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Cellulase is an enzyme capable of breaking cellulose. If humans were able to produce cellulase in our stomach would we be able to digest grass? If not, what more things would we need in order to digest grass?
No. Even for an herbivore, digesting grass, or plants to be more general, is hard, because they contain cellulose. Herbivores have different parts of a stomach, whereas humans only have one compartment, so in herbivores, the plants (or grass) enter the first part of their stomachs called the rumen, which contains a salty solution that breaks down cellulose, then, the herbivore will regurgitate the processed food, or cud, from the rumen, into their mouths, and break down the cellulose even MORE. The cud then enters back into the rumen and then digests as normal. So even if a human had cellulase, they would need a rumen, to break down the cellulose further.
Isolation of cellulose—producing microbes from the intestine of grass carp (Ctenopharyngodon idellus)
The cellulase activities of bacterial strains in the intestine of grass carp were analyzed, using filter paper and absorbent cotton as substrates and measuring the concentration of glucose by calorimetry. Six strains were isolated and determined high cellulase activity in all grass carp. Strains showed different abilities to produce cellulase, which suggests that they interact in the grass carp intestine to digest cellulose. The presence of cellulose activity suggests that grass carp have the ability to digest cellulose in the diet. The cellulase enzymatic activity increased dramatically after 6 days of culture and reached its peak at the 7th day. Microbes are probably the main source of cellulase in grass carp diets.
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You’ve probably seen cows enjoying a nice mouthful of grass, but why can't we do the same?
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Michael: Cows really do have it made. I mean, basically everywhere they look, there&rsquos grass for them to eat! We humans don&rsquot have it as easy. You could try eating grass, but it wouldn&rsquot do much for you. So what&rsquos the difference? If cows can digest grass, why can&rsquot we?
Well, cows have the tools to digest the cellulose in grass, but we don&rsquot. Our digestive systems just aren&rsquot equipped for it. Cellulose is a complex carbohydrate that consists of long chains of glucose units. It makes up plant cell walls, which is why it&rsquos found in basically all plants, like spinach, kale, and grass. Most of the plants we eat do also have some nutrients we can digest. But grass is basically all cellulose, and that cellulose is really hard to break down. Well, unless you&rsquore an animal like a cow.
When a cow munches on some grass, it travels down its esophagus, and into its four-chambered stomach. And for every serving of grass, this actually happens more than once. After the grass gets digested a little, it passes into one of the stomach chambers called the reticulum where it forms chunks called cud. Then the chunks are regurgitated. They&rsquore brought back up to the cow&rsquos mouth so the cow can grind them up a bit more and break down the food even further. Eventually, the cow swallows the food again, which makes its way back to the stomach. Sounds delicious.
It&rsquos not just cows that do this&mdashother animals, like sheep and goats, regurgitate their food too. They&rsquore called ruminants. The main area of the stomach is the largest chamber of the four: the rumen. It&rsquos where the grass-digesting magic happens. See, it&rsquos not actually the cow that&rsquos digesting the cellulose in the grass, it&rsquos the tiny microbes living inside the cow&rsquos rumen. These guys do their job without oxygen, in a process known as anaerobic cellulose digestion. It involves two main steps: enzyme production and fermentation.
In enzyme production, the microbes in the rumen secrete certain enzymes, like cellulase, which helps break down the cellulose. One of the main ways that&rsquos done is by hydrolyzing the cellulose, where a chemical reaction involving water breaks the cellulose up into smaller carbohydrates like glucose. But the enzymes are the real stars of the show, acting as catalysts that kickstart the reaction.
From there, the leftover, smaller carbohydrates are fermented, meaning that they&rsquore metabolized and converted into fatty acids like acetic acid, the acid in vinegar butyric acid, which is found in milk and propionic acid, an acid that&rsquos also often used as a food preservative. These later get absorbed as nutrients. After all that, the partially-digested grass eventually reaches the abomasum, which is the acidic part of the stomach that&rsquos similar to ours.
Here, the food is digested even more and eventually enters the cow&rsquos small and large intestines. So, the main players in grass digestion are the microbes. Humans can&rsquot digest grass because we don&rsquot have those microbes to produce the enzymes we&rsquod need to break down cellulose.
We do have the enzymes to digest other carbohydrates, like starch and simple sugars &mdash we&rsquore just missing the ones that digest cellulose. But what if you just take the same microbes that are in a cow&rsquos rumen, put them in your stomach, and let them do their thing? That probably wouldn&rsquot work, because your stomach is way too acidic for the cellulose-digesting process to happen. The
pH of your stomach is normally around 1 to 3, which is very acidic. The pH of the rumen, where the grass-digesting microbes live in cows, is closer to a more neutral 6 or 7. The microbes stop breaking down cellulose at a pH of 5.5 or lower, so putting them in your stomach wouldn&rsquot give you the ability to digest grass.
But, there are two other potential homes for these microbes: your small and large intestines. But neither is a good choice. The pH in your small intestine is much more neutral, but the microbes might try to compete with you for the nutrients in the digested food.
And your large intestine wouldn&rsquot be able to absorb the nutrients from the grass, so putting microbes in it to break down cellulose wouldn&rsquot make much of a difference. Another option might be to just swallow some cellulase &mdash like how people who are lactose-intolerant can take a pill with the lactase enzyme, which allows them to have dairy. But researchers haven&rsquot yet developed a practical method for extracting the enzymes that would allow you to digest grass, and even if they did, we don&rsquot know what effects it would have on your health. So as convenient as it would be to graze on your front lawn for dinner, it&rsquos probably best to leave the grass to the cows.
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Last week I watched the History Channel’s second installment of Evolution – this time on the evolution of guts. Pharyngula has a write up of the episode. One item that especially interested me was the bit about how cows and other ruminants evolved to eat grass. I’ve been involved over the years in several debates about whether humans can gain any benefit from drinking wheatgrass, as is sold at Jamba Juice and other health food outlets. As I wrote in Wheatgrass madness over three years ago, I don’t think we can, primarily because we can’t digest grass. The presenters of this program appeared to agree with me.
The first point the presenter made was that grass is a low-quality food – something that seems inconsistent with the claims of wheatgrass proponents that somehow wheatgrass is “superfood”. Its low quality is probably one reason cows have to eat so much of it – watch them and that’s all they do all day. It seems unlikely one small 2oz shot would make much difference, even if we could absorb its nutrients.
The second point was that only ruminants can digest grass, and I see no reason wheatgrass should be an exception. I’ve had several wheatgrass proponents respond that the juicing process breaks down the cell walls to make the nutrients available to humans, but the History Channel program made it clear there’s more to it than that. To demonstrate this they showed a surgically fistulated cow – a cow with a surgically implanted hole in its side that allows researchers to get their hands right into its gut. It sounds a little disgusting (and looks even more so - see the picture on the right), but apparently the cow is fine (although to be honest, no one actually asked the cow what it thought). In the TV program they showed you the inside of the cow’s gut through the fistula, and someone actually put a hand in to withdraw some of the partially fermented grass. (Ew!) Anyway, the conclusion was that the gut of the cow, in addition to containing unique bacteria that are necessary to break down the cellulose to smaller molecules, also acts as a fermentation vat for the grass-digestion process. This lengthy fermentation process produces cellulase enzymes that break down the cellulose. And according to The New Scientist , this fermentation process is necessarily slow to prolong contact with the microbes so they have enough time to do their job. So not only does it seem unlikely that humans can get any benefit from wheatgrass, even cows don’t get an immediate benefit – it takes days fermenting. Claims of an “instant high” or that its nutrients are “ assimilated into the blood in about 20 minutes ”, are clearly false.
Ruminants swallow the grass they graze almost without chewing and it passes down the oesophagus to the rumen and reticulum. Here liquid is added and the muscular walls churn the food. These chambers provide the main fermentation vat of the ruminant stomach. Here bacteria and single-celled animals start to act on the cellulose plant cell walls. These organisms break down the cellulose to smaller molecules that are absorbed to provide the cow or sheep with energy. In the process, the gases methane and carbon dioxide are produced. These cause the “burps” you may hear cows and sheep making.
Not only do the micro-organisms break down the cellulose but they also produce the vitamins E, B and K for use by the animal. Their digested bodies provide the ruminant with the majority of its protein requirements.
Not only do I see no reason to modify my original wheatgrass post, this Evolution program seemed to confirm what I originally thought, namely that wheatgrass is useless as food for humans.
Wheatgrass Myths v. Facts
From the above, I think we can say that most of the favorable claims for wheatgrass are myths at best. Some facts about wheatgrass:
- Wheatgrass is a low quality food that does not contain anything even remotely close to "all of the vitamins and most of the minerals we need". (See Jamba Juice's wheatgrass nutrition data: 7% daily values (%DV) of Vitamin C 10% iron zero percent of everything else. Zero! Sheesh - eat an orange.)
- Wheatgrass does not contain enzymes that aid human digestion. Any enzymes that wheatgrass contains are enzymes that help wheatgrass metabolize its food wheatgrass enzymes do not help you digest wheatgrass or anything else. (They may both be called "enzymes", but they are different molecules.)
- Special microbes and enzymes are necessary in the gut to break down the cellulose in grass. Humans do not have these enzymes, and so humans can't digest grass, whether it is juiced first or not.
- The digestion process, even in ruminants, is necessarily slow. This means that (a) human digestion would not be slow enough to digest wheatgrass (even if human guts had the necessary microbes, which they don't), and (b) if it was that slow (and had the microbes), wheatgrass would still not be absorbed into the body "within 20 minutes" as is claimed.
- Likewise, claims of a "high" after drinking wheatgrass are due either to the power of suggestion or some physiological reaction due to the body not being able to digest what it was just fed.
- Because it cannot be digested, wheatgrass cannot possibly be an energizer, build your blood (whatever that means), cleanse your body, or do any of the other magical things claimed for it.
So there you have it. On the plus side, aren't you impressed I didn't use any version of "Holy Cow" in the headline?
Can humans digest cellulose?
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.) indigestible 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.
One of the comments indicated the reader is confused as to whether termites have the necessary enzymes to digest cellulose. The answer indicates, correctly, that they do not have the enzymes (innately). Instead, they have a symbiotic relationship with a bacteria that provides the needed enzymes. In other words, they have them, but only because a friendly organism supplies them with them.
If humans had cellulase would they be able to digest grass? - Biology
There are two types of hydrogen bonds in cellulose molecules: those that form between the C3 OH group and the oxygen in the pyranose ring within the same molecule and those that form between the C6 OH group of one molecule and the oxygen of the glucosidic bond of another molecule. Ordinarily, the beta-1,4 glycosidic bonds themselves are not too difficult to break. However, because of these hydrogen bonds, cellulose can form very tightly packed crystallites. These crystals are sometimes so tight that neither water nor enzyme can penetrate them only exogluconase, a subgroup of cellulase that attacks the terminal glucosidic bond, is effective in degrading it. The inability of water to penetrate cellulose also explains why crystalline cellulose is insoluble. On the other hand, amorphous cellulose allows the penetration of endogluconase, another subgroup of cellulase that catalyzes the hydrolysis of internal bonds. The natural consequence of this difference in the crystalline structure is that the hydrolysis rate is much faster for amorphous cellulose than crystalline cellulose. The process of breaking the glucosidic bonds that hold the glucose basic units together to form a large cellulose molecule is called hydrolysis because a water molecule must be supplied to render each broken bond inactive. In addition to crystallinity, the chemical compounds surrounding the cellulose in plants, e.g. lignin, also limit the diffusion of the enzyme into the reaction sites and play an important role in determining the rate of hydrolysis. Sometimes, wood chips are pretreated with acid at approximately 160°C to strip hemicellulose and lignin before they are treated with an enzyme or a mixture of enzymes. In general, 20 to 70% yield of glucose can be expected after 24 hours.
The conversion of cellulose into glucose is now known to consist of two steps in the enzyme system of Trichoderma viride. In the first step, beta-1,4 glucanase breaks the glucosidic linkage to cellobiose, which is a glucose dimer with a beta-1,4 bond as opposed to maltose, a counterpart with an alpha-1,4 bond. Subsequently, this beta-1,4 glucosidic linkage is broken by beta-glucosidase: The kinetics of cellulose hydrolysis has been widely studied, and Michaelis-Menten types of rate expressions with substrate or product inhibition terms have been proposed to describe the observed reaction kinetics.
A wide variety of fungal and bacterial species produce cellulase and transport the enzyme across the cell membrane to the outside environment. Although it is common to refer to a mixture of compounds that can degrade cellulose as cellulase, it is really composed of more than one distinctive enzymes. Recent research has shown that one of the components is relatively inert with the ability of recognizing and attaching itself to the surface of the cellulose mass, in addition to the ability of recognizing and holding onto another protein component that exhibits enzymatic activities. Thus, the chance of reaction is significantly enhanced by a proximity effect, because the active enzyme is held onto the surface of a solid substrate by an inert protein which acts as a glue.
The species most often used to study the production of cellulase are white-rot fungal cultures of Trichoderma ressei and Trichoderma viride. We all have seen a piece of rotting wood. And perhaps without knowing it, we are actually quite accustomed to the appearance and action of this fungi. As in Experiment No. 1, it is only natural that the most promising place to search for cellulase is in a piece of rotting wood. The microorganisms responsible for this enzyme can easily be isolated from a piece of rotting wood, or from a termite's gut if bacterial species are desired. Other fungal species often used are Fusarium solani, Aspergillus niger, Penicillium funicolsum, and Cellulomonas sp. The bacterial species Clostridium thermocellum and Clostridium thermosaccharolyticum also represent promising candidates for cellulase production because they are thermophilic (less contamination problem and faster rate at a high temperature), anaerobic (no oxygen transfer limitation), and ethanologenic (conversion of cellulose to ethanol via glucose with a single culture). In general, different species of microorganisms produce different cellulolytic enzymes.
List of Reagents and Instruments
- Erlenmeyer flasks
- Graduated cylinder
- Pipets, 1ml, 10ml
- Test tubes
- Incubator or thermostated shaker
- Temperature bath (or heat source -- Bunsen burner or hot plate)
- Filter holder
- Filter paper
- Cellulose source (filter paper, wood chips, carboxymethyl cellulose, cotton)
- Cellulase, buffered at pH=5.00۪.01, 10g/l solution
- HCl, 5% solution
- H2SO4, 5% solution
- Reagents for sugar analysis
- Enzymatic Hydrolysis: Repeat the same procedures for shredded wood chips (a complex and impure mixture of cellulose, lignin, and a variety of others), carboxymethyl cellulose (a model amorphours-structured cellulose), and cotton (90 % cellulose, mostly crystalline-structured). If time permits and if there is extra enzyme solution, try other sources of biomass and waste materials such as newsprint, grass, straw, and corn stalk. See Note 1.
- Shred a 10 cm 2 piece of cellulose filter paper and weigh 0.1 g. (As opposed to other type of papers with binding materials, a piece of cellulose filter paper without wetting agents has minimum impurities and is almost pure in cellulose. The result of a quantitative analysis using a filter paper would have been very unreliable had impurities leached out into the filtrate.
- Submerge the shredded paper in 10 ml of the buffered cellulase solution in a test tube. Note the starting time.
- Incubate the mixture at 40°C. (The enzyme is most active at a temperature of 40°C and a pH of approx. 4.5.)
- This reaction should last for approximately 24 hours. Take 1 ml samples at some predetermined appropriate intervals. Note that one does not have much to waste because the starting sample is small. (A volume of 1 ml is actually considered as a huge sample when working with biochemicals.)
- Stop the hydrolysis reaction in the sample. The first method of stopping the reaction is to deprive the mixture of substrate. This can be easily achieved by filtering out the residual solid material from the solution. The individual samples may be stored frozen for later analysis. The samples are thawed and brought to room temperature before they are subjected to measurements. However, this first method is not applicable to soluble cellulose, e.g., CMC. Alternatively, the enzymatically catalyzed reactions can be halted either by adding a strong enzyme inhibitor or by raising the temperature of the mixture to 90°C for 5-10 minutes in a heated bath to inactivate the enzyme.
- Measure the glucose concentrations of the samples with the dinitrosalicylate colorimetric method. (Reference: Gail Lorenz Miller, Use of dinitrosalicylic acid reagent for determination of reducing sugar, Analytical Chemistry,31, 427, 1959.) See Note 2.
- Acid Hydrolysis (Sulfuric Acid): Use the same cellulose sources as in enzymatic hydrolysis.
- Add 0.2g cellulose to 10ml of 5% H2SO4 solution in a lightly capped test tube. See Note 3. One may choose to carry out the reaction at 90°C instead of at room temperature.
- This reaction should last for 2 hours. Take 1 ml samples at some predetermined appropriate intervals.
- Stop the hydrolysis reaction in the sample by neutralizing the acid and slightly reversing the pH with the addition of a small volume of a concentrated potassium hydroxide solution. Make a quick calculation to see how much KOH is needed for this purpose. Note that one has to keep a close track of the volume of KOH solution added because this information will be needed to calculate the glucose concentration in the original undiluted sample.
- Measure the glucose concentration of the alkaline sample.
- Acid Hydrolysis (Hydrochloric Acid): Substitute 5% sulfuric acid with 5% hydrochloric acid and repeat the same procedures as in sulfuric acid.
There has been a large amount of research work done on the digestion of cellulose into glucose. The generated glucose can be used to produce single cell protein as food for livestock or even for humans. Glucose can also be used as the starting raw material in the production of a wide variety of chemicals and fuels. This is usually carried out with the help of microorganisms. For example, glucose can be easily fermented to ethanol by Saccharomyces cerevisiae (yeast) or Pseudomonas mobilis (bacterium). Ethanol can be used as gasoline or processed further to make other common petrochemicals. Another example is the conversion of glucose into solvents such as acetone and butanol by Clostridium acetobutylicum. Because the volume of cellulose is so overwhelming and because the resource is renewable, the world will likely to depend on it more heavily for food, fuel, chemical supplies, and raw materials in the future. It has the great potential of alleviating the need for petroleum, whose supply is fast dwindling.
Thus, the ability to manipulate this organic chemical has extremely important implications. A breakthrough in the investigation of cellulose digestion processes will not only have an enormous impact on the world food supply, economy, and geopolitical balance of power, it will also greatly influence the various types and ways products are produced by the chemical industry and enjoyed by the end users. This experiment introduces a student in biochemical engineering to one of tomorrow's technologies with the most far-reaching impacts.
As demonstrated in this experiment, the breaking down some of the cellulose is really not very difficult. However, translating a process from a laboratory scale to a commercial scale is not so trivial. First of all, the entire operation has to be both technically sound and economically feasible. In order for a process to be actually adapted, it, of course, has to be technically possible first. In addition, it must offer some clear advantage over all other competing processes. This advantage is almost always measured in the form of a larger profit margin, irrespective of the political system in which the process is to be employed. Note that in calculating the profit, one must duly include various costs that are sometimes not obvious nor easy to estimate, e.g. the public images, institutional responsibilities, and environmental impacts. Unprofitable processes are a waste of natural and human resources and must not survive. As a chemical engineers, whether conducting basic research or designing a plant, one is continually reminded of the economical impact.
Two typical approaches to effect a similar end result are studied in this experiment. However, one should keep in mind that there are numerous other competing approaches, and one is constantly faced with multiple choices. For example, acetic acid can be produced by fermentation means or chemical synthesis. So are a wide range of pharmaceuticals. As a matter of fact, life is rarely simple and straight forward enough that there is only one choice.
Bonnethead Sharks Consume and Digest Seagrass
A small coastal shark called the bonnethead shark (Sphyrna tiburo) eats copious amounts of seagrass (Thalassia testudinum) and has adaptations in its digestive system to process vegetation, according to new research.
Leigh et al investigated the digestive function of bonnethead sharks in order to determine whether they can digest and assimilate nutrients from seagrass. Image credit: Mills Baker / CC BY 2.0.
The bonnethead shark is a member of the hammerhead shark genus Sphyrna in the family Sphyrnidae.
This species is commonly found in shallow estuaries and bays over seagrass, mud and sandy bottoms at depths from 33 to 263 feet (10-80 m).
It ranges from New England, where it is rare, to the Gulf of Mexico and Brazil and from southern California to Ecuador. It is common in the inshore waters of the Carolinas and Georgia in summer, and off Florida and in the Gulf of Mexico in spring, summer, and fall.
On average, bonnethead sharks are about 2-3 feet (61-91 cm) long, with a maximum size of about 5 feet (1.5 m). Females tend to be larger than males. The body is grey-brown above and lighter on the underside.
The first evidence for plant-eating bonnethead sharks came from an unusual discovery published in 2007.
Dana Bethea, a research ecologist with NOAA Fisheries in Florida, and colleagues examined the stomach contents of bonnethead sharks in the Gulf of Mexico, and were surprised to find that more than half of the material they had ingested was seagrass.
However, it was unclear if the sharks were actually consuming grass and extracting nutrients from it, or just accidentally swallowing it as they hunted for crabs and shrimp hiding in the vegetation.
This work inspired University of California Irvine researcher Samantha Leigh and co-authors to investigate the ability of bonnetheads to digest plants.
“We captured several individuals and brought them into the laboratory at Florida International University, where they were fed a diet of 90% seagrass for several weeks,” the scientists said.
“The seagrass had been labeled with stable isotope carbon-13, so when the sharks consumed it, we could test for a signature of carbon-13 in the sharks’ tissues and see if nutrients from the seagrass were actually taken up into the body.”
“We also collected the shark’s feces, to see how much of the seagrass nutrients (such as carbohydrates, proteins, etc) was simply excreted undigested.”
“Further, we looked at digestive enzymes in the intestines of the bonnetheads, to see if they even have any ability to break down plant material.”
“A purely carnivorous animal should have no mechanism to digest plants, but if the bonnethead sharks eat seagrass regularly, they should have enzymes for this purpose.”
The results were conclusive: carbon-13 from the labeled seagrass was found in the shark’s blood, so they were fully digesting and incorporating nutrients from the grass into their bodies, not just excreting it as waste.
Out of the total grass consumed, about half was actually digested and broken down by the gut, and half was excreted undigested.
The team also found that the sharks had the digestive enzyme b-glucosidase in their guts, which breaks down cellulose, an important component of plant matter.
This is the first finding of plant-specific digestive enzymes in sharks.
On top of this, the bonnetheads seemed perfectly content on their 90% vegetarian diet.
“We observed no negative health effects, and the sharks even gained weight during the study,” the researchers noted.
“While in the wild bonnethead sharks would likely eat less than 90% seagrass, the ability to thrive on such a high plant diet is further support for their ability to obtain nutrients from seagrass.”
“Adaptations for omnivory may allow the bonnetheads to be generalists as opposed to strictly predators, giving them flexibility to consume both plants and protein,” Leigh said.
“We always think of sharks as these apex predators, but here is this shark that is not really acting like an apex predator at all… but more like an omnivore.”
It is still unknown whether the sharks intentionally consume grass in the wild by grazing or if they ingest plants accidentally and have adapted a digestive mechanism to take advantage of that.
“A greater concern is what may happen to bonnethead sharks if these seagrass meadows, which are currently threatened, are destroyed,” the authors said.
“While bonnetheads are not currently endangered, this research indicates seagrass is an important part of their diet, in addition to their habitat.”
The structure of cellulose
Cellulose is usually described by chemists and biologists as a complex carbohydrate (pronounced car-bow-HI-drayt). Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen that function as sources of energy for living things. Plants are able to make their own carbohydrates that they use for energy and to build their cell walls. According to how many atoms they have, there are several different types of carbohydrates, but the simplest and most common in a plant is glucose. Plants make glucose (formed by photosynthesis) to use for energy or to store as starch for later use. A plant uses glucose to make cellulose when it links many simple units of glucose together to form long chains. These long chains are called polysaccharides (meaning "many sugars"
and pronounced pahl-lee-SAK-uh-rydes), and they form very long molecules that plants use to build their walls.
It is because of these long molecules that cellulose is insoluble or does not dissolve easily in water. These long molecules also are formed into a criss-cross mesh that gives strength and shape to the cell wall. Thus while some of the food that a plant makes when it converts light energy into chemical energy (photosynthesis) is used as fuel and some is stored, the rest is turned into cellulose that serves as the main building material for a plant. Cellulose is ideal as a structural material since its fibers give strength and toughness to a plant's leaves, roots, and stems.
Consider Corn Anatomy
Corn kernels are the seeds of the plant. The tough outer covering of the kernel is called the pericarp, or bran coat. This portion of the kernel remains largely intact in your digestive tract. Inside the bran coat are the germ and the endosperm.
The germ is the would-be plant portion at the base of the kernel. The endosperm, which makes up the majority of the kernel, provides nourishment to the germ.
Corn is a classified as a starchy vegetable, with 98 to 99 percent of the corn's starch located in the endosperm, according to an article published in the February 2016 issue of the journal Comprehensive Reviews in Food Science and Food Safety.
Turning Waste Into Food: Cellulose Digestion
Fiber constitutes an essential element in the human diet. It has been shown to prevent cholesterol absorption and heart disease and help control diabetes (1). The National Academy of Sciences Institute of Medicine recommends the adult male consume at least 38 grams of soluble fiber per day – the only kind of fiber humans can digest (1). The other more abundant type of fiber, insoluble fiber, passes through the human digestive system virtually intact and provides no nutritional value.
What if humans could digest fiber? Cellulose, the main type of insoluble fiber in the human diet, also represents the most abundant organic compound on Earth (2). Almost every plant has cell walls made from cellulose, which consists of thousands of structurally alternating glucose units (Fig. 1). This configuration gives cellulose its strength but prevents it from interacting with human enzymes. Cellulose contains just as much energy as starch because both molecules consist of glucose subunits. It is only possible to use that energy by burning wood and other cellulose materials. However, if that energy were physiologically available, humans could lower their food consumption and produce much less digestive waste than they currently do.
Figure 1: Structure of 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).
Figure 2: The organs of the human digestive system.
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.
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?
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 be 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.
Cellulose has many forms, some of which are more complex and harder to break down than others. Some of the microbes in the rumen, such as Fibrobacter succinogenes, produce cellulase that breaks down the more complex forms of cellulose in straw while others such as Ruminococci produce extracellular cellulase that hydrolyzes the simpler amorphous type of cellulose (7). Conveniently, cellulose hydrolysis produces several byproducts, such as cellobiose and pentose disaccharides, which are useful to rumen microbes. The reactions produce other byproducts such as methane, which is eventually passed out of the ruminant (7). Thus, the microbes and ruminants live symbiotically so that the microbes produce cellulase to break down cellulose for the ruminants while gaining a food source for their own sustenance.
Figure 3: The ruminant digestive system
The various microbes within ruminants may hydrolyze certain types of cellulose, but ruminants still cannot eat wood or cotton. Termites, on the other hand, can feed on various types of wood. It was believed for a long time that termites also depended on microorganisms that lived inside their bodies to digest cellulose for them, but research in the late 1990s showed that certain types of termites had the ability to produce enough cellulases and xylanases in the midgut to support their own survival (8). However, other species of termites do not have the capacity to produce enough cellulase independently and must depend on microbes from the domains Archaea, Eubacteria and Eucarya to break down cellulose. Regardless of the various levels of termite independence, there exists a symbiotic relationship between termites and over 400 species of microorganisms, analogous to that of ruminants and their microbes (8). The termite gut is even designed to provide energy-yielding substrates for the microbes (8).Both protists and fungi are attributed to the production of supplementary enzymes, but their specific roles and mechanisms are still being debated because isolating pure cultures has proven technically difficult. Despite the ubiquity of these microbes and the benefits they bring to ruminants and termites, research has yet to fully elucidate their mechanisms.
People have long been interested in tapping into the energy in cellulose. However, most companies and research groups are only focused on ways to harness that energy as biofuel and not as food. Major research is aimed at converting cellulosic material into ethanol, although that process is still inefficient and requires refinement.
Cellulose must first be hydrolyzed into smaller sugar components such as glucose, pentose or hexose before it can be fermented into bioethanol (9). One method uses acids to hydrolyze cellulose but this can destroy much of the sugar in the process. Another way to hydrolyze cellulose is by mimicking the microorganisms inside ruminants and termites. Bioenergy engineers can use the enzymes produced by microbes to break down cellulose. However, enzymes have biological limitations and implement natural feedback inhibition that poses a problem for industrial manufacturing (9). Other technical barriers to efficient enzymatic hydrolysis include the low specific activity of current commercial enzymes, the high cost of enzyme production, and a lack of understanding of the mechanisms and biochemistry of the enzymes (9).
Companies and governments all over the are eager to invest heavily in research to turn biomass into biofuel, which could bring enormous benefits to the world economy and environment. Biomass is readily available, biodegradable, and sustainable, making it an ideal choice as a source of energy for both developed and developing countries. This could also help reduce waste problems plaguing society today. The United States produces 180 million tons of municipal waste per year, and about fifty percent of this is cellulosic and could potentially be converted into energy with the right technology (10).
Cellulose Digestion in Humans
The benefits of turning cellulose into biofuel are just as relevant when considering engineering humans to digest cellulose as a food source. Right now, technology focuses on controlling cellulose hydrolysis and processing in factories, but perhaps in the future humans could serve as the machine for extracting energy from cellulose, especially since the enzymes used to hydrolyze cellulose are hard to isolate in large quantities for industrial use. Termites themselves are tiny creatures, but as a colony, they can break down houses and entire structures. A healthy human digestive system already carries an estimated 1 kg of bacteria, so adding a couple of extra harmless types should not pose a problem (11).
Termites and ruminants serve as a great example of how organisms can use microbes effectively. However, the human body would need some adjustments to introduce the microbes into the body. Our stomach is much too acidic for most microbes to survive. The acid, among other secretions and enzymes, follows the food into the small intestine, where the microbes might end up competing with us for food. By the time the food has reached the large intestines, only the cellulosic material is left for dehydration and possibly hydrolysis. However, our large intestines lack the ability to absorb the sugars that the microbes would produce from hydrolysis. Perhaps another organ could be added to the end of the human gastrointestinal tract to especially accommodate cellulose-digesting microbes. Modern medicine allows safe inter-species transplantation, but the ideal solution would be to genetically engineer humans to develop the organs themselves to avoid he complications of surgery and organ transplantation. Genetic engineering for the purpose of treating disease and illness is still undergoing intense debate, so nonessential pursuits such as cellulose digestion will not be possible until the scientific and medical communities accept genetic engineering as a safe and practical procedure.
A simpler solution would be to take supplements similar to the ones used to treat lactose intolerance. Cellulose broken down in the stomach can be absorbed as glucose. Extracting the right enzymes to work in the human stomach can bypass the problems of supporting microbes inside the human body. Additionally, since the process would occur inside the human body, the limitations that posed a problem for commercial hydrolysis of cellulose would become necessary biological controls. In the case of lactose intolerance, lactase is easily extracted from yeast fungi such as Kluyveromyces fragilis, so perhaps the easiest solution for cellulose indigestion is to extract the appropriate enzyme from the right microbes (12). As mentioned previously, the commercial extraction of enzymes is not yet practical. As previously stated, this field of human enhancement does not receive much research because companies and funding institutions are much more interested in the lucrative biofuel industry. Consequently, many questions remain unasked and unanswered. For example, what would the removal of cellulose weight from stool do to the process of defecation? What other effects might the microbes have on the human body? How do we deal with the other byproducts of cellulose hydrolysis such as methane production?
These questions could be analyzed through observation. Other mammals have survived many millennia by digesting cellulose with microbes, and since humans are mammals, there are no underlying reasons why human bodies cannot be compatible with these organisms. The microbes that currently reside in the human body already produce gases inside the digestive system, ten percent of which is methane (3). Methane production used to be viewed as a problem at cattle ranches and dairy farms, but methane itself is a highly energetic biogas that can be used as fuel. Harnessing it might prove difficult considering that current social graves do not favor open flatulence even for the sake of renewable energy. However, certain diets richer in alfalfa and flaxseed have been proven to reduce methane production in cows, which could potentially solve that problem (13).
Vegetation, which is severely lacking in the modern diet, is the major source of insoluble fiber. Vegetables contain many vitamins, nutrients, and soluble fiber, which has numerous health benefits as mentioned in the introduction. Adding these foods to our diet after adding cellulose-digesting capabilities could help assuage the obesity epidemic and significantly improve human health.
Ultimately, improving human digestion could vastly reduce waste generated by humans and increase the efficiency of human consumption. We only need to better observe and understand those particular microbes to integrate them into our bodies, which are already structurally favorable for such a change. With the successful integration of microbes, we could cut down on food intake by making use of the energy in previously indigestible cellulose, reduce cellulosic waste by turning it into food, solve problems of food shortages by making algae, grass, straw, and even wood edible, and eventually turn human bodies into a source of renewable energy.
4. H. B. Melvin, Pediatrics. 118, 1279-1286 (2006).
6. D. C. Church, Digestive Physiology and Nutrition of Ruminants (O & B Books, Corvallis, Oregon, 1979).
7. R. L. Baldwin, R.L., Modeling Ruminant Digestion and Metabolism (Chapman & Hall, London, UK, 1995).
8. T. Abe, D. E. Bignell, M. Higashi, Ed., Termites: Evolution Sociology, Symbiosis, Ecology (Kluwer Academic Publishers, Dordrecht, Netherlands, 2000).
9. A. Demirbas, Biofuels (Springer-Verlag London Limited, London, UK, 2009).
By the end of this section, you will be able to do the following:
- Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants
- Explain carbohydrate classifications
- List common monosaccharides, disaccharides, and polysaccharides
Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet. Grains, fruits, and vegetables are all natural carbohydrate sources that provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.
The stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule represents carbohydrates . In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Scientists classify carbohydrates into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides (mono- = “one” sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R’), it is a ketose. Depending on the number of carbons in the sugar, they can be trioses (three carbons), pentoses (five carbons), and/or hexoses (six carbons). (Figure) illustrates monosaccharides.
The chemical formula for glucose is C6H12O6. In humans, glucose is an important source of energy. During cellular respiration, energy releases from glucose, and that energy helps make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn provides energy requirements for the plant. Humans and other animals that feed on plants often store excess glucose as catabolized (cell breakdown of larger molecules) starch.
Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are isomers) because of the different arrangement of functional groups around the asymmetric carbon. All these monosaccharides have more than one asymmetric carbon ((Figure)).
What kind of sugars are these, aldose or ketose?
Glucose, galactose, and fructose are isomeric monosaccharides (hexoses), meaning they have the same chemical formula but have slightly different structures. Glucose and galactose are aldoses, and fructose is a ketose.
Monosaccharides can exist as a linear chain or as ring-shaped molecules. In aqueous solutions they are usually in ring forms ((Figure)). Glucose in a ring form can have two different hydroxyl group arrangements (OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the ring formation process). If the hydroxyl group is below carbon number 1 in the sugar, it is in the alpha (α) position, and if it is above the plane, it is in the beta (β) position.
Disaccharides (di- = “two”) form when two monosaccharides undergo a dehydration reaction (or a condensation reaction or dehydration synthesis). During this process, one monosaccharide’s hydroxyl group combines with another monosaccharide’s hydrogen, releasing a water molecule and forming a covalent bond. A covalent bond forms between a carbohydrate molecule and another molecule (in this case, between two monosaccharides). Scientists call this a glycosidic bond ((Figure)). Glycosidic bonds (or glycosidic linkages) can be an alpha or beta type. An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane, and a beta bond is formed when the OH group on the carbon-1 is above the ring plane.
Common disaccharides include lactose, maltose, and sucrose ((Figure)). Lactose is a disaccharide consisting of the monomers glucose and galactose. It is naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is comprised of glucose and fructose monomers.
A long chain of monosaccharides linked by glycosidic bonds is a polysaccharide (poly- = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of joined monomers. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.
Plants store starch in the form of sugars. In plants, an amylose and amylopectic mixture (both glucose polymers) comprise these sugars. Plants are able to synthesize glucose, and they store the excess glucose, beyond the their immediate energy needs, as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a food source for humans and animals. Enzymes break down the starch that humans consume. For example, an amylase present in saliva catalyzes, or breaks down this starch into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.
Glucose starch comprises monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond. As (Figure) illustrates, unbranched glucose monomer chains (only α 1-4 linkages) form the starch whereas, amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).
Glycogen is the storage form of glucose in humans and other vertebrates and is comprised of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen breaks down to release glucose in a process scientists call glycogenolysis.
Cellulose is the most abundant natural biopolymer. Cellulose mostly comprises a plant’s cell wall. This provides the cell structural support. Wood and paper are mostly cellulosic in nature. Glucose monomers comprise cellulose that β 1-4 glycosidic bonds link ((Figure)).
As (Figure) shows, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. While human digestive enzymes cannot break down the β 1-4 linkage, herbivores such as cows, koalas, and buffalos are able, with the help of the specialized flora in their stomach, to digest plant material that is rich in cellulose and use it as a food source. In some of these animals, certain species of bacteria and protists reside in the rumen (part of the herbivore’s digestive system) and secrete the enzyme cellulase. The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in ruminants’ digestive systems. Cellulases can break down cellulose into glucose monomers that animals use as an energy source. Termites are also able to break down cellulose because of the presence of other organisms in their bodies that secrete cellulases.
Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, the exoskeleton, which protects their internal body parts (as we see in the bee in (Figure)). This exoskeleton is made of the biological macromolecule chitin , which is a polysaccharide-containing nitrogen. It is made of repeating N-acetyl-β-d-glucosamine units, which are a modified sugar. Chitin is also a major component of fungal cell walls. Fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.
Registered Dietitian Obesity is a worldwide health concern, and many diseases such as diabetes and heart disease are becoming more prevalent because of obesity. This is one of the reasons why people increasingly seek out registered dietitians for advice. Registered dietitians help plan nutrition programs for individuals in various settings. They often work with patients in health care facilities, designing nutrition plans to treat and prevent diseases. For example, dietitians may teach a patient with diabetes how to manage blood sugar levels by eating the correct types and amounts of carbohydrates. Dietitians may also work in nursing homes, schools, and private practices.
To become a registered dietitian, one needs to earn at least a bachelor’s degree in dietetics, nutrition, food technology, or a related field. In addition, registered dietitians must complete a supervised internship program and pass a national exam. Those who pursue careers in dietetics take courses in nutrition, chemistry, biochemistry, biology, microbiology, and human physiology. Dietitians must become experts in the chemistry and physiology (biological functions) of food (proteins, carbohydrates, and fats).
Benefits of Carbohydrates
Are carbohydrates good for you? Some often tell people who wish to lose weight that carbohydrates are bad and they should avoid them. Some diets completely forbid carbohydrate consumption, claiming that a low-carbohydrate diet helps people to lose weight faster. However, carbohydrates have been an important part of the human diet for thousands of years. Artifacts from ancient civilizations show the presence of wheat, rice, and corn in our ancestors’ storage areas.
As part of a well balanced diet, we should supplement carbohydrates with proteins, vitamins, and fats. Calorie-wise, a gram of carbohydrate provides 4.3 Kcal. For comparison, fats provide 9 Kcal/g, a less desirable ratio. Carbohydrates contain soluble and insoluble elements. The insoluble part, fiber, is mostly cellulose. Fiber has many uses. It promotes regular bowel movement by adding bulk, and it regulates the blood glucose consumption rate. Fiber also helps to remove excess cholesterol from the body. Fiber binds to the cholesterol in the small intestine, then attaches to the cholesterol and prevents the cholesterol particles from entering the bloodstream. Cholesterol then exits the body via the feces. Fiber-rich diets also have a protective role in reducing the occurrence of colon cancer. In addition, a meal containing whole grains and vegetables gives a feeling of fullness. As an immediate source of energy, glucose breaks down during the cellular respiration process, which produces ATP, the cell’s energy currency. Without consuming carbohydrates, we reduce the availability of “instant energy”. Eliminating carbohydrates from the diet is not the best way to lose weight. A low-calorie diet that is rich in whole grains, fruits, vegetables, and lean meat, together with plenty of exercise and plenty of water, is the more sensible way to lose weight.
For an additional perspective on carbohydrates, explore “Biomolecules: the Carbohydrates” through this interactive animation.
Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders. Scientists classify carbohydrates as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that form as a result of dehydration reactions, forming disaccharides and polysaccharides with eliminating a water molecule for each bond formed. Glucose, galactose, and fructose are common monosaccharides whereas, common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched. Cellulose is an example of an unbranched polysaccharide whereas, amylopectin, a constituent of starch, is a highly branched molecule. Glucose storage, in the form of polymers like starch of glycogen, makes it slightly less accessible for metabolism however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause the cell to uptake excessive water.