Many natural materials are polymers, including the cellulose of wood and the carbohydrates in food. These are called biopolymers, and they are giant molecules made of chains or networks of linked small organic molecules. Four of the most important biological molecule classes are proteins, lipids, nucleic acids and carbohydrates. This article shares a general overview of the carbon atoms called carbohydrates, what they are and why they are important.
Carbohydrates: Definition and Structure
As the name suggests, carbohydrates are hydrates of carbon. This means that they have carbon, hydrogen and oxygen. In general, the formula for a carbohydrate molecule is CH2O, and the elemental ratio for a carbohydrate is 1:2:1 for C:H:O.
To make a polymer of any sort, you need monomers. Monomers are the individual units that are the building blocks of the longer chain. The monomers of carbohydrates are monosaccharides. Monosaccharides can be found in either a straight carbon chain or in a ring. All monosaccharides have the formula C6H12O6. As such, they are structural isomers. Examples of different monosaccharides will be given in the section below. Monosaccharides are what is known as simple sugars.
Disaccharides are formed through a dehydration reaction that allows for monosaccharides to come together. Continued dehydration results in the addition of more monomers to form a polysaccharide.
Simple carbohydrates are called simple sugars. Polysaccharides are what is known as complex carbohydrates. Chemically, that means three or more linked sugars. Examples of common disaccharides and polysaccharides are given below.
Monosaccharides: Definition and Examples
Glucose is the most common carbohydrate and one of the most important. Glucose is an aldose (in the chain form, it contains an aldehyde) and a hexose (six-carbon sugar). In aqueous solution, glucose is primarily found in the cyclic form.
Glucose is made by plants during photosynthesis by using carbon dioxide and sunlight. Glucose molecules are made into starch for storage in plants. People get glucose by eating plants.
Humans use glucose for energy. The processes of glycolysis and cellular respiration allow for the cell to break down glucose molecules as a way of extracting energy from the food that you eat. As a result of cellular respiration, you end up regenerating ATP molecules, which are the energy currency of the body.
Fructose is another common monosaccharide. You may have heard of “high fructose” in foods and drinks. This is the fructose they are talking about! Fructose is a hexose just like glucose, but it is a ketose instead of an aldose. The common table sugar is a disaccharide made of a glucose molecule and a fructose molecule. Table sugar is chemically known as sucrose, and you can read more about that below.
Galactose is another monosaccharide that is an aldose and a hexose. It is commonly found as a monosaccharide in peas.
Galactosemia is a disease in which the enzyme needed to convert galactose into glucose is not present. This is a big problem for newborns if not caught early. Without the enzyme, blood galactose levels are high and can cause a variety of symptoms, including weight loss and jaundice. If found early, then all sources of galactose can be removed from the child’s diet. Eventually, the child develops an alternative pathway for metabolizing galactose.
You may have heard of this sugar as part of another type of important biological macromolecule: nucleic acids. Ribose and deoxyribose are important parts of nucleic acids. They make up the sugar-phosphate backbone. Ribose is found in RNA, and deoxyribose is found in DNA.
Disaccharides: Definition and Examples
Disaccharides are formed through a dehydration reaction. For example, when two glucose molecules form a disaccharide, the alcohol group on one glucose and the hydrogen from the alcohol group on another alcohol form water. What is left is a bond through an oxygen between the two monomers. This linkage is called a glycosidic linkage or glycosidic bond.
Disaccharides cannot be metabolized by humans. Why? Well, a disaccharide is too big to pass through the cell membrane. As a result, any disaccharides (or larger chains) must first be broken down by an enzyme. After this, the constituent monosaccharides can be metabolized.
You are probably very familiar with sucrose. Sucrose is also known as table sugar or just sugar. Sucrose is the sugar that you add to the mixture when you’re trying to make a cake.
Sucrose is made when glucose and fructose form a disaccharide. As fructose is sweeter than sucrose, when sucrose is hydrolyzed into its component parts, the solution becomes even sweeter. Bees hydrolyze sucrose into glucose and fructose. That’s why honey is so sweet.
In order to break down sucrose, you need to have the enzyme sucrase.
Lactose is the sugar found in milk. It is found in the milk of humans and other mammals. Lactose is composed of galactose and glucose. It is not nearly as sweet as sucrose (as you could probably guess, since milk is not as sweet as sugar!).
In order to break down lactose into its component parts, you need to have the enzyme lactase. Individuals who do not have lactase are termed “lactose intolerant.” Many people are somewhat lactose intolerant; they just might not know it.
Maltose is a disaccharide formed by two glucose monomers. In order to break down maltose, the human body needs the enzyme maltase.
Polysaccharides: Definition and Examples
Polysaccharides are chains of monosaccharides joined by many glycosidic linkages. Polysaccharides are the most abundant carbohydrate molecules found in living organisms because they serve a variety of functions including providing structure to plants and storing energy.
They are formed by the reaction between the alcohol (-OH) group of one glucose molecule and the alcohol group on another glucose molecule, creating linkages. These linkages are made over and over again between monosaccharides. This type of reaction is called a condensation polymerization reaction (or dehydration synthesis) because a water molecule is formed from the OH functional group of one alcohol and the H from the other alcohol.
Starch is one of the most important carbohydrates as it makes up a large portion of the human diet. Starch is a polymer that stores glucose in plants, particularly tubers (think potatoes). While humans can eat starch for energy, plants can also break down starch and use the glucose for energy when they cannot photosynthesize as much.
This polysaccharide is made of amylose (chains of glucose) and amylopectin (branched chains of glucose).
In order to break down starch, the human body makes use of many enzymes collectively knowns as amylases. These enzymes break down the long chains of starch into smaller units that the human body can metabolize. Amylases can be found in your mouth. That’s why digestion starts in your mouth.
Cellulose is a carbohydrate found in plants. It is a vital structural component of the walls of plant cells. You can also find it in different vegetable fibers like those of cotton. Without cellulose, plants would be a lot floppier.
Cellulose is a linear polymer of glucose. The particular conformation of the glycosidic linkages in cellulose is not digestible by humans. As such, humans can eat starch but not cellulose.
Glycogen is the polysaccharide that animals use to store energy. Glycogen, which is made up of highly branched chains of glucose, is primarily stored in the liver. When you aren’t eating anything or are in a fasting state, your body uses the glycogen that is in these stores to obtain the glucose you need as a source of energy.
About the Author
Riti Gupta holds a Honors Bachelors degree in Biochemistry from the University of Oregon and a PhD in biology from Johns Hopkins University. She has an interest in astrobiology and manned spaceflight. She has over 10 years of biology research experience in academia. She currently teaches classes in biochemistry, biology, biophysics, astrobiology, as well as high school AP Biology and Chemistry test prep.