Disaccharides are the carbohydrates that are made up of two monosaccharide subunits. They act as a source of monosaccharides. Sometimes, they also used as an alternative to monosaccharides as they share various common properties.
In this section of the notes, we will discuss the structure, classification, and properties of disaccharides. We will also study some important disaccharides in detail.
As mentioned earlier, disaccharides are made when two monosaccharide subunits are combined. The two similar or different monosaccharide molecules are attached via a glycosidic bond to form a disaccharide. As a water molecule is released in this condensation process, it is also known as a dehydration reaction.
The structure of disaccharide is much similar to that of a monosaccharide. They also consist of multiple hydroxyl groups and a functional group that could be an aldehyde or a ketone. The only difference is the presence of a glycosidic bridge connecting the two molecules.
In a glycosidic bridge or bond, the two carbon atoms of two different molecules are joined using an oxygen atom. The oxygen atom acts as a bridge connecting the two different carbons of two different monosaccharide molecules in a glycosidic bond.
Based on the reducing power, disaccharides are classified into two categories.
They can act as reducing agents and can donate electrons to the recipients in the redox reaction. In these disaccharides, one of the monosaccharides retains its free functional group that can participate in the redox reaction. The functional group of only one monosaccharide is consumed in the formation of the glycosidic bond. An example of reducing disaccharide is maltose.
These disaccharides do not behave as a reducing agent because they do not have a free aldehydic or ketonic functional group. The functional groups of both the monosaccharides are consumed in the process of glycosidic bond formation. Sucrose is an example of a non-reducing disaccharide.
Following are the common properties of disaccharides;
- Water Solubility
Due to the presence of a large number of hydroxyl groups, disaccharides are easily soluble in water. These hydroxyl groups form hydrogen bonds with the water molecules when dissolved in aqueous solutions.
Disaccharides are polar compounds. The polarity of disaccharides is due to abundant hydroxyl groups and hydrogen atoms attached to carbon atom. Each hydroxyl group in a disaccharide carries a partial negative charge while each hydrogen atom carries a partial positive charge.
Just like monosaccharides, they also sweet in taste. They are more commonly used sweetening agents in our households and industries than the monosaccharides. Sucrose, also called table sugar, is an example.
- Diffusion ability
Disaccharides are large molecules with a size greater than the size of the pores or openings present in the cell membranes. Thus, they cannot cross the cell membranes. They must be broken down into monosaccharides to be transported from one cell to another.
As we have studied the general structure and properties of disaccharides, let us now jump to the discussion of some common and important disaccharides found in nature.
Sucrose is the most important carbohydrate belonging to the class of disaccharides. It is also known as table sugar or cane sugar. Its molecular formula is C12H22O11.
It is made up of a glucose and a fructose molecule joined together by a glycosidic bond. This glycosidic bond is formed between the carbon 1 of glucose and the carbon 2 of fructose. It is formed between the functional groups of two molecules.
The fructose molecule in sucrose has a beta orientation while the glucose molecule has alpha orientation. In alpha orientation, the OH group of the anomeric carbon is oriented on the side of the ring opposite to that of 6th carbon. On the other hand, in beta orientation, the OH group of the anomeric carbon is on the same side of the ring as the 6th carbon. (The anomeric carbon is the carbon atom of the aldehydic or ketonic functional group).
Thus, the glycosidic bond in sucrose is called alpha-1 beta-2 glycosidic bond.
Sucrose has the following properties.
- It is sweeter than glucose.
- It is soluble in water.
- It is a white crystalline solid in appearance.
- It is non-reducing sugar because no free functional group is available to act as a reducing agent.
- It is dextrorotatory meaning that it can bend light in the right direction when passed through it.
- It yield glucose and fructose on hydrolysis.
Sucrose is mainly obtained from fruits and nectars. The amount of sucrose in high in the ripening fruits but it decreases as the fruits are ripened. Sucrose is present in large amounts in sugar cane. This is the reason why it is also called cane sugar.
In the human body, sucrose is digested in the small intestine by an enzyme called invertase or sucrase. This enzyme breaks sucrose into glucose and fructose molecules that are absorbed in the blood and carried to the liver for metabolism.
Following are the commercial as well as household uses of sucrose.
- It is used as table sugar
- It is used as a sweetening agent in confectionery and desserts.
- It is used to make inverted sugar syrup.
Inverted sugar syrup is made when sucrose is heated in the presence of water. It consists of glucose and fructose in equal amounts that are not chemically bound together. It is more commonly used in industries as a sweetening agent because it is sweeter than sucrose, glucose or fructose.
Lactose is another important disaccharide commonly consumed by human beings. It is also known as milk sugar. Its molecular formula is the same as of sucrose i.e. C12H22O11.
Lactose is made up of glucose and galactose molecules attached via a glycosidic bond. It is a C1-C4 glycosidic bond as it attaches the first carbon of glucose to the fourth carbon of galactose.
Both the glucose and galactose molecules have alpha orientation in lactose. Therefore, the glycosidic bond between them is also called alpha 1-4 glycosidic bond.
Lactose is a crystalline white solid having the following properties;
- It is soluble in water, but its solubility is less than sucrose.
- It is less sweeter than sucrose.
- It is also dextrorotatory, rotating light rays to the right.
Contrary to sucrose, lactose is purely animal in origin. It is only made by the lactating mammary glands of mammals. The enzyme necessary for its synthesis is only active during the phase of lactation. Lactose is the most abundant sugar found in milk and milk products like cow milk, goat milk, human milk, etc.
It is digested in the human body by lactase enzyme present in the small intestine. The glucose and galactose molecules are then sent to the liver for further metabolism.
- It is used in infant formula milks as sugar
- It is used in pharmaceutical industries as an ingredient
- It is used in the beverage industry to sweeten stout bear
Lactose intolerance is a medical condition that develops in some people after consumption of milk or milk products. Some people inherently lake the lactase enzyme in their digestive system and thus, cannot digest lactose. When such people consume milk or milk products, the undigested lactose remains in the alimentary canal. The osmotic effect of lactose causes the malabsorption of water and the patient presents with diarrhea. Other signs and symptoms associated with lactose intolerance are bloating, cramps, flatulence, and vomiting, etc.
Lactulose is a medically important artificial disaccharide. It does not exist in nature and is made artificially by the pharmaceutical industries.
It is made up of galactose and fructose molecules attached by beta 1-4 glycosidic bond. Both the galactose and fructose molecules have beta orientation in lactulose.
Human body lakes enzyme to digest lactulose. It is used medically in two conditions; constipation and hepatic encephalopathy.
Lactulose helps to relieve constipation by its osmotic effect that prevents the absorption of water molecules in the gastrointestinal tract.
It is also beneficial in hepatic encephalopathy as it traps the ammonia in the form of ammonium ions. The intestinal flora convert lactulose into lactic acid by the process of fermentation. The acidic environment thus created converts ammonia molecules into ammonium ions. These ammonium ions cannot be absorbed into the blood and are excreted along with feces.
Maltose is a disaccharide that is not abundantly found in nature. Its molecular formula is the same as that of lactose and sucrose, i.e. C12H22O11.
Maltose is a disaccharide made up of two subunits of glucose. Both the glucose molecules are attached via a 1-4 glycosidic bond. This bond attaches the carbon 1 of one glucose molecule to carbon 4 of the second glucose molecule.
The first glucose molecule has alpha orientation. The second glucose molecule retains its aldehydic functional group and can have either an alpha or beta orientation.
maltose has the following properties;
- It is a reducing sugar due to free aldehydic group of glucose.
- It is soluble in water.
- It has a sweet taste.
- Upon hydrolysis, it yields two glucose molecules.
It is only rarely found in nature in some fruits. A major source of maltose is ‘malt’ which is obtained when the grains are kept in water for a long time. In the human body, maltose is formed during the digestion of starch. It is the major break down product of starch.
Maltose can be digested in the human intestine by an enzyme known as maltase. Upon digestion, maltose yields two glucose molecules that are further processed in the liver.
Trehalose is a disaccharide commonly found in insects, bacteria, fungi, and other organisms. It is also known as mycose for its association with fungi.
It is made up of two glucose molecules that are joined by a 1-1 glycosidic bond. The aldehydic groups of both the glucose molecules take part in making this glycosidic bond.
Both the glucose molecules have an alpha orientation in trehalose.
The properties of trehalose are the same as that of other disaccharides except the reducing power. Trehalose is a non-reducing sugar like sucrose, because it does not have a free functional group.
Trehalose can be easily digested in the human intestine due to the presence of trehalase enzyme. It breaks trehalose into two molecules of glucose.
Disaccharides are the class of carbohydrates that are made up of two monosaccharide subunits.
Like monosaccharides, they also have multiple hydroxyl groups. The aldehydic or ketonic functional group may or may not be present in them.
They are formed as a result of a condensation reaction between two monosaccharides.
Disaccharides also have a glycosidic bond in their structure that keeps together the two monosaccharide subunits.
Depending on the reducing capability, they are classified as;
- Reducing disaccharides
- Non-reducing disaccharides
All disaccharides share the following properties;
- They are polar compounds
- They are readily soluble in water due to hydrogen bonding
- They have sweet taste
- They cannot diffuse through cellular membranes
Important disaccharides include sucrose, lactose, lactulose, maltose, and trehalose.
Sucrose is common sugar made up of glucose and fructose.
- It is non-reducing sugar.
- It is present in fruits, berries, nectar etc.
- It is used as a sweetening agent in homes as well as in industries.
- It is used to make inverted sugar syrup.
Lactose is milk sugar present only in milk of mammals.
- It is made up of glucose and galactose.
- Lactose is a reducing sugar.
- Lactose intolerance is an important medical phenomenon associated with it.
Lactulose is a synthetic disaccharide medically used in constipation and hepatic encephalopathy.
Maltose is a less common disaccharide present in nature.
- It is a breakdown product of starch digestion.
- It is a reducing sugar made up of two glucose molecules.
- It can be digested in the human body by maltase enzyme.
Trehalose is a disaccharide found in insects, fungi, bacteria and some plants.
- It is made up of two glucose molecules attached by 1-1 glycosidic linkage.
- Trehalose is a noon-reducing sugar like sucrose.
- It can also be digested in the human body by trehalase enzyme present on cells of the small intestine.
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