Why Can Humans Digest Starch But Not Cellulose? Read Here

The way these two polymers- starch and cellulose, behave is significant but very different in our bodies. You can eat and break down starch, but this is different for cellulose. So, why can humans digest starch but not cellulose?

If cellulose is so important, why can’t we digest it? Does it affect us? These are questions I will review and give adequate answers to in this article.

What is starch?

Starch is an organic white, powdered chemical that all green plants produce. It is soft, white and doesn’t have a taste. It is granular and insoluble in alcohol, cold water, and other solvents.

The basic chemical formula for starch is (C6 H10 O5)n. You synthesize starch in the green leaves of plants using extra glucose produced during photosynthesis to provide plants with a reserve food supply.

In addition, storage organs like the cassava plant roots, tubers of potatoes, the stem pith of sago, and seeds of corn, rice, and wheat hold starch. It can also store in the chloroplast in the form of granules.

When necessary, starch breaks down into constituent monomer glucose units in the presence of certain enzymes and water. It all diffuses from the cell to nourish the plant tissues.

Starch from plants breaks down into its component sugar molecules in humans and other animals, which in turn provides the tissues with energy.

Most of the starch sold is made from corn, but you can also use tapioca, potato starch, and wheat.

You can make the commercial starch by crushing or grinding tubers and seeds that contain starch, then mixing the shaft with water. The result of what you have done is a paste, free from impurities and then dried.

Apart from the basic nutritional benefits, you can use starch to brew and act as a thickening agent in baked foods.

SEE: Find Out Here if Cornstarch Go Bad After a Long While

What is cellulose?

Cellulose is a polysaccharide or complex carbohydrate made up of at least 3000 glucose units. It is the most prevalent of all naturally occurring organic substances and the fundamental structural element of plant cell walls.

Cellulose makes up around 33% of vegetable matter, 90% of cotton, and 50% of food. It is a food source for herbivorous animals like cows and horses because they can hold on to it long enough for the microorganisms in their gastrointestinal tract to break it down.

Termites have gut protozoans that can also break cellulose down. Despite all this, it is indigestible to humans.  

Why can humans digest starch but not cellulose?

Humans can digest starch but not cellulose because of certain enzymes in our bodies. We have enzymes that hydrolyze the alpha glycosidic linkages of starch but not the beta glycosidic linkages of cellulose.

This means that your body contains enzymes that can break starch down into glucose to fuel your body. The enzyme amylase can break the glycosidic connection between glucose monomers, but only if they are linked through the alpha form.

The reason is because of the variation of the bonding between cellulose and starch. Cellulose contains beta-1, 3 bonds that can not digest alpha-1, 4, and alpha-1, 6 bonds. The alpha bonds are present in starch and glycogen.

Cellulose is not soluble and largely forms crystalline and insoluble fibers. The fibers require a very unique way of degrading a set of enzymes called endo, exocellulase, and beta-glucosidases.

The ruminant animals can break down cellulose effectively because they host cellulolytic microorganisms. They host bacteria and fungi in their GI tract, and it helps.

When it comes to biological systems, you must understand that enzymes are extremely selective and often ignore particular molecules. This change happens when there is a change in the arrangements of substituents.

SEE: Find Out Here if Cornstarch Go Bad After a Long While

What are the differences between starch and cellulose?

The difference between starch and cellulose is their petrochemical configuration, especially around the glycosidic bond. While the units of starch connect by the alpha linkage, the units in cellulose connect by the beta – as explained earlier.

The difference in the unit and general orientation makes the cellulose stronger and harder to break down than starch. Amylase, particularly, is an enzyme responsible for breaking polysaccharides which are starch into dimeric units.

It is highly specific for alpha-oriented starch and would not hydrolise beta-oriented cellulose.

We can digest starch using the alpha-amylases because they hydrolyze alpha-1, 4, and alpha-1, 6 bonds. Humans do not have cellulose, so we can’t hydrolyze beta-1, 4.

If humans are herbivores or vertebrates like cattle and other ruminants, it would be easy to use cellulose as food.

The rumen of ruminant animals harbors bacteria with the ability to secrete starch. If we consume raw grass and green plants like cows, we would be able to digest cellulose.

However, there are some bacteria from the gastrointestinal tracts of humans and even other animal species that can use cellulose as an energy source. This bacteria can break the beta bond.

Why didn’t humans evolve to digest cellulose?

While digesting cellulose may give humans an advantage, it doesn’t mean our systems will evolve to break it down. For anything to go through the process of evolution, important mutations must be present or occur.

This mutation must occur or be present before any selection; if not, the trait will not evolve. It is chemically hard to digest cellulose. When you check the fossil record, you will see that when cellulose first evolved, very few things were able to break it down.

The first set of things to digest was not very good at it, resulting in lots of atmospheric carbon locked into the cellulose by plants that were like trees. The plants were the first to evolve in the ability to manufacture cellulose on a large scale, but they died.

When they died, their cellulose did not even degrade quickly at all. At the end of this, the atmospheric content of oxygen on earth increased and resulted in the popular giant insects of the Carboniferous era.

All the plants that died with cellulose still present in them did not rot; they piled up in great laws of organic woody material that ended up turning into coal. You can trace a very large amount of the coal reserves on earth to this period.

In time, some microorganisms evolved to break cellulose down; these groups include some bacteria, protozoans, and fungi. These groups are present in animals.

Apart from this, nothing else on earth succeeded in evolving the ability to digest cellulose. It shows that breaking cellulose down biochemical is very hard. If humans can’t digest cellulose, the main ingredient in plant cell walls, why do we need to eat vegetables?

Contrary to popular belief, humans do not need vegetables. From time before, we have survived in excellent perfect health, eating meat and blubber.

Many doctors in the early years reported that many Inuit people they came in contact with to treat were among the healthiest people they had ever seen.

The doctors also stated in published reports that they did not find travels of diabetes, heart disease, or cancer among people Inuit. This changed when these people started eating processed foods.

We only seem to need vegetables because almost every other food is processed and damaging to our systems. With all this, there are still processed vegetables in the market. Only a minimal amount of cellulose is in the cell walls of plants.

Should you stop eating vegetables?

No, you shouldn’t. The rest of the cell contains enzymes, minerals, sugars, antioxidants, vitamins, and other substances good for you. That’s why you should also eat vegetables to get the minerals and vitamins the body requires.

Particularly the nutrients we can not get from any other food source. These nutrients are preserved inside indigestible cell walls. This means you need to break down the cell walls to absorb them and make the nutrients available to the digestive system.

It’s possible to do this with your teeth, but they are not enough. When you eat uncooked vegetables, half of the nutrients pass through your digestive system without absorption. That’s because it is still locked in a tight net of cellulose.

It is due to digestion that we have cooking. The process of cooking breaks down hard and strong cell walls. Cooking also allows us to extract the highest quantity of nutrition from the vegetables we eat.

Regardless, you can not still digest cellulose, but you can make use of its significant benefits.

One of these benefits is dietary fiber, which enables us to maintain our colon health. It also keeps our stool soft and regulates and feeds the bacterial community that stays there.

SEE: Unique Ways to Reuse Your Vegetables


What can digest cellulose?

Animals like cows, termites, horses, and other ruminant animals can digest cellulose.

How do animals digest cellulose?

Animals do not have the enzymes that break cellulose down, but they host microbes that can digest it.

How many forms of starch exist?

Two forms of starch exist, namely, amylase and amylopectin.

Where is starch naturally found?

Starch is found in vegetables and many grains like wheat, rice, and maize.


Now that the article has answered the question, why can humans digest starch but not cellulose? You now understand that even though humans can not digest cellulose, thankfully, it still functions in your systems as fiber.

Fiber acts as an assist to your digestive system. It keeps food moving through your gut and pushes waste out of your body.

Thank you for reading.

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