Why are ruminant able to digest cellulose?

Ruminants, including cows, sheep, goats, and deer, are herbivores that possess a specialized digestive system, commonly known as the foregut fermentation system, that enables them to efficiently process cellulose, a complex carbohydrate that plant cell walls are largely made of.

The foregut fermentation system consists of four main compartments:

1. Rumen: The rumen is a large fermentation vat that serves as the central hub of the digestive process. It is home to a diverse population of billions of microorganisms, including bacteria, protozoa, and fungi, which collectively form the rumen microbiome.

2. Reticulum: Connected to the rumen, the reticulum is a honeycomb-like structure that functions as a "sorting station" for ingested food.

3. Omasum: The omasum, also known as the "leaf book," further absorbs water and nutrients from the digesta.

4. Abomasum: The abomasum is the "true stomach" of the ruminant, where gastric secretions and enzymes are released to break down proteins and prepare the digesta for further digestion and absorption in the small intestine.

The process of cellulose digestion in the ruminant digestive system involves the following steps:

1. Ingestion and Regurgitation: Ruminants begin by consuming large quantities of plant material, which they swallow without thorough chewing. This partially chewed food, called cud, is stored temporarily in the rumen.

2. Fermentation in the Rumen: The cud is regurgitated and mixed with saliva before being returned to the rumen for fermentation. The rumen microbes utilize enzymes, such as cellulases, to break down cellulose into simpler sugars, such as glucose and volatile fatty acids.

3. Microbial Protein Synthesis: The rumen microorganisms use the breakdown products of cellulose, along with other nutrients present in the diet, to synthesize microbial proteins. These proteins become a valuable source of amino acids that contribute to the ruminant's nutritional needs.

4. Absorption and Digestion: The digesta, which is a complex mixture of microbial cells, partially fermented plant material, and other nutrients, passes through the reticulum, omasum, and finally into the abomasum. In the abomasum, gastric secretions and enzymes break down the microbial cells, plant fibers, and other digestible components. The nutrients are then absorbed in the small intestine, and the remaining indigestible material is eliminated as waste.

The rumen fermentation process not only enables ruminants to efficiently digest cellulose, but it also contributes to the production of volatile fatty acids, which serve as an important source of energy for the ruminant. The symbiotic relationship between the ruminant and its rumen microbes is crucial for accessing the nutritional value of the fibrous plant-based diet, allowing these herbivores to thrive in environments where cellulose-rich food is abundant.