A monogastric organism has a simple double-chambered stomach (one stomach). Examples of monogastric herbivores are horses, rabbits, gerbils, and hamsters. Examples of monogastric omnivores include humans, pigs, and rats. Furthermore, there are monogastric carnivores such as dogs and cats.
A monogastric organism is comparable to ruminant organisms (which has a four-chambered complex stomach), such as cattle, goats, or sheep. 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 by 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. Great apes 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. (Note that horses have no (or negligible amounts of) amylase in their saliva). 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 are stored in the gall bladder (note that horses do not have a gall bladder and bile is directly secreted into the small intestine) and secreted once the contents of the stomach have reached the small intestines where most fats are broken down. The pancreas secretes enzymes and alkali to neutralize the stomach acid.
Humans lack the enzyme necessary to digest cellulose. Hay and grasses are particularly abundant in cellulose, and both are indigestible by humans (although humans can digest starch). Animals such as termites and herbivores such as cows, koalas, and horses all digest cellulose, but even these animals do not themselves have an enzyme that digests this material. Instead, these animals harbor microbes that can digest cellulose.
The termite, for instance, contains protists (singlecelled organisms) called mastigophorans in their guts that carry out cellulose digestion. The species of mastigophorans that performs this service for termites is called Trichonympha, which, interestingly, can cause a serious parasitic infection in humans.
Animals such as cows have anaerobic bacteria in their digestive tracts which digest cellulose. Cows are ruminants, or animals that chew their cud. Ruminants have several stomachs that break down plant materials with the help of enzymes and bacteria. The partially digested material is then regurgitated into the mouth, which is then chewed to break the material down even further.
The bacterial digestion of cellulose by bacteria in the stomachs of ruminants is anaerobic, meaning that the process does not use oxygen. One of the by-products of anaerobic metabolism is methane, a notoriously foul-smelling gas. Ruminants give off large amounts of methane daily. In fact, many environmentalists are concerned about the production of methane by cows, because methane may contribute to the destruction of ozone in Earth’s stratosphere.
Although cellulose is indigestible by humans, it does form a part of the human diet in the form of plant foods. Small amounts of cellulose found in vegetables and fruits pass through the human digestive system intact. Cellulose is part of the material called “fiber” that dieticians and nutritionists have identified as useful in moving food through the digestive tract quickly and efficiently.
Diets high in fiber are thought to lower the risk of colon cancer because fiber reduces the time that waste products stay in contact with the walls of the colon (the terminal part of the digestive tract). The major component in the rigid cell walls in plants is cellulose. Cellulose is a linear polysaccharide polymer with many glucose monosaccharide units. The acetal linkage is beta which makes it different from starch.
Humans are unable to digest cellulose
This peculiar difference in acetal linkages results in a major difference in digestibility in humans. Humans are unable to digest cellulose because the appropriate enzymes to breakdown the beta acetal linkages are lacking. (More on enzyme digestion in a later chapter.) Undigestible cellulose is the fiber which aids in the smooth working of the intestinal tract.
Animals such as cows, horses, sheep, goats, and termites have symbiotic bacteria in the intestinal tract. These symbiotic bacteria possess the necessary enzymes to digest cellulose in the GI tract. They have the required enzymes for the breakdown or hydrolysis of the cellulose; the animals do not, not even termites, have the correct enzymes. No vertebrate can digest cellulose directly.
Even though we cannot digest cellulose, we find many uses for it including: Wood for building; paper products; cotton, linen, and rayon for clothes; nitrocellulose for explosives; cellulose acetate for films.
The structure of cellulose consists of long polymer chains of glucose units connected by a beta acetal linkage. The graphic on the left shows a very small portion of a cellulose chain. All of the monomer units are beta-D-glucose, and all the beta acetal links connect C # 1 of one glucose to C # 4 of the next glucose.
In this section contains a lot of information and many terms that are probably new to you. Set aside the detail for the moment, read the caption and try to get an overall impression. You should see that it shows the digestive systems of two herbivores, simplified and stretched out, rather than being folded up into the minimum space as they would be inside an actual animal.
None of the other internal organs of the animals is shown. One of the great benefits of diagrams over photographs or accurate scale drawings is that in a diagram, you can just concentrate on the aspects you want to show and omit all other details. As you read the text, you will be introduced to different aspects of this diagram, so try to glance from the text to the diagram as you meet the various points.
The enzyme needed to digest cellulose is called cellulase. Enzymes are often named like this, i.e. by taking part of the name of the substance that they digest and replacing the end of the word by ‘-ase’. For example, proteins are digested by types of enzymes called proteases and the sugar in milk, lactose, is digested by the enzyme lactase. Animals that live on plant material have detectable cellulase activity in their digestive systems.
However, detailed genetic studies have shown that the vast majority of animals do not possess the gene for making cellulase. So the enzyme is clearly not synthesised by the animal itself. The gene for cellulase does appear to exist in a few animals, namely some insects (such as vine weevil larvae, cockroaches, termites and silkworms). Some earthworms, a wood-boring clam, and a fish called the grass carp.
This very odd assemblage of creatures has no close taxonomic relationship, indicating that the ability to produce cellulase probably evolved independently several times. Why it has not evolved more frequently is mysterious – it appears never to have evolved in mammals.
Mammals that need cellulase have to make use of some additional species that do possess the gene – namely microbes, mostly bacteria and fungi. But also some protoctists (tiny organisms, most of them composed of just a single cell with a nucleus). Parts of the mammal’s digestive system are modified to form large sacs where huge colonies of these microbes break down, or ferment, the cellulose.
The Human Digestive System
Disregarding cellulose digestion, human digestion is still a very efficient process (Fig. 2). Even before food enters the mouth, saliva glands automatically start secreting enzymes and lubricants to begin the digestive process. Amylase breaks down starches in the mouth into simple sugars. And teeth grind up the food into smaller chunks for further digestion.
After swallowing the food, hydrochloric acid and various enzymes work on the food in the stomach for two to four hours. During this time, the stomach absorbs glucose, other simple sugars, amino acids, and some fat-soluble substances (3). The mixture of food and enzymes, called chyme, then moves on to the small intestines where it stays for the next three to six hours.
In the small intestines, pancreatic juices and liver secretions digest proteins, fats and complex carbohydrates. Most of the nutrition from food is absorbed during its journey through over seven feet of small intestines. Next, the large intestines absorb the residual water and electrolytes and store the leftover fecal matter.
Deficiency of enzymes or hormones
Although the human digestive system is quite efficient, discrepancies among the human population exist concerning what individuals can or cannot digest. For example, an estimated seventy percent of people cannot digest the lactose in milk and other dairy products. Because their bodies gradually lost the ability to produce lactase (4). Humans can also suffer from various other enzyme or hormone deficiencies that affect digestion and absorption, such as diabetes.
Comparative studies show that the human digestive system is much closer to that of herbivores rather than carnivores. Humans have the short and blunted teeth of herbivores and relatively long intestines-about ten times the length of their bodies. The human colon also demonstrates the pouched structure peculiar to herbivores (5). Yet, the human mouth, stomach, and liver can secrete enzymes to digest almost every type of sugar except cellulose, which is essential to a herbivore’s survival.
In the case of lactose intolerance, lactase supplements can easily rectify the deficiency. So what rectifies the inability to digest cellulose?
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