and leaves over culms, when they are available. How much of the panda's energy actually comes from microbial fermentation rather than from the cell contents released by chewing remains an area of active disagreement.