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Carbohydrates are one of the three essential macronutrients needed by the body to obtain energy, apart from proteins and fats. At the chemical level, carbon, hydrogen and oxygen together form carbohydrates. The general formula of carbohydrates is Cx(H2O)y, where 2:1 is the fixed ratio of oxygen and hydrogen. The occurrence of carbohydrates is primarily found in plant-based foods. They also occur in dairy products in the form of lactose. Foods high in carbohydrates include bread, beans, potatoes, rice, and cereals.
In other words, carbohydrates are the most abundant organic molecules in nature that serve as the primary source of energy for living organisms. The structure of Carbohydrates is defined as a polyhydroxy aldehyde or ketone (or their derivatives) with the general formula Cn(H2O)n.
- During the digestion process, carbohydrates are broken down into glucose, also known as blood sugar.
- Carbohydrates are used as the primary energy source for the brain and muscles.
- To maintain the cells, tissues and organs, this sugar is utilised by our body.
- Carbohydrates are also involved in fat metabolism and the prevention of ketosis.
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Classification of Carbohydrates
Carbohydrates are primarily classified into two categories, namely, simple and complex carbohydrates. The former includes Monosaccharides, Disaccharides and Oligosaccharides, whereas the latter contains Polysaccharides.
- Monosaccharides: Simplest carbohydrates (glucose, fructose, galactose, ribose, etc.)
- Disaccharides: Two monosaccharide units joined by a glycosidic linkage (maltose, lactose, sucrose)
- Poly saccharides: Many monosaccharide units (starch, glycogen, cellulose, chitin)
Carbohydrates Classification, Structure & Function With One Shot Video
Multiple Choice Questions on Carbohydrates
Understanding the classification, structure, properties, and biochemical significance of carbohydrates is crucial for both board exams and competitive exams such as NEET. Here are some most important multiple-choice questions on Carbohydrates given below:
1. The general formula of carbohydrates is
- CnH2nOn
- Cn(H2O)n
- Both A and B are correct
- CnH2n+2On
Ans: C
2. Which of the following is a ketotriose?
- Glyceraldehyde
- Dihydroxyacetone
- Erythrose
- Ribulose
Ans: B
3. The number of optical isomers possible for glucose is
- 2
- 4
- 8
- 16
Ans: D (2n where n = 4 chiral carbons)
4. Fructose on treatment with Benedict’s solution gives positive test because
- It is a ketose
- It tautomerises to glucose and mannose in an alkaline medium
- It has a free aldehyde group
- It reduces Tollen’s reagent directly
Ans: B
5. Which disaccharide is present in milk?
- Maltose
- Lactose
- Sucrose
- Cellobiose
Ans: B
6. Sucrose is non-reducing because
- It contains no free anomeric carbon
- Both anomeric carbons are involved in the glycosidic linkage
- It is a ketose
- It is made of two glucose units
Ans: B
7. The glycosidic linkage in maltose is
- α-1→4
- β-1→4
- α-1→2
- α-1→2
Ans: A
8. Which of the following is a non-reducing sugar?
- Glucose
- Maltose
- Sucrose
- Lactose
Ans: C
9. Osazone formation is shown by
- Glucose, fructose and mannose
- Glucose and fructose only
- Glucose and mannose only
- All monosaccharides
Ans: A (All aldoses and ketoses having at least two hydroxyl groups adjacent to the carbonyl group)
10. The storage polysaccharide in animals is
- Starch
- Glycogen
- Cellulose
- Inulin
Ans: B
11. Cellulose is a polymer of
- α-D-glucose with α-1→4 linkages
- α-D-glucose with α-1→4 linkages
- α-D-glucose with β-1→2 linkages
- β-D-fructose with β-2→6 linkages
Ans: B
12. Amylopectin differs from amylose in having
- Α-1→4 linkages only
- Both α-1→4 and α-1→6 linkages
- Β-1→4 linkages
- No branching
Ans: B
13. Which sugar is called invert sugar?
- Glucose
- Fructose
- Mixture of glucose and fructose after hydrolysis of sucrose
- Sucrose
Ans: C
14. The sugar present in DNA is
- Ribose
- Deoxyribose
- Fructose
- Galactose
Ans: B
15. Mutarotation is shown by
- Sucrose
- Cellulose
- Glucose
- Starch
Ans: C
16. Epimers differ in configuration at
- C-1
- Any one specific carbon other than the anomeric carbon
- C-2 and C-4
- All chiral carbons
Ans: B
17. Glucose and mannose are
- C-2 epimers
- C-4 epimers
- Anomers
- Enantiomers
Ans: A
18. Which of the following is the sweetest sugar?
- Sucrose
- Glucose
- Fructose
- Lactose
Ans: C
19. Inulin is a polymer of
- Glucose
- Fructose
- Galactose
- Mannose
Ans: B
20. Humans cannot digest cellulose because they lack
- Amylase
- Cellulase
- Invertase
- Maltage
Ans: B
21. Which of the following is a non-reducing sugar?
- Glucose
- Fructose
- Sucrose
- Maltose
Ans: C
22. Which of the following pairs contains only reducing sugars?
- Sucrose, lactose
- Maltose, lactose
- Sucrose, starch
- Sucrose, fructose
Ans: B
23. Which disaccharide will give a positive test with Benedict’s reagent?
- Sucrose
- Trehalose
- Lactose
- None of these
Ans: C
24. The only non-reducing disaccharide commonly asked in NEET is
- Maltose
- Lactose
- Cellobiose
- Sucrose
Ans: D
25. Which of the following will NOT give a red precipitate with Fehling’s solutions?
- Glucose
- Maltose
- Sucrose
- Lactose
Ans: C
Frequently Asked MCQs on Carbohydrates in Competitive Exams
Ques 1. Which class of carbohydrates cannot be hydrolysed further?
- Monosaccharides
- Polysaccharides
- Disaccharides
- Proteoglycan
Click here for the answer
Ans. a) Monosaccharides
Explanation: Monosaccharides cannot be broken down into smaller units, since they are already simple sugars consisting of a single polyhydroxy aldehyde or ketone unit. The acetal or Ketal unit of one monosaccharide reacts with the hydroxyl group of the next adjacent monosaccharide. In the process of hydrolysis, bonds are broken, and energy is released.
Ques 2. Maltose is a disaccharide of
- Fructose and lactose
- Glucose and glucose
- Glucose and galactose
- Glucose and lactose
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Ans. b) Glucose and glucose
Explanation: Also called malt, maltose is a disaccharide made up of two alpha-D-glucose units. An alpha 1,4 glycosidic bond is formed between the two units of glucose. During the process of digestion, maltose is broken down into two glucose molecules by the enzymes present in the lining of the small intestine. These simple molecules are absorbed by the body.
Ques 3. A sweetener used in sugarless gums and candies
- Ribitol
- Xylitol
- Inositol
- Mannitol
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Ans. b) Xylitol
Explanation: Except for Xylitol, all the others are sugar alcohols. Found in most plant materials, Xylitol is a naturally occurring alcohol found in most plant materials. Xylitol is widely used as a substitute for sugar and in sugar-free chewing gums, mints, and other candies. On the other hand, ribitol, inositol and mannitol are sugar alcohols.
Ques 4. Starch consists of
- Branched amylose and branched amylopectin
- Unbranched amylose and branched amylopectin
- Unbranched amylose and unbranched amylopectin
- None of the above
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Ans. b) unbranched amylose and branched amylopectin
Explanation: Found as a stored carbohydrate in plants, starch is a colourless, odourless polymer. It is made up of glucose monomers joined together to form polysaccharides. The two types of polysaccharide molecules in starch are amylose and amylopectin. Amylose is composed of several D-glucose units. It is an unbranched, straight chain. Whereas amylopectin is a D-glucose polymer made up of multiple components. Hence, it is a branched polymer.
Ques 5. The reserve food material of green algae is
- Starch
- Floridian starch
- Laminarin
- Chrysolaminarin
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Ans. a) Starch
Explanation: Floridian starch is the reserve food material of red algae, whereas Laminarin is the reserve food material of brown algae. Chrysolaminarin is the reserve food material of diatoms.
Reserve foods can be described as important stores of energy in plants that can be used for ATP production whenever the plant requires it. These reserves are mainly composed of fats, carbohydrates or proteins in cells and tissues.
Ques 6. Which of the following is also known as inverted sugar?
- Sucrose
- Fructose
- Dextrose
- Glucose
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Ans. a) Sucrose
Explanation: Sucrose produces an equimolar combination of D-glucose and D-fructose during its hydrolysis with hot dilute acid. This phenomenon is also called the inversion of sucrose or invertase; hence, sucrose is also known as invert sugar. One molecule of glucose and one molecule of fructose together make up sucrose.
Ques 7. Name the major storage form of carbohydrates in animals.
- Starch
- Chitin
- Glycogen
- Cellulose
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Ans. c) Glycogen
Explanation: The major storage form of carbohydrates in animals is glycogen. It is mostly found in the muscles and the liver of animals. A highly branched form of amylopectin, here, branching occurs after every 10 glucose residues. It is also an essential energy reserve found in some microorganisms, like bacteria. Glycogen gets broken down into glucose whenever energy is required by the body.
Ques 8. The only carbohydrate which does not have any chiral carbon atoms is
- Glyceraldehyde
- Erythrose
- Dihydroxyacetone
- Erythrulose
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Ans. c) Dihydroxyacetone
Explanation: The carbonyl group of Dihydroxyacetone is on carbon number 2. It is a simple ketone sugar. All sugars except for dihydroxyacetone have stereoisomers as they have at least one chiral carbon. A chiral atom can be defined as a carbon atom that makes its bonds with four different types or groups of atoms.
Ques 9. What is the name of the drug which inhibits the Na+/K+ pump across the cell membrane?
- Taxol
- Quinone
- Vinblastine
- Ouabain
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Ans. d) Ouabain
Explanation: A common glycoside, ouabain, inhibits the action of enzymes that pump Na+/k+ ions across cell membranes. Glycosides are formed by condensation between monosaccharides. When the anomeric hydroxyl group of a monosaccharide undergoes condensation with the hydroxyl group of a second molecule, the formation of glycosides takes place.
Ques 10. Sugars which differ from each other only around a single carbon atom are called epimers.
- True
- False
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Ans. a) True
Explanation: Common sugars are mostly similar. The basic difference among them is at a single carbon atom, i.e. differing by stereochemistry. A carbohydrate, epimers vary in one position for the placement of the -OH group. For eg: D-glucose and D-galactose. An epimer differs in configuration at only one chiral centre.
Ques 11. A molecule of amylopectin contains 1500 glucose residues and is branched after every 30 residues. How many reducing ends are there?
- 5
- 0
- 2
- 1
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Ans. d) 1
Explanation: Amylopectin is an α-glucose residue joined by both α (1→4) linkages and α (1→6) branch points. Along a chain of α (1→4) linked glucose, branches occur at every twelve to thirty residues. Hence, amylopectin has one reducing end and many nonreducing ends. So, no matter how many residues there are, a molecule of amylopectin has only one reducing end.
Ques 12: Which of the following glycosidic linkages is found in maltose?
- Glucose (α-1 – 2β) Fructose
- Glucose (α1 – 4) Glucose
- Galactose (β1 – 4) Glucose
- Glucose (β1 – 4) Glucose
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Ans. b) Glucose (α1 – 4) Glucose
Explanation: In maltose, α-1, the 4-glycosidic bond is present as the α-anomeric form of C-1 on one sugar and the hydroxyl oxygen atom on C-4 of the other sugar is joined by a glycosidic linkage. Formed between a sugar molecule, or carbohydrate and an OR group, glycosidic linkages are covalent bonds.
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