Introduction to Carbohydrates Until Now We Have Focused on Alcohols That Contain Only One Hydroxyl Group but Organic Compounds C

Until now we have focused on alcohols that contain only one hydroxyl group but organic compounds can have more OH groups in their structure. One example of such compound is glucose, which is a simple sugar with the molecular formula C6H12O6. Green plants are capable of synthesizing glucose from carbon dioxide (CO2) and water (H2O) by using solar energy in the process known as photosynthesis:

Plants can use the glucose, which is a monosaccharide, for energy or convert it to larger carbohydrates, such as starch or cellulose, which are polysaccharides. All carbohydrates consist of carbon, hydrogen, and oxygen atoms and are polyhydroxy aldehydes or ketones (or compounds that can be broken down to form such compounds.) A chemical structure of glucose is in Figure 1.

Figure 1. Glucose contains five OH groups and one aldehyde group in its open straight-chain form.

Starch provides energy for later use, perhaps as nourishment for a plant’s seeds, while cellulose is the structural material of plants. We can gather and eat the parts of a plant that store energy— seeds, roots, tubers, and fruits—and use some of that energy ourselves. Carbohydrates are also needed for the synthesis of nucleic acids and many proteins and lipids.

Animals, including humans, cannot synthesize carbohydrates from carbon dioxide and water and are therefore dependent on the plant kingdom to provide these vital compounds. We use carbohydrates not only for food (about 60–65% by mass of the average diet) but also for clothing (cotton, linen, rayon), shelter (wood), fuel (wood), and paper (wood).

Carbohydrates are usually classified according to their structure as monosaccharides, oligosaccharides or polysaccharides. The term saccharide comes from Latin (saccharum, sugar) and refers to the sweet taste of some simple carbohydrates. The three classes of carbohydrates are related to each other through hydrolysis.

More information about the carbohydrates see e.g. Virtual Textbook of Organic Chemistry,  https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Cyclic structures of monosaccharides Maps/Map%3A_O rganic_Chemistry_(McMurry)/Chapter_25%3A_Biomolecules%3A_Carbohydrates/25.001_Intr oduction

The preferred structural form of many monosaccharides is a cyclic hemiacetal. A straight-chain monosaccharide forms a cyclic structure, when OH group on the fifth carbon atom reacts with the carbonyl group, Figure 2(b). Carbonyl oxygen atom may be pushed either up or down, giving rise to two stereoisomers, as shown in Figure 3.

Figure 2. D-Glucose can be represented with a Fischer projection (a), three dimensionally (b) or a cyclic Haworth projection (c).

The structure shown on the left side of Figure 3, with the OH group on the first carbon atom projected downward, represent what is called the alpha (α) form. The structures on the right side, with the OH group on the first carbon atom pointed upward, is the beta (β) form. These two stereoisomers of a cyclic monosaccharide are known as anomers; they differ in structure around the anomeric carbon—that is, the carbon atom that was the carbonyl carbon atom in the straight- chain form. The α form melts at 146°C and has a specific rotation of +112°, while the β form melts at 150°C and has a specific rotation of +18.7°.

Figure 3. In an aqueous solution, monosaccharides exist as an equilibrium mixture of three forms. The interconversion between the forms is known as mutarotation (Latin mutare, meaning “to change”). At equilibrium, the mixture consists of about 36% α-D-glucose, 64% β-D-glucose, and less than 0.02% of the open-chain aldehyde form. The observed rotation of this solution is +52.7°.

More information about the cyclic structures of monosaccharides see e.g. Virtual Textbook of Organic Chemistry,  https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Maps/Map%3A_ Organic_Chemistry_(McMurry)/Chapter_25%3A_Biomolecules%3A_Carbohydrates/25.05_Cy clic_Structures_of_Monosaccharides%3A_Anomers

Disaccharides, especially maltose

Previously, you learned that monosaccharides can form cyclic structures by the reaction of the carbonyl group with an OH group. These cyclic molecules can in turn react with another alcohol (Recall the alcohol reaction how to produce ether). Disaccharides (C12H22O11) are sugars composed of two monosaccharide units that are joined by a carbon–oxygen-carbon linkage known as a glycosidic linkage. This linkage is formed from the reaction of the anomeric carbon of one cyclic monosaccharide with the OH group of a second monosaccharide, Figure 4.

Figure 4. The formation of the glycosidic bond between two glucose molecules.

The disaccharides differ from one another in their monosaccharide constituents and in the specific type of glycosidic linkage connecting them. There are three common disaccharides: maltose, lactose, and sucrose. All three are white crystalline solids at room temperature and are soluble in water. We’ll consider maltose in more detail.

Maltose is a reducing sugar. Thus, its two glucose molecules must be linked in such a way as to leave one anomeric carbon that can open to form an aldehyde group. The glucose units in maltose are joined in a head-to-tail fashion through an α-linkage from the first carbon atom of one glucose molecule to the fourth carbon atom of the second glucose molecule (that is, an α-1,4-glycosidic linkage; see Figure 1). The bond from the anomeric carbon of the first monosaccharide unit is directed downward, which is why this is known as an α-glycosidic linkage. The OH group on the anomeric carbon of the second glucose can also be in the β position.

Maltose occurs to a limited extent in sprouting grain. It is formed most often by the partial hydrolysis of starch and glycogen. In the manufacture of beer, maltose is liberated by the action of malt (germinating barley) on starch; for this reason, it is often referred to as malt sugar. Maltose is about 30% as sweet as sucrose. The human body is unable to metabolize maltose or any other disaccharide directly from the diet because the molecules are too large to pass through the cell membranes of the intestinal wall. Therefore, an ingested disaccharide must first be broken down by hydrolysis into its two constituent monosaccharide units. In the body, such hydrolysis reactions are catalyzed by enzymes such as maltase. The same reactions can be carried out in the laboratory with dilute acid as a catalyst, although in that case the rate is much slower, and high temperatures are required. Whether it occurs in the body or a glass beaker, the hydrolysis of maltose produces two molecules of D-glucose. This is a reverse reaction for the reaction shown in Figure 4.

More information about the disaccharides see e.g. Chemistry LibreText  https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Maps/Map%3A_O rganic_Chemistry_(McMurry)/Chapter_25%3A_Biomolecules%3A_Carbohydrates/25.08_Disa ccharides