Disaccharides(Sucrose, Maltose, Lactose) – Definition, Types, Examples, FAQs

A disaccharide is two monosaccharides united into one, as one might expect from the di- prefix, which means “two.” A disaccharide is formed when two monosaccharides combine in a chemical process known as dehydration synthesis, which results in the loss of a water molecule. The condensation reaction is another term for this phenomenon. The disaccharide sucrose is created when the monosaccharides fructose and glucose are combined.

What are Disaccharides?

Disaccharides are the carbohydrates that on hydrolysis carry two identical or disparate monosaccharides. Their general formula is C12H22O11.  The consequential building blocks belonging to disaccharides are sucrose, maltose, and lactose. On hydrolysis with dilute acids or enzymes, these carry the following two molecules of monosaccharides.  

C12H22O11 (sucrose) + H2O C6H12O6 (glucose) + C6H12O6 (fructose)

C12H22O11 (Maltose) + H2O ⭢ C6H12O6 (glucose) + C6H12O6 (glucose)

C12H22O11 (Lactose) + H2O ⭢ C6H12O6 (glucose) + C6H12O6 (Galactose)

The disaccharides are fashioned up of two molecules of monosaccharides associated with each other by the condensation reaction. The associating is crystallized exactly as hemiacetals react with alcohols to solidify acetal with the elimination of a water molecule.  

In a similar way, the hydroxyl groups of hemiacetals of two monosaccharides nits condense to form a disaccharide. The two monosaccharide units are linked to each other through an oxygen atom by a bond calledglycosidic linkage.

Examples of Disaccharides

Sucrose

It’s the most familiar disaccharide and is extensively apportioned in plants specifically sugar cane and sugar beet. It’s fashioned either from cane sugar or sugar beet. The sugar acquired from sugar beet is called beet sugar. It’s a tintless, crystalline, and sweet substance. It’s veritably soluble in water and its aqueous solution is dextrorotatory having (a) p = 66. °. On hydrolysis with dilute acids or enzyme invertase, cane sugar gives an equimolar admixture of)-glucose and D- (-)-fructose. 

C12H22O11 (Sucrose) +H₂O ⭢ C6H12O6 D-(+)-Glucose + C6H12O6 D-(-)-Fructose

Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. D-(-)-fructose has a higher concrete gyration than D-(+)-glucose. Thus, the attendant result upon hydrolysis is laevorotatory nature with distinct gyration of (-39.9°). Since there’s a modification in the sign gyration from Dextro before hydrolysis to leave after hydrolysis, the reaction is called inversion reaction and the admixture (glucose and fructose) is called invert sugar.  

Sucrose solution is fermented by yeast when the enzyme invertase hydrolysis the sucrose to glucose and fructose and enzyme zymase proselytizes these monosaccharides to ethanol.  

C11H22O11 (sucrose) + H2O ⭢ C6H12O6 (glucose) + C6H12O6 (fructose)

C6H12O6 (glucose or fructose) + H2O ⭢ 2C2H6OH (Ethanol) + 2CO2

Maltose

It is known as malt sugar. It is the principal disaccharide obtained by the partial hydrolysis of starch by an enzyme present in malt (sprouted barley seeds).

2(C6H10O5)n (Starch) + nH2O ⭢  nC12H22O11 (Maltose)

On Hydrolysis, one mole of maltose gives two moles of α-D glucose. It is a reducing sugar. It is composed of α-D-glucose units which are condensed together through C1 of one unit and C4 of the other unit. Both glucose relatives are in pyranose form.

Lactose

Lactose occurs in milk and, thus, it’s also called milk sugar.  Lactose on hydrolysis with dilute acid gives an equimolar mixture of β-D glucose and B-D-galactose. It’s reducing sugar. Thus, it’s formulated of β-D-glucose and β-D-galactose units. These units are held together by the glycosidic connection between the C₁ of galactose and the C4 of the glucose unit.  

Lactose gets hydrolyzed by emulsion, an enzyme that especially hydrolyses glycosidic linkages.  

Sugar and Sweetness

The monosaccharides and disaccharides possess a sweet flavour and, thus, they’re called sugars. The degree of sweetness, still, varies as shown below-

SugarRelative sweetness
Lactose16
Maltose32
Galactose32
Glucose74
Sucrose100
Fructose173

As is see-through from the table, fructose is the sweetest of all the natural sugars.  Nevertheless, it may be noted that in addition to carbohydrates, some other chemical compounds are also veritably sweet. For exemplification,  

  • Saccharin (applied as a man-made sweetener) is about 500 times as sweet as sucrose.
  • The protein monoallelic is around 2000 times sweeter than sucrose.
  • The peptide aspartame is around 160 times sweeter than sucrose.

FAQs on Disaccharides

1. Is a diet consisting mainly of rice an adequate diet? Why or why not? 

Ans. Adequate and good quality food is necessary for physical and mental development. This food enter with energy for our daily work. Nutritious food should contain carbohydrates, fats, proteins, and vitamins. Rice alone cannot supply these nutrients. Therefore, a diet mainly of rice is not an adequate diet.

2. What is meant by the inversion of sugar?

The alteration of the especial rotation of sugar from dextrorotatory to laevorotatory is called the inversion of sugar. 

3. Amylose and cellulose are both straight chains polysaccharides containing only D-glucose units. What is the structural difference between the two?

Amylose is a straight-chain polysaccharide having D-glucose units joined together by C-glycoside linkage involving C, of one glucose unit, and C, of the next glucose unit. On the other hand, cellulose is a straight t polysaccharide having D-glucose units joined by B-glycosidic linkages between C, of one glucose unit and C next glucose unit.

4. What is reducing and non-reducing sugars? What is the structural feature characterizing reduce sugars?

Sugars that can degrade Tollen’s reagent or Fehling’s solution are called reducing sugars. The sugars which de reduce Tollen’s reagent or Fehling solution are called non-reducing sugars.  

The distinctive structural affection of reducing sugars in the sight of either-hydroxy aldehyde g (-CHOH-CHO) similar as glucose, mannose, galactose, etc. α-hydroxy keto group (-CO-CH ₂ OH) as present in the fruit.

5. What are the products of Hydrolysis of sucrose?

Glucose and fructose are the products of the Hydrolysis of sucrose.

6. What’s the nomenclature given away to the linkage which holds simultaneously monosaccharide units in polysaccharides?

Glycosidic linkage is the linkage that holds simultaneously monosaccharide units in polysaccharides.  

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.