Introduction
Definition of Disaccharides is a compound that consists of two units of monosaccharides like glucose, fructose, galactose, and mannose. Disaccharides are generally formed in plants and they can be separated from plant material.
Formation of disaccharides
Condensation reaction and glycosidic bond formation
- In the presence of acid, a glycosidic bond is formed between two monosaccharide units, and one molecule of water is liberated, this reaction is called condensation reaction.
- The hydroxyl group of the number one carbon atom (C-1) is known as the glycosidic hydroxyl. It is very reactive and readily forms a glycosidic link with a hydroxyl group of another sugar molecule.
- When such linkage occurs one H-atom and one OH group are eliminated to form H2O. This process is condensation and is in fact dehydration.
Monosaccharide units involved in disaccharide formation
Formation of Sucrose
It consists of one α-D-glucose and one β-D-fructose unit linked by α1 → β2 linkage.
Formation of Lactose
Lactose is made up of one glucose and one galactose unit linked by β-1, 4 linkages.
Formation of Maltose
It is made up of two glucose units linked by α-1, 4 linkages.
Common types of disaccharide
Sucrose
Occurrence: It is widely distributed in higher plants.
Source: Sugar cane, sweet potatoes, and sugar beets are the major sources of sucrose.
Role in plant: Vascular plants like sugarcane produce sucrose. It is a nutrient for plants that transports all parts of plants by phloem.
Human diet: Sucrose is a commonly used nutritive sweetener. We take sucrose naturally through fruits and vegetables. We also used sucrose as added sugar in beverages and other sweet products.
Lactose
Occurrence: Lactose or milk sugar occurs in the milk of mammals and is synthesized in the mammary glands. It may also be present in the urine during pregnancy.
Source: Milk products are the source of lactose.
Role in plants: Lactose is not found in plants so this disaccharide has no role in plants.
Human diet: It is a nutrient diet for infants. Lactose is an important newborn child for early growth and development.
Lactose intolerance and its implications:
- Lactose intolerance is a disorder of lactose digestion in humans. It is caused by a defect in the enzyme lactase (β-galactosidase).
- If someone suffers from lactose intolerance he cannot digest lactose resulting in people suffering from gastrointestinal disturbance.
Maltose
Occurrence: Maltose is found in germinating seeds and malt.
Source: It is not found independently in nature. It is produced during the digestion of starch by α-amylase and ptyalin enzymes.
Role in plants: In plants, amylase enzymes convert starch to maltose.
Human diet: Maltose has no specific role in the human body. We take maltose as a starch form which is converted to maltose. It is used in making bread, cake, pastries, etc.
Its uses in the brewing industry: In the brewing industry maltose is used for making beer. Maltose enhances the fermentation speed.
Physical and chemical properties of disaccharides
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Solubility
All disaccharides are highly soluble in water because they have a large number of hydroxyl groups. This hydroxyl group formed a hydrogen bond with water molecules.
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Sweetness
Disaccharides are sweet in nature. Sucrose is sweeter than other sugars.
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Reducing and non-reducing
- Maltose and lactose both reduce sugar because they have a free aldehyde or keto group.
- Non-reducing disaccharides with no free aldehyde or keto group e.g. sucrose, trehalose.
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Hydrolysis
It is the reverse reaction of the condensation reaction. The molecule cleaved with the incorporation of the elements of water is called hydrolysis. By hydrolysis, disaccharides broke their constituent residues.
Examples
Maltose
It is hydrolyzed by dilute acid or the enzyme maltase into two molecules of α-D-glucose.
Sucrose
Sucrose is hydrolyzed to D-glucose and D-fructose. Invertase enzymes catalyze this process.
Lactose
Lactose is hydrolyzed by lactase to glucose and galactose.
Digestion and absorption of disaccharides
Role of enzymes (Lactase, Sucrase, and maltase)
They all are intestinal digestive enzymes. Are synthesized in the endoplasmic reticulum and then transported to the small intestine brush border.
Digestion of disaccharides occurs only in the small intestine where these enzymes hydrolyze the disaccharides to monosaccharides after they are absorbed easily.
Sucrase: It is capable of hydrolyzing a large quantity of sucrose to glucose and fructose.
Lactase: Lactase also known as β galactosidase. It hydrolyzed the milk sugar or lactose to glucose and galactose.
Maltase: Maltase enzyme cleaves maltose into two glucose units.
Absorption of Monosaccharides in the small intestine
The absorption of monosaccharide takes place in the duodenum and upper jejunum of small by following method
Simple diffusion: Simple monosaccharides are absorbed by simple diffusion. It depends on the sugar concentration gradient between mucosal cells and blood plasma.
Active transport: Glucose and galactose are actively absorbed as compared to fructose and they are used for energy production. Fructose is not absorbed by simple diffusion. Fructose absorption is faster than pentoses.
Biological significance of disaccharides
1. The energy source of the cell
Disaccharides are a major source of energy. For producing energy cells hydrolyze disaccharides into a simple sugar that is used as fuel in metabolic cycles like glycolysis.
2. Role in transporting carbohydrates in plants
- Through disaccharides plants transport fructose, glucose, and galactose from all cells.
- Plants produce carbohydrates like sucrose which are transported to two other parts by phloem.
- Their sucrose is hydrolyzed to produce energy. Carbohydrates are important for plant growth, flowering, etc.
3. Role in the health and nutrition
- Carbohydrates are the main dietary source of both plants and animals.
- It generates major metabolites like glucose, and fructose which are essential for generating energy and growth.
- The body needs a sufficient quantity of glucose like the human brain needs 120g of glucose per day otherwise it leads to coma or death.
4. Impact on blood sugar level
Under normal conditions, the hormone insulin takes the glucose from the blood and by the cell, it’s used to produce energy. Under abnormal conditions, the insulin is unable to be taken up by the cells hence sugar level rises in the blood.
Applications of disaccharides
1. Drug formulation
Disaccharides are used in drug formulation. Maltose is used to produce syrups. Fructose is used to produce HFCS (high fructose corn syrup).
2. Food industries
In food industries, they are used as sweetening agents. Sucrose is a most common example because it is sweeter than other sugars except fructose.
Disaccharides are used to produce syrups, honey, beverages, baking cakes, cookies, and other dairy products.
3. As a stabilizing agent
Disaccharide increases storage stability by lowering the water activity of the product. They inhibit microbial growth by binding water in jams and jellies.
Summary
- Disaccharides are a type of carbohydrate.
- They contain two monosaccharide units which are linked with glycosidic bonds.
- Maltose, Sucrose, and Lactose are the common types of disaccharides.
- Disaccharides are formed by a condensation reaction where two simple sugar molecules are linked together by a glycosidic bond and one water molecule is liberated.
- Maltose consists of two glucose units joined by α-1, 4 linkages.
- Sucrose consists of one α-D-glucose and one β-D-fructose unit linked by α1 → β2 linkage.
- Lactose (milk sugar) consists of one glucose unit and one galactose unit linked by β-1, 4 linkages.
- Digestion and absorption of disaccharides take place in the small intestine.
- Disaccharides are a major source of energy and nutrients in plants and animals and they are important in the food industry and producing many types of syrups.
References
2. Lehninger Principle of Biochemistry, 4th edition (David L. Nelson & Michael M. Cox)
3. Carbohydrate Chemistry and Biochemistry, structure and mechanism ( Michael L. Sinnott)
