17 3.4 Carbohydrates

Created by: CK-12/Adapted by Christine Miller

The Cellulose of Our Lives

Created by: CK-12/Adapted by Christine Miller

Image shows a pile of jeans of various shades of blue.
Figure 3.4.1 Jeans are made of cotton, and cotton is made of cellulose.

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?

 are the most common class of biochemical compounds. They include sugars and starches. Carbohydrates are used to provide or store , 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 Each monosaccharide consists of six carbon atoms, as shown in the model of the monosaccharide glucose shown in Figure 3.4.2.

Figure 3.4.2 A model of the monosaccharide glucose. 

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 . The simplest sugars consist of a single . They include glucose, fructose, and galactose.  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.

Image shows molecular diagrams of glucose, fructose, galactose, deoxyribose and ribose.
Figure 3.4.3 Five important monosaccharides.

Other sugars contain two molecules and are called . These include sucrose (table sugar), maltose, and lactose. Sucrose is composed of one fructose molecule and one glucose molecule, maltose is composed of two glucose molecules, and lactose is composed of one glucose molecule and one galactose molecule. Lactose occurs naturally in milk. Some people are lactose intolerant because they can’t digest lactose. If they drink milk, it causes gas, cramps, and other unpleasant symptoms, unless the milk has been processed to remove the lactose.

Complex Carbohydrates

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

Starch

Image shows potatoes in several colours and sizes.
Figure 3.4.4 Potatoes store glucose made via photosynthesis in the form of 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 and other simple sugars. The leaves of potato plants make sugars by , 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  with energy. Starches are easily and quickly digested with the help of digestive  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.

Glycogen

Animals do not store energy as starch. Instead, animals store extra energy as the complex carbohydrate glycogen.  is a of . 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  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.

Diagram shows the way in which the liver controls homeostasis of blood sugar by either storing glucose as glycogen when blood sugar levels are too high, or releasing glucose from glycogen when blood sugar levels are too low.
Figure 3.4.5 Your liver plays an important role in balancing blood sugar levels. Glycogen in your liver can either collect glucose out of your blood stream to lower blood sugar, or release glucose into the bloodstream to increase blood sugar.

Cellulose

Image shows a field of ripe cotton. Waist height dried out brownish plants have white balls of cotton growing from where the flowers once were.
Figure 3.4.6 Cotton fibres represent the purest natural form of cellulose, containing more than 90 per cent of this polysaccharide.

is a polysaccharide consisting of a linear chain of several hundred to many thousands of linked 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.

Certain animals, including termites and ruminants such as cows, can digest cellulose with the help of microorganisms that live in their gut. Humans cannot digest cellulose, but it nonetheless plays an important role in our diet. It acts as a water-attracting bulking agent for feces in the digestive tract and is often referred to as “dietary fibre.”  In simpler terms, it helps you poop.

Chitin

Image shows a ladbug perched on a mushroom.
Figure 3.4.7 Chitin is an important structural component in fungal cell walls and the exoskeletons of insects.

 is a long-chain polymer of a derivative of . 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.

In Figure 3.4.7, both the exoskeleton of the ladybug and the cell walls of the mushroom are made partly of the complex carbohydrate chitin.

The Right Molecule for the Job

Starch, glycogen, cellulose and chitin are all made from the .  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.

Image shows molecules of starch, glycogen and cellulose.
Figure 3.4.8 Starch, glycogen and cellulose are all made of many linked monomers of glucose. The shape and bonding of these monomers affects the function of the molecule.

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.

Image shows a bowl of kidney beans.
Figure 3.4.9 Beans are an excellent source of both soluble and insoluble fibre.

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.

How much fibre do you need for good health? That depends on your age and gender. The Institute of Medicine recommends the daily fibre intake for adults shown in Table 3.4.1 below. Most dietitians further recommend a ratio of about three parts of insoluble fibre to one part of soluble fibre each day. Most fibre-rich foods contain both types of fibre, so it usually isn’t necessary to keep track of the two types of fibre as long as your overall fibre intake is adequate.

Table 3.4.1

Recommended Daily Fibre Intake for Males and Females

Recommended Daily Fibre Intake for Males and Females
Gender Age 50 or Younger Age 51 or Older
Male 38 grams 30 grams
Female 25 grams 21 grams

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.

 

Image shows a nutrition label. It lists information about calories, fat, cholesterol, sodium, carbohydrates, protein and vitamins. This example shows that the food contains 4 grams of dietary fibre per serving.
Figure 3.4.10 You can determine how much dietary fibre is in your food by reading the nutrition label.

Table 3.4.2

Carbohydrate Comparison

Name
Class
Function
Location
Glucose Monosaccharide Energy for cells Cells
Starch Polysaccharide Energy storage Plant cells
Glycogen Polysaccharide Energy storage Animal cells
Cellulose Polysaccharide Structural component in cell walls Plant cells
Chitin Polysaccharide Structural component in cell walls and exoskeletons Fungi and arthropods

 

3.4 Summary

  • are the most common class of biochemical compounds. The basic building block of carbohydrates is the monosaccharide, which consists of six carbon atoms.
  • Sugars are sweet, short-chain, soluble carbohydrates that are found in many foods and supply us with energy. Simple sugars, such as , consist of just one . Some sugars, such as sucrose (or table sugar), consist of two monosaccharides. These are called .
  • , or , consist of hundreds — or even thousands — of monosaccharides. They include , , , and . They generally either store energy or form structures, such as cell walls, in living things.
  • Starch is a complex carbohydrate that is made by plants to store energy. Potatoes are a good food source of dietary starch, which is readily broken down into its component sugars during digestion.
  • Glycogen is a complex carbohydrate that is made by animals and fungi to store energy. Glycogen plays a critical part in the homeostasis of blood glucose levels in humans.
  • Cellulose is the single most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Like most other animals, humans cannot digest cellulose, but it makes up most of the crucial dietary fibre in the human diet.
  • Chitin is a complex carbohydrate, similar to cellulose, that makes up organic structures, such as the cell walls of fungi and the exoskeletons of insects and other arthropods.

3.4 Review Questions

  1. What are carbohydrates? Describe their structure.
  2. Compare and contrast sugars and complex carbohydrates.
  3. If you chew on a starchy food (such as a saltine cracker) for several minutes, it may start to taste sweet. Explain why.
  4. True or False: Glucose is mainly stored by lipids in the human body.
  5. Name three carbohydrates that contain glucose as a monomer.
  6. Jeans are made of tough, durable cotton. Based on what you know about the structure of carbohydrates, explain how you think this fabric gets its tough qualities.
  7. Which do you think is faster to digest — simple sugars or complex carbohydrates? Explain your answer.
  8. True or False: Cellulose is broken down in the human digestive system into glucose molecules.
  9. ___________ fibre dissolves in water, __________ fibre does not dissolve in water.
  10. What are the similarities and differences between muscle glycogen and liver glycogen?
  11. Which carbohydrate is used directly by the cells of living things for energy?
  12. Which of the following is not a complex carbohydrate?
    • Chitin
    • Starch
    • Disaccharide
    • None of the above

3.4 Explore More

How do carbohydrates impact your health? – Richard J. Wood, TED-Ed, 2016

Why is cotton in everything? – Michael R. Stiff, TED-Ed, 2020

Attributions

Figure 3.4.1

Pile of Jeans by Marco Verch, on Flickr, is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 3.4.2

e-from-xtal-1979-Alpha-D-glucose-from-xtal-1979-3D-balls by Ben Mills [Benjah-bmm27] on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.4.3

Monosasccharides by OpenStax College on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

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 3.4.5

Homeostasis_of_blood_sugar by Christine Miller [christinelmiller] 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

License

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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