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organisms that consume cellulose

organisms that consume cellulose

organisms that consume cellulose

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Monogastric

A monogastric organism has a simple single-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.

Gastrointestinal function

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 gallbladder (note that horses do not have a gallbladder 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.

organisms that consume cellulose
organisms that consume cellulose

Acetal Functional Group:

Carbon # 1 is called the anomeric carbon and is the center of an acetal functional group. A carbon that has two ether oxygens attached is an acetal.

The Beta position is defined as the ether oxygen being on the same side of the ring as the C # 6. In the chair structure this results in a horizontal or up projection. This is the same definition as the -OH in a hemiacetal.

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Compare Cellulose and Starch Structures:

Cellulose:

Beta glucose is the monomer unit in cellulose. As a result of the bond angles in the beta acetal linkage, cellulose is mostly a linear chain.

Starch:

Alpha glucose is the monomer unit in starch. As a result of the bond angles in the alpha acetal linkage, starch-amylose actually forms a spiral much like a coiled spring.

Fiber in the Diet:

Dietary fiber is the component in food not broken down by digestive enzymes and secretions of the gastrointestinal tract. This fiber includes hemicelluloses, pectins, gums, mucilages, cellulose, (all carbohydrates) and lignin, the only non-carbohydrate component of dietary fiber.

High fiber diets cause increased stool size and may help prevent or cure constipation. Cereal fiber, especially bran, is most effective at increasing stool size while pectin has little effect. Lignin can be constipating.

Fiber may protect against the development of colon cancer, for populations consuming high fiber diets have a low incidence of this disease. The slow transit time (between eating and elimination) associated with a low fiber intake would allow more time for carcinogens present in the colon to initiate cancer. But constipated people do not have a higher incidence of colon cancer than fast eliminators, so fiber’s role in colon cancer remains unclear.

Pectin and rolled oats

Dietary fiber may limit cholesterol absorption by binding bile acids. High fiber diets lower serum cholesterol and may prevent cardiovascular disease. Some fibers, such as pectin and rolled oats, are more effective than others, such as wheat, at lowering serum cholesterol.

Dietary fiber is found only in plant foods such as fruits, vegetables, nuts, and grains. Whole wheat bread contains more fiber than white bread and apples contain more fiber than apple juice, which shows that processing food generally removes fiber.

Cellulose Digestion

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.

organisms that consume cellulose
organisms that consume cellulose

Earth’s stratosphere

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 then regurgitated into the mouth, which 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 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).

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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 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.

The ability to digest is different in humans

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?

organisms that consume cellulose
organisms that consume cellulose

Ruminants and Termites

Ruminants-animals such as cattle, goats, sheep, bison, buffalo, deer, and antelope. Regurgitate what they eat as cud and chew it again for further digestion (6). Ruminant intestines are very similar to human intestines in their form and function (Fig. 3). The key to specialized ruminant digestion lies in the rumen. Ruminants, like humans, also secrete saliva as the primary step in digestion. But unlike humans, they swallow the food first only to regurgitate it later for chewing. Ruminants have multi-chambered stomachs, and food particles must made small enough to pass through the reticulum chamber into the rumen chamber. Inside the rumen, special bacteria and protozoa secrete the necessary enzymes to break down the various forms of cellulose for digestion and absorption.

Why don’t ruminal bacteria digest cellulose faster?

Major constraints to cellulose digestion are caused by cell-wall structure of the plant. And by limited penetration of the nonmotile cellulolytic microbes into the cell lumen.

Thank you for staying with this post “organisms that consume cellulose” until the end.

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