Organic Compounds: Carbohydrates

1. Chemistry of Carbon

In previous lessons, you learned about the inorganic molecules that are essential to life: water, salt, and acids & bases. Starting in this lesson and continuing over the next four lessons, you will learn about organic molecules that are important to the structure and function of the human body.

Recall that organic molecules contain covalently bonded carbon and hydrogen atoms. These carbon skeletons (think of the “framework” of carbon) form four types of macromolecules that are important to human life:

• Carbohydrates
• Lipids
• Protein
• Nucleic acids

In many carbon structures, set groups of covalently bonded atoms are found together. These groups, called functional groups, function together as a single unit in a chemical reaction. You can think of functional groups as tightly knit “cliques” (groups of friends) whose members are unlikely to be parted. Five functional groups are important in human physiology; these are the hydroxyl, carboxyl, amino, methyl, and phosphate groups (see the table below).

Functional group	Structural formula	Importance
Hydroxyl	—O—H	Hydroxyl groups are polar. They are components of all four types of organic compounds discussed in this unit. They are involved in dehydration synthesis reactions* and hydrolysis reactions*.
Carboxyl		Carboxyl groups are found within fatty acids, amino acids, and many other acids.
Amino	—N—H₂	Amino groups are found within amino acids, the building blocks of proteins.
Methyl	—C—H₃	Methyl groups are found within amino acids.
Phosphate	—P—O₄²⁻	Phosphate groups are found within phospholipids and nucleotides.

reflect

* Remember from a past lesson that dehydration synthesis is a reaction in which one reactant gives up an atom of hydrogen and another reactant gives up a hydroxyl group (OH) in the synthesis of a new product. A hydrolysis reaction is a decomposition reaction in which a molecule of water disrupts a compound, breaking its bonds.

Carbon’s affinity for covalent bonding means that many distinct and relatively stable organic molecules nevertheless readily form larger, more complex molecules. Any large molecule is referred to as macromolecule (macro, large), and the organic compounds in this section all fit this description. However, some macromolecules are made up of several “copies” of single units called monomer (mono, one; mer, part). Like beads in a long necklace, these monomers link by covalent bonds to form long polymers (poly, many). Almost every organic macromolecule forms monomers and polymers.

terms to know

Functional Group

Any group of covalently bonded atoms in an organic molecule that function as one unit in a chemical reaction (i.e., hydroxyl, carboxyl, amino, methyl, phosphate).

Macromolecule

Any large molecule.

Monomer

A single unit of a macromolecule.

Polymer

Many monomers linked together by covalent bonds.

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2. Carbohydrates

before you start

What do you think of when you hear the word “carbohydrate”?

Do you think of bagels, bread, pasta, potatoes, and sweet desserts? If so, you are correct. All of those are foods rich with carbohydrates.

If you recall, carbohydrates are one of the four macromolecules (large molecules) that are important for the function of the human body. In the duration of this lesson, you’ll learn about their chemical structure, various forms, and functions in the human body. In future lessons, you’ll learn about the other three macromolecules: lipids, proteins, and nucleic acids.

Let’s start with the terminology—what does “carbohydrate” mean? The term carbohydrate means “hydrated carbon,” where carbo refers to the atom carbon and the root hydro- indicates water. When bonded together, these form a carbohydrate, a molecule composed of carbon hydrogen and oxygen. The majority of carbohydrates have a 1:2:1 proportion of carbon, hydrogen, and oxygen, making the chemical formula for a “generic” molecule of carbohydrate (CH₂O)ₙ

Scientifically speaking, carbohydrates are referred to as saccharides, a word meaning “sugars.” Three forms of saccharides are important in the body. Monosaccharides are the monomers of carbohydrates. Disaccharides (di, two) are made up of two monomers. Polysaccharides are the polymers, and can consist of hundreds to thousands of monomers.

2a. Monosaccharides

A monosaccharide is a monomer of carbohydrates. Five individual monosaccharides are important in the body. Three of these are the hexose sugars, so called because they each contain six atoms of carbon and form a hexagon shape. These are glucose, fructose, and galactose, each with the chemical formula C₆H₁₂O₆ with slightly different orientations. Glucose is the most common monosaccharide in the human body. Fructose is known as fruit sugar and is found in fruits and many plants. Galactose is known as milk sugar and is found in many dairy products. The remaining two monosaccharides are the two pentose sugars, each of which contains five atoms of carbon and have the chemical formula C₅H₁₀O₅. They are ribose and deoxyribose and play roles in information storage in the cell which will be covered in depth in a future lesson.

2b. Disaccharides

A disaccharide is a pair of monosaccharides bonded together by what is called a glycosidic bond (glyco, sugar). Each combination of the above monosaccharides forms a different disaccharide. Three disaccharides shown below are important to humans, sucrose, lactose, and maltose. Sucrose, commonly referred to as table sugar is one glucose and one fructose molecule bonded together. Lactose, or milk sugar, is one glucose and one galactose molecule bonded together. Maltose, or malt sugar, is two glucose molecules bonded together.

Disaccharides are formed by combining monosaccharides via dehydration synthesis reactions. You consume these in your diet, however, your body cannot use disaccharides directly. Instead, in the digestive tract, the bond is broken via hydrolysis reactions and they are split into their component monosaccharides before being absorbed and utilized.

2c. Polysaccharides

Polysaccharides are sugars that can contain a few to a thousand or more monosaccharides. There are three polysaccharides that are important to the body.

• Starches are polymers of glucose that are stored as straight (amylose) or branched (amylopectin) chains in plant-based foods and are relatively easy to digest.
• Glycogen is also a branched polymer of glucose, but it is stored in the tissues of animals, especially in the muscles and liver. It is not considered a dietary carbohydrate because very little glycogen remains in animal tissues after slaughter; however, the human body stores excess glucose as glycogen, again, in the muscles and liver.
• Cellulose, a polysaccharide that is the primary component of the cell wall of green plants, is the component of plant food referred to as “fiber.” In humans, cellulose/fiber is not digestible because the enzyme is not present to break it down. However, dietary fiber has many health benefits. It helps you feel full so you eat less, it promotes a healthy digestive tract, and a diet high in fiber is thought to reduce the risk of heart disease and possibly some forms of cancer.

2d. Functions of Carbohydrates

The body obtains carbohydrates from plant-based foods. Grains, fruits, and legumes and other vegetables provide most of the carbohydrate in the human diet, although lactose is found in dairy products.

Although most body cells can break down other organic compounds for fuel, all body cells can use glucose. Moreover, nerve cells (neurons) in the brain, spinal cord, and through the peripheral nervous system, as well as red blood cells, are limited almost exclusively to the use of glucose for fuel.

In addition to being a critical fuel source, carbohydrates are present in very small amounts in cells’ structure. For instance, some carbohydrate molecules bind with proteins to produce glycoproteins, and others combine with lipids to produce glycolipids, both of which are found in the membrane that encloses the contents of body cells and discussed in future lessons.

terms to know

Carbohydrate

A macromolecule made of carbon, hydrogen, and oxygen in a relative 1:2:1 ratio.

Saccharide

The scientific term for carbohydrates or sugars.

Monosaccharide

A monomer of carbohydrates.

Disaccharide

Two monomers of carbohydrates bonded together with a glycosidic bond.

Polysaccharide

Many monomers of carbohydrates bonded together.

Starch

A polymer of glucose and the storage form of carbohydrates in plants.

Glycogen

A polymer of glucose and the storage form of carbohydrates in animals.

Cellulose

A polymer of glucose and the cell wall structure of plants.