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A monogastric organism has a simple single-chambered stomach, compared to a ruminant organism, which has a four-chambered complex stomach. Examples of monogastric animals include omnivores such as humans, rats, and pigs, carnivores such as dogs and cats, and herbivores such as horses and rabbits. Herbivores with monogastric digestion can digest cellulose in their diets by way of symbiotic gut bacteria. However, their ability to extract energy from cellulose digestion is less efficient than in ruminants.
Herbivores digest cellulose via microbial fermentation. Monogastric herbivores which can digest cellulose nearly as well as ruminants are called hindgut fermenters, while ruminants are called foregut fermenters. These are subdivided into two groups based on the relative size of various digestive organs in relationship to the rest of the system: colonic fermenters tend to be larger species such as horses and rhinos, and cecal fermenters are smaller animals such as rabbits and rodents. The great apes (bonobos, chimpanzees, gorillas, and orangutans), in contrast to humans, derive significant amounts of phytanic acid from the hindgut fermentation of plant materials.
Monogastrics cannot digest the fiber molecule cellulose as efficiently as ruminants, though the ability to digest cellulose varies amongst species.
A monogastric digestive system works as soon as the food enters the mouth. Saliva moistens the food and begins the digestive process. After being swallowed, the food passes from the esophagus into the stomach, where stomach acid and enzymes help to break down the food. Bile salts stored in the gall bladder empty the contents of the stomach into the small intestines where most fats are broken down. The pancreas secretes enzymes and alkali to neutralize the stomach acid.
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