What allows grass eaters to digest cellulose?

What allows grass eaters to digest cellulose?

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Scientists are saying that it was a small step for the Panda to move from a meat diet to a grass diet. The article only refers to differences in the Panda's skull, presumably for better chewing.

I've also heard that the Japanese are able to digest Nori because they have the right gut biome because Nori has been part of their diet and the appropriate bacteria has developed over time.

Does that suggest that if the cellulose of bamboo and other grasses were pre-chewed by mechanical means and the right bacteria were introduced into the human gut that we would likewise be able to digest grasses?

Related story

You might find the answers to a similar previous question interesting:

Human Digestion of Cellulose?

Summary of those answers: It seems that some humans do host cellulose-digesting symbiotic bacteria, and do digest some of their cellulose. According to Cellulose and the Human Gut, finely ground cellulose with low lignin content is most readily digested by humans who host the right bacteria.

No there is something else missing besides bacteria

humans often already have the right bacteria but they don't digest most of the cellulose they ingest because digesting cellulose with bacteria is SLOW. Too slow to keep up with normal mammalian digestion. You need a specialized gut that either has a large complex stomach to keep recycling the cellulose in the system (foregut fermenters) or has large storage chambers to keep the cellulose a long time and allow it to be digested slowly (hindgut fermentation).

humans have neither. To make matters worse Humans have huge caloric needs compared to other animals of the same size. Brains are calorie hogs and our brains are so large they consume 1/4 of all our calories. That is 3 pounds of our mass consumes a quarter of all the calories we eat. honestly without the advent of cooking (which drastically improves the caloric availability of food) human brains could never have reached the size they did.

Further reading

More further reading

Activity of cellulolytic enzymes in the contents of reticulorumen and caecocolon of roe deer (Capreolus capreolus)

Selective ruminants, which prefer easily digestible plants, cannot digest fibrous forage as well as grass eaters. Low enzyme activity or short retention time of ingesta particles in fermentation chambers appeared to be responsible for reduced cellulose breakdown. Seasonal activity of cellulolytic enzymes, cellulose concentration and protozoa population in reticulorumen (RR) and caecocolon (CC) of roe deer as a typical concentrate selector were investigated. Cellulase activities were lowest in winter when cellulose concentration in RR contents were highest. Highest enzyme activities and lowest cellulose concentration were measured in early spring. Cellulolytic activities were significantly correlated with the number of protozoa in RR. Only one entodinomorphic genus was identified in the RR. The enzyme activities in CC were far lower compared with those in RR. Low cellulose digestion in the RR cannot be compensated for by cellulose breakdown in the CC. The reduced cellulose digestion of roe deer may be attributed to the short retention time of food particles in spring and summer, whereas decreased colonisation of microorganisms in the rumen may be the main reason for low cellulose breakdown in winter.

How do humans digest cellulose?

Cellulose is difficult for most animals to digest. Humans are unable to digest cellulose due to the lack of the enzyme needed to cleave its &beta( 1 &minus 4 1-4 1&minus4 ) glycosidic bond (see molecular structure in Figure 1).

One may also ask, how is cellulose broken down? Cellulolysis is the process of breaking down cellulose into smaller polysaccharides called cellodextrins or completely into glucose units this is a hydrolysis reaction. Horses use cellulose in their diet by fermentation in their hindgut via symbiotic bacteria which produce cellulase to digest cellulose.

Beside above, how do cows digest cellulose?

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.

Yes, like animal cells, plant cells are full of proteins. The problem is that most plant proteins&mdashand many other nutrients&mdashcan't be easily accessed because plant cells are surrounded by cell walls made of cellulose. Since human beings lack the cellulase enzyme, they cannot effectively digest cellulose.

How Can Roaches Eat so Many Disgusting Things?

Cockroaches enjoy a symbiotic relationship with some kinds of bacteria that live in their digestive systems. The bacteria use the roach as a host and in return, provide the roach with nutrients and help it to digest many nasty substances.

One study found that a huge part of the genome of American cockroaches is dedicated to metabolizing toxic or dangerous materials. Their bodies have adapted over millions of years to surviving in harsh conditions and eating whatever’s available.

Enzymatic Hydrolysis of Cellulose

The enzymatic method uses bacteria secreted proteins to hydrolyze cellulose. This method involves a lot of enzymes, namely cellulase. Research has shown that these enzymes play different roles cooperatively in the hydrolysis of cellulose: some cleave the cellulose chain from the middle into fragments containing 4-5 glucoses, some breakdown these fragments into smaller units of two glucoses, and some finally turn these small units into single glucose. [4] However, the research of enzymatic hydrolysis is still at the early stage. There are many important questions remain unanswered, such as the number of critical enzymes needed for the hydrolysis, the crystal structures of the enzymes, the exact mechanisms of hydrolysis, etc. In the next 10-20 years, the research in this area is expected to make great progresses, therefore a lot of protein engineering work will be accomplished to enhance the efficiency and lower the cost of the hydrolysis of cellulose.

What allows grass eaters to digest cellulose? - Biology

Unit Six. Animal Life

25. The Path of Food Through the Animal Body

Most animals lack the enzymes necessary to digest cellulose, the carbohydrate that functions as the chief structural component of plants. The digestive tracts of some animals, however, contain prokaryotes and protists that convert cellulose into substances the host can digest. Although digestion by gastrointestinal microorganisms plays a relatively small role in human nutrition, it is an essential element in the nutrition of many other kinds of animals, including insects like termites and cockroaches and a few groups of herbivorous mammals. The relationships between these microorganisms and their animal hosts are mutually beneficial and provide an excellent example of symbiosis.

Cows, deer, and other herbivores called ruminants have large, divided stomachs. By following the path food takes in figure 25.14, we can explore the areas of the stomach. Food enters the stomach by way of the rumen 1. The rumen, which may hold up to 50 gallons, serves as a fermentation vat in which prokaryotes and protists convert cellulose and other molecules into a variety of simpler compounds. The location of the rumen at the front of the four chambers is important because it allows the animal to regurgitate and rechew the contents of the rumen (see how the arrow leaves the stomach after looping through the rumen and reenters), an activity called rumination, or “chewing the cud.” The cud is then swallowed and enters the reticulum 2, from which it passes to the omasum 3 and then the abomasum 4, where it is finally mixed with gastric juice. Hence, only the abomasum is equivalent to the human stomach in its function. This process leads to a far more efficient digestion of cellulose in ruminants than in mammals that lack a rumen, such as horses.

Figure 25.14. Four-chambered stomach of a ruminant.

The grass and other plants that a ruminant, such as a cow, eats enter the rumen, where they are partially digested. From there, the food may be regurgitated and rechewed. The food is then transferred through the last three chambers. Only the abomasum secretes gastric juice.

In some animals such as rodents, horses, and lago- morphs (rabbits and hares), the digestion of cellulose by microorganisms takes place in the cecum, which is greatly enlarged in these animals (see the rabbit, a nonruminant herbivore, in figure 25.15). Because the cecum is located beyond the stomach, regurgitation of its contents is impossible. However, rodents and lagomorphs have evolved another way to digest cellulose that achieves a degree of efficiency similar to that of ruminant digestion. They do this by eating their feces, thus passing the food through their digestive tract a second time. The second passage makes it possible for the animal to absorb the nutrients produced by the microorganisms in its cecum. Animals that engage in this practice of coprophagy (from the Greek words copros, excrement, and phagein, eat) cannot remain healthy if they are prevented from eating their feces. The organization of the digestive system reflects the diet of the animal. Thus the large cecum of the rabbit reflects a diet of plants. In contrast, the insec- tivore and carnivore in figure 25.15 digest primarily protein from animal bodies therefore, they have a reduced or absent cecum. Ruminant herbivores, as described earlier, have a large four-chambered stomach and also a cecum, although most digestion of vegetation occurs in the stomach.

Figure 25.15. The digestive systems of different mammals reflect their diets.

Herbivores require long digestive tracts with specialized compartments for the breakdown of plant matter. Protein diets are more easily digested thus, insectivorous and carnivorous mammals have shorter digestive tracts with few specialized pouches.

Cellulose is not the only plant product that vertebrates can use as a food source because of the digestive activities of intestinal microorganisms. Wax, a substance indigestible by most terrestrial animals, is digested by symbiotic bacteria living in the gut of honeyguides, African birds that eat the wax in bees’ nests. In the marine food chain, wax is a major constituent of copepods (crustaceans in the plankton), and many marine fish and birds appear to be able to digest wax with the aid of symbiotic microorganisms.

Another example of the way intestinal microorganisms function in the metabolism of their animal hosts is provided by the synthesis of vitamin K. All mammals rely on intestinal bacteria to synthesize this vitamin, which is necessary for the clotting of blood. Birds, which lack these bacteria, must consume the required quantities of vitamin K in their food. In humans, prolonged treatment with antibiotics greatly reduces the populations of bacteria in the intestine under such circumstances, it may be necessary to provide supplementary vitamin K.

Key Learning Outcome 25.7. Much of the food value of plants is tied up in cellulose, and the digestive tract of many animals harbors colonies of cellulose-digestive microorganisms.

How Cows Eat Grass

Digestion is the process our bodies use to break down and absorb nutrients stored within food, but the ability to digest food is not the same for all animals. Cows, for example, have a very different digestive system than our own, and this allows them to thrive on a menu predominantly made up of grass.

Let’s explore how cows are able to eat grass. The key to this ability lies in the stomach. After we chew and swallow our food, the stomach serves as a holding tank where digestion begins and food starts being separated into individual nutrients. Next, food passes into the small intestine where the breakdown continues and where the body absorbs nutrients. This basic digestive process is also true of cows, but there are a few extra steps along the way.


Cows are unique in that they have fewer teeth than other animals. In the front of the mouth, teeth (known as incisors) are only located on the bottom jaw. In place of the top incisors, there is a hard leathery pad (known as the “dental pad”). In addition, cattle have a relatively immobile upper lip (compared to goats and sheep). Because of this unique oral anatomy, a cow uses its tongue to grasp a clump of grass and then bite it off. Teeth in the back of the mouth (known as molars) are located on the top and bottom jaws. Plant materials sometimes contain tough stems, but because a cow chews food in a side-to-side motion, the molars shred the grass into small pieces that are more easily digested.

The Stomach

Diagram 1. Stomach of the Cow
A = Esophagus B = Reticulum C = Rumen
D = Omasum E = Abomasum F = Small Intestine Begins

Diagram 2. Stomach of the Dog
A = Esophagus E= Stomach Body F = Small Intestine Begins

Diagram 3. The Reticulo-rumen.

All three diagrams courtesy of Sudz Publishing

On the right, a cow’s stomach is shown in Diagrams 1 and 3, and a dog’s stomach is shown in Diagram 2. Use the letters that label the stomach parts in Diagrams 1 and 2 to identify the similarities and differences between the two stomachs. Notice that the letters do more than identify the structures they also map the path food travels on its digestive journey. The dog’s stomach is a lot like our own. See how many more structures there are in the cow’s stomach? In the cow, rather than having a single pouch, there are four interconnected pouches, each with a unique function.

When a cow first takes a bite of grass, it is chewed very little before it is swallowed. This is a characteristic feature of the digestion in cows. Cows are known as “ruminants” because the largest pouch of the stomach is called the rumen. Imagine a large 55-gallon trashcan. In a mature cow, the rumen is about the same size! Its large size allows cows to consume large amounts of grass. After filling up on grass, cows find a place to lie down to more thoroughly chew their food. “But they have already eaten,” you might be thinking. This is true, but cows are able to voluntarily “un-swallow” their food. This process of swallowing, “un-swallowing”, re-chewing, and re-swallowing is called “rumination,” or more commonly, “chewing the cud.” Rumination enables cows to chew grass more completely, which improves digestion.

The reticulum is directly involved in rumination. The reticulum is made of muscle, and by contracting, it forces food into the cow’s esophagus which carries the food back to the mouth. The reticulum (letter B, Diagram 1) is sometimes called the “honeycomb” because of its distinct honeycomb-like appearance. See Figure 1 for a close-up look.

With a simple stomach, the dog, and even man, cannot digest many plant materials. A cow’s rumen is different because it functions like a large food processor. In fact, millions of tiny organisms (mainly bacteria) naturally live in the rumen and help the cow by breaking down plant parts that cannot be digested otherwise. These tiny organisms then release nutrients into the rumen. Some nutrients are absorbed right away others have to travel to the small intestine before being absorbed. To help the cow’s body capture and absorb all these nutrients, the inside of the rumen is covered by small finger-like structures (called papillae). In Figure 2, notice that the rumen wall resembles a shag carpet or the imitation wool on the inside of a winter coat. The papillae give the rumen wall this texture.

There is little separation between the first two sections of a cow’s stomach, the reticulum and the rumen (Diagram 3), so food and water pass back and forth easily. The next pouch in the stomach is the omasum (letter D, Diagram 1). This pouch acts like a giant filter to keep plant particles inside the rumen while allowing water to pass freely. By keeping grass pieces and other feed inside the rumen, bacteria have more time to break them down, providing even more nutrients for the cow. Figure 3 shows the multiple layers of the omasum.

After the grass pieces and other feed are broken down to a small enough size, they eventually pass through the omasum and enter the abomasum (letter E, Diagram 1). The prefix “Ab-,” means from, off, or away from. The abomasum, then, is located just beyond the omasum. Refer back to Diagrams 1 and 2 and notice that the center of the dog’s stomach and the abomasum of the cow’s stomach are both labeled with the letter “E”. This illustrates a similarity in function. You see, the abomasum has the same basic function as the stomach of the dog, man, or other mammal, which is the production of acids, buffers, and enzymes to break down food. After passing through the abomasum, partially digested food enters the small intestine where digestion continues and nutrients are absorbed.

Figure 1. The Reticulum.
Photo courtesy of Dr. Karen Petersen, Univ. of Washington, Dept. of Biology
Figure 2. Rumen Papillae.
Photo courtesy of Dr. Karen Petersen, Univ. of Washington, Dept. of Biology
Figure 3. The Omasum.
Photo courtesy of Dr. Karen Petersen, Univ. of Washington, Dept. of Biology

The Benefits

The rumen efficiently extracts nutrients from food other animals cannot digest. For this reason, cows can eat plant materials (such as seed coats, shells, and stems) that remain after grains are harvested for human consumption. These remaining materials are sometimes called “by-products.” Feeding by-products helps farmers and businesses save money by not having to pay to dispose of these extra materials and make money by selling the by-products as animal feed.

When oil is extracted from grains (for example, soybean oil from soybean seed and Canola oil from rapeseed), or grains are used to brew alcohol or make fuel-ethanol, plant by-products are made. Although key nutrients (like fat, sugar, and protein) are removed from the plant materials during processing, when used properly, these by-products can be fed to cows. The complex nature of their four-compartment stomachs and their rumen bacteria allow cows to eat and thrive on plant by-products that other animals cannot digest.

The better we understand the cow’s digestive system, the better we are able to formulate diets and manage our herds for the optimal production of the nutritious meat and milk we routinely enjoy. So, the next time you have a cool glass of milk, a cup of ice cream, or a juicy hamburger, you will know that these products came from cows fed grass, grain, or by-products, and you will know, How Cows Eat Grass.

Why don't animals eat wood when they eat grass? (insects excluded)

Beavers in fact, do eat wood, although they don't (contrary to comic opinion) eat the whole thing. They're interested only in the cambium, bark, and smaller shoots and twigs. As /u/cathlock said they have the bacteria and gut to manage it, but even then it's not a highly nutritious food, even less than most grazing animals manage on. As a result, you can't get very big and still meet your nutritional needs with something that is extremely tough to digest, or you need to spend most of your time eating and chewing cud.

Insects do eat wood, but not most of them many insects which bore into wood are eating sap or hunting for the larvae of other insects.

Wood has lignin, which gives wood its strength. Aside from being so hard to bite off and chew, most animals do not have the enzymes necessary to digest lignin (like what termites have). Also, in the case of termites, they have symbionts (mostly bacteria) inside their guts that help digest lignin further. Animals who eat grass don't have that.

The role of bacteria is sort of a second-order explanation here bacteria allow animals to extract sufficient nutrients from food that is deficient in some vitamins or mostly contains hard-to-digest carbohydrates. But the important distinction here is the difference between proper wood and grass/cambium/buds as food sources in the first place.

Wood is dead tissue. Unlike living tissue, it doesn't contain enzymes (=protein, =nitrogen, in however small the quantities) or simple sugars or even much in the way of mineral nutrients. Nutrients that are present are locked up by the structural material. This does contain cellulose, which is digestible by some gut microbes, but it is primarily composed of lignin, which has a more irregular structure that requires highly specialized enzymes to break down and takes a lot of moisture and a lot of time for not a lot of reward. It can't be done quickly enough to fuel an animal.

Incidentally, you can imagine a hierarchy of plant food quality, with something like a seed or nut on the high end--full of calories and nutrients--and wood on the far end. The lower the food quality, the more of it you need to eat to extract enough nutrients to survive. Ruminants solve this problem by eating medium-quality foods and extracting a lot out of each bit, while herbivores that don't have rumens either eat their own poop to increase extraction rates (rabbits), or just eat a lot of food and get as much as they can while passing it through quickly (elephants). None of those strategies can handle wood as a main food source (elephants poop a lot of wood because they eat it along with more nutritious tree bits).

I do research with cellulose! All plants have cellulose in them, this is what creates pathways for water and nutrients to be transported and also a structure around a plant cell to form the cell wall. However, different plants (and organs within each plant) have different functions (like leaves vs a tree trunk). These functions dictate how the cell walls are formed, the ratios between cellulose/lignin/hemicellulose needed to maintain its structure, the degree of crystallinity of the cellulose, the hierarchical arrangement of the nanoparticles that bind together to form the cell wall, the angle of alignment of each layer of cellulose within the cell wall, etc. Basically there are a crazy number of variables that determine the structure and behavior of plant cells, so the animals that eat cellulose-based materials won't be equipped to consume all of them.

TLDR: these plants are all made from cellulose, but they are still very different, so an animal would not be able to consume all of them the same way.

You could also look at it from an evolutionary point of view, grass grows fast and can regrow after being eaten to ground level, trees however grow slow, and if you eat the bark you kill the tree. If many grazing animals ate tree there would be none left.

That is shown in the case of elephants, that do eat trees, if the number of elephants increases too much, the trees die and so do the elephants.

We have a similar problem here in Australia, with Koalas if a local population gets too high they start to kill off the trees they need to survive. Trees just cant sustain high population grazing.

Until a proper biologist (ask me about materials and physics/chemistry) takes the lead I shall try to answer thine question! [warning ahead: sorry for crappy english]

As far as I know, it's mostly because of the bacterial life in their stomach. You see, mostly plant live-matter is made out of cellulose, which is hard already to digest. When digesting grass, it is much easier for the bacteries to decompose the "bad stuff" and give the animal (let's say, cow) either subproducts of their own like sugars, or dire access to the grass nutrients.

When dealing with dead (or alive) "hard wood" (trunks, branches. ) the process is much more energy-defficient and exausting, not really being worth the effort (and needing different chemicals and bacteries to actually do anything to the wood itself).

But, AFAIK, sometimes animals like horses or cattle would feed on small, sweet and soft roots. So heavily depends on the wood composition.

tl:dr: Bacterial life is what actually "digest" plants. Since digesting wood is not energy-efficient, there are little to no bacteries focused on that, henceforth there are a few animals able to eat wood.

Animals cannot digest Cellulose

Most plants synthesize cellulose which they use to give strength to their bodies and make them withstand the vagaries of storms. It gives the shape to the trees and branches then expand the tree.

Cellulose becomes the wood when trees become mature. Furniture is made of this material. It is obvious that cellulose is insoluble in water otherwise no one will use it in construction and furniture.

Cellulose and starch and other carbohydrates consist of glucose molecules which are arranged in chains of different styles. In starch, two chains are intertwined.

These chains get separated on boiling the starch in water. This caused the chains to disperse in the water increasing its viscosity. Such starch is called pre-gelatinized starch.

On the other hand, cellulose structure is such that chains are not dispersed and wood remains unaffected in water. Glucose is the basic unit of sugar which is used by animals like humans for obtaining energy to keep the body running.

How do then we assimilate higher sugars like starch. They have to broken down to glucose units. The enzymes found in humans and other animals allow them to digest and metabolize many, but not all, biomolecules. Cellulose is one example of a molecule that defies digestion in many animals.

But the slight difference in the way the glucose molecules are hooked together in starch compared with how they are hooked together in cellulose makes a big difference in their digestibility.

Humans and many other higher animals have the enzyme required to break the bonds in starch, releasing glucose. The particular enzyme is called alpha-amylase.

But because the shape of the linkage is different in cellulose, the same enzyme will not work. In fact, where cellulose is concerned, humans do not have an enzyme that will work.

As it turns out, most humans eat a fair amount of cellulose in the form of fruits and vegetables. Although we cannot digest it, the cellulose serves as roughage or fiber that gives food bulk and keeps it moving through the digestive system. In the end, all of the undigested material ends up being eliminated as feces.

Maybe you are wondering how animals such as cattle, sheep, deer, and goats thrive on a diet of grass or other cellulose-rich food. Can they digest cellulose when humans cannot?

The answer is no. None of these animals have the enzymes required to digest cellulose. Instead they rely on colonies of microorganisms living in their digestive systems.

These simple microorganisms have the correct enzymes to digest the cellulose and to reassemble the products into starches and proteins. From these products, grazing animals acquire their nutrients. The special relationship between these animals and their resident microbes is called symbiosis—two organisms living with each other to the benefit of both.


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Watch the video: Digestion in Grass Eating Animals (January 2023).