Created by: CK-12/Adapted by Christine Miller

The Cellulose of Our Lives

Created by: CK-12/Adapted by Christine Miller

Where would we be without our jeans? They have been the go-to pants for many people for decades, and they are still as popular as ever. Jeans are made of denim, a type of cotton fabric. Cotton is a soft, fluffy fibre that grows in a protective case around the seeds of cotton plants. The fibre is almost pure cellulose. Cellulose is the single most abundant biochemical compound found in Earth’s living things, and it’s one of several types of carbohydrates.

What Are Carbohydrates?

Carbohydrates are the most common class of biochemical compounds. They include sugars and starches. Carbohydrates are used to provide or store energy, among other uses. Like most biochemical compounds, carbohydrates are built of small repeating units, or monomers, which form bonds with each other to make larger molecules, called polymers. In the case of carbohydrates, the small repeating units are known as monosaccharides. Each monosaccharide consists of six carbon atoms, as shown in the model of the monosaccharide glucose shown in Figure 3.4.2.

Sugars are the general name for sweet, short-chain, soluble carbohydrates, which are found in many foods. Their function in living things is to provide energy. The simplest sugars consist of a single monosaccharide. They include glucose, fructose, and galactose. Glucose is a simple sugar that is used for energy by the cells of living things. Fructose is a simple sugar found in fruits, and galactose is a simple sugar found in milk. Their chemical structures are shown in Figure 3.4.3. All monosaccharides have the formula C₆H₁₂O₆.

Some carbohydrates consist of hundreds — or even thousands! — of monosaccharides bonded together in long chains. These carbohydrates are called polysaccharides (“many saccharides”). Polysaccharides are also referred to as complex carbohydrates. Complex carbohydrates that are found in living things include starch, glycogen, cellulose, and chitin. Each type of complex carbohydrate has different functions in living organisms, but they generally either store energy or make up certain structures in living things.

Starch

Starch is a complex carbohydrate that is made by plants to store energy. For example, the potatoes pictured in Figure 3.4.4 are packed full of starches that consist mainly of repeating units of glucose and other simple sugars. The leaves of potato plants make sugars by photosynthesis, and the sugars are carried to underground tubers where they are stored as starch. When we eat starchy foods such as potatoes, the starches are broken down by our digestive system into sugars, which provide our cells with energy. Starches are easily and quickly digested with the help of digestive enzymes such as amylase, which is found in the saliva. If you chew a starchy saltine cracker for several minutes, you may start to taste the sugars released as the starch is digested.

Animals do not store energy as starch. Instead, animals store extra energy as the complex carbohydrate glycogen. Glycogen is a polysaccharide of glucose. It serves as a form of energy storage in fungi (as well as animals), and it is the main storage form of glucose in the human body. In humans, glycogen is made and stored primarily in the cells of the liver and muscles. When energy is needed from either storage area, the glycogen is broken down to glucose for use by cells. Muscle glycogen is converted to glucose for use by muscle cells, and liver glycogen is converted to glucose for use throughout the rest of the body. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of lipids, which are the primary form of energy storage in animals.

Glycogen plays a critical part in the homeostasis of glucose levels in the blood. When blood glucose levels rise too high, excess glucose can be stored in the liver by converting it to glycogen. When glucose levels in the blood fall too low, glycogen in the liver can be broken down to glucose and released into the blood.

Cellulose

Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands of linked glucose units. Cellulose is an important structural component of the cell walls of plants and many algae. Human uses of cellulose include the production of cardboard and paper, which consist mostly of cellulose from wood and cotton. The cotton fibres pictured are about 90 per cent cellulose.

Chitin

Chitin is a long-chain polymer of a derivative of glucose. It is found in many living things. For example, it is a component of the cell walls of fungi; the exoskeletons of arthropods, such as crustaceans and insects ; and the beaks and internal shells of animals, such as squids and octopuses. The structure of chitin is similar to that of cellulose.

The Right Molecule for the Job

Starch, glycogen, cellulose and chitin are all made from the monomer glucose.  So how are they all so different?  Their difference in structure and function is related to how they are linked together.  Starch is linked in long chains with a small amount of branching, glycogen is linked in many branching chains, and chitin and cellulose form long single chains that pack together tightly.  Each of these variations of linking the same monomer, glucose, together creates a different way the molecule can be used.  As shown in the Figure 3.4.8 diagram, starch and glycogen have many exposed “ends” of their chains.  These are areas where a glucose molecule can easily be removed for use as energy, whereas cellulose does not.  For this reason, glycogen and starch are well-suited for energy storage in organisms while cellulose is not.  Conversely, cellulose packs many monomers together in a sort of mesh that is very strong — this is why it is a great option for building strong cell walls.

Feature: My Human Biology

You probably know that you should eat plenty of fibre, but do you know how much fibre you need, how fibre contributes to good health, or which foods are good sources of fibre? Dietary fibre consists mainly of cellulose, so it is found primarily in plant-based foods, including fruits, vegetables, whole grains, and legumes. Dietary fibre can’t be broken down and absorbed by your digestive system. Instead, it passes relatively unchanged through your gastrointestinal tract and is excreted in feces (otherwise known as poop). That’s how it helps keep you healthy.

Fibre in food is commonly classified as either soluble or insoluble fibre.

• Soluble fibre dissolves in water to form a gel-like substance as it passes through the gastrointestinal tract. It lowers blood levels of cholesterol and glucose, which is beneficial for your health. Good sources of soluble fibre include whole oats, peas, beans, and apples.
• Insoluble fibre does not dissolve in water. This type of fibre increases the bulk of feces in the large intestine, and helps keep food wastes moving through, which may help prevent or correct constipation. Good sources of insoluble fibre include whole wheat, wheat bran, beans, and potatoes.

Use food labels like the one shown below in Figure 3.4.10 and online fibre counters to find out how much total fibre you eat in a typical day. Are you consuming enough fibre for good health? If not, consider ways to increase your intake of this important substance. For example, substitute whole grains for refined grains, eat more legumes (such as beans), and try to consume at least five servings of fruits and vegetables each day.

Table 3.4.2

Carbohydrate Comparison

Figure 3.4.4

Potatoes by Jean Beaufort, on Public Domain Pictures.net, is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 3.4.6

Cotton by David Nance for Agricultural Research Service, the research agency of the United States Department of Agriculture, on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.4.7

Ladybug on a mushroom /Fungi in the Woods by Benjamin Balázs on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.4.8

Carbohydrate structure comparison [Three Important Polysaccharides] by OpenStax College is on Wikimedia Commons, used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 3.4.9

Beans by Milada Vigerova on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.4.10

FDA Nutrition Facts Label 2014, by US Food and Drug Administration, on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Table 3.4.1

Recommended Daily Fibre Intake for Males and Females is from OpenStax, used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license..

Table 3.4.2

Carbohydrate Comparison is from OpenStax. used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

References

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 2.18. Five important monosaccharides [image]. In Anatomy and Physiology.  OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 2.20. Three important polysaccharides [image]. In Anatomy and Physiology.  OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

Mayo Clinic. (n.d.). Lactose intolerance [online article]. Mayo Foundation for Medical Education and Research (MFMER). https://www.mayoclinic.org/diseases-conditions/lactose-intolerance/symptoms-causes/syc-20374232

TED-Ed. (2016, January 11). How do carbohydrates impact your health? – Richard J. Wood. YouTube. https://youtu.be/wxzc_2c6GMg

TED-Ed. (2020, January 23). Why is cotton in everything? – Michael R. Stiff. https://www.youtube.com/watch?v=tKLJ6KQAcjI