Free Carbohydrates MCQs with Answers

1. Sucrose is formed from glucose and fructose by which type of reaction?

• A. Hydrolysis
• B. Condensation
• C. Oxidation
• D. Neutralisation

Explanation: A condensation reaction joins the two monosaccharides through a glycosidic linkage and releases one molecule of water. Hydrolysis is the reverse process and would split sucrose back into glucose and fructose, which is what sucrase does in the small intestine. No change in oxidation state and no acid-base pairing is involved.

2. Cellulose and starch are both polymers of glucose, yet humans can digest only starch. The reason is that

• A. cellulose contains beta 1,4 linkages that human enzymes cannot hydrolyse
• B. cellulose molecules are too large to enter the gut
• C. cellulose is a protein rather than a carbohydrate
• D. starch contains no glycosidic bonds at all

Explanation: Starch is built from alpha glucose and its alpha 1,4 linkages are hydrolysed by amylase, while cellulose is built from beta glucose and its beta 1,4 linkages need cellulase, an enzyme humans do not produce. The polymer size is not the obstacle, since starch molecules are also very large. Both are carbohydrates and both contain glycosidic bonds.

3. Which of the following is a reducing sugar?

• A. Sucrose
• B. Starch
• C. Maltose
• D. Cellulose

Explanation: Maltose keeps a free anomeric carbon, so its open-chain form has an aldehyde group that reduces Benedict's reagent to a brick red precipitate. In sucrose both anomeric carbons are locked into the glycosidic bond, which is why it is the classic non-reducing disaccharide. Starch and cellulose are polysaccharides with only one free end in an enormous molecule, so no detectable reduction occurs.

4. Glucose and fructose have the same molecular formula, C6H12O6, but different structures. They are therefore

• A. isomers
• B. polymers
• C. isotopes
• D. identical sugars

Explanation: Isomers share a molecular formula but differ in the arrangement of their atoms, and here glucose carries an aldehyde group while fructose carries a ketone group, which is why only glucose forms a six membered ring. The difference in structure gives a different taste and a different metabolic route despite the identical formula. Isotopes are forms of one element differing in neutron number.

5. Glycogen is a better short term storage molecule in animals than starch is in plants partly because glycogen

• A. contains no glycosidic bonds
• B. is soluble and diffuses freely out of the cell
• C. is built from beta glucose
• D. is far more branched, so many ends can be hydrolysed at once

Explanation: Extensive branching gives a large number of terminal glucose units, so enzymes can release glucose very rapidly when an active animal needs it. Being a large polymer it is also osmotically inactive and compact enough to store in liver and muscle. Starch is less branched and is adequate for a plant, which does not need sudden bursts of energy.

6. The bond formed between two amino acids is called

• A. Glycosidic bond
• B. Peptide bond
• C. Ester bond
• D. Phosphodiester bond

Explanation: A peptide bond (amide bond) forms between the carboxyl group of one amino acid and the amino group of another, with loss of water.

7. Starch and glycogen are polymers of

• A. β-glucose
• B. α-glucose
• C. Fructose
• D. Galactose

Explanation: Both starch and glycogen are polymers of α-glucose, linked by α-1,4 and α-1,6 glycosidic bonds.

8. The enzyme that digests starch in the mouth is

• A. Pepsin
• B. Trypsin
• C. Salivary amylase
• D. Lipase

Explanation: Salivary amylase (ptyalin) begins starch digestion in the mouth, breaking it into maltose.

9. Which of the following has the highest energy content per gram?

• A. Carbohydrate
• B. Protein
• C. Lipid
• D. Nucleic acid

Explanation: Lipids provide about 9 kcal/g, while carbohydrates and proteins provide about 4 kcal/g each.

10. DNA and RNA differ in all EXCEPT

• A. Type of sugar
• B. Presence of uracil vs thymine
• C. Being nucleic acids
• D. Single vs double stranded

Explanation: Both DNA and RNA are nucleic acids. They differ in sugar (deoxyribose vs ribose), bases (T vs U), and structure (double vs single stranded).