Conceptual and Tricky MCQs on Carbohydrates, Lipids, Nucleic Acids, Amino Acids, and Proteins

Suitable for GPAT, NIPER, AIIMS Pharmacist, Railway Pharmacist, SSC, ESIC, and State Pharmacist examinations.

Short Notes: Carbohydrates, Lipids, Nucleic Acids, Amino Acids, and Proteins

Carbohydrates

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, generally following the formula (CH₂O)ₙ.. They are the primary source of energy in the human body and provide approximately 4 kcal/g of energy. Carbohydrates are classified into monosaccharides (glucose and fructose), disaccharides (sucrose, lactose, and maltose), oligosaccharides, and polysaccharides (starch, glycogen, and cellulose). Glucose is the major fuel for the brain and red blood cells. Glycogen serves as the storage form of carbohydrate in animals, while starch is the storage form in plants.

Lipids

Lipids are a heterogeneous group of hydrophobic organic compounds that are insoluble in water but soluble in organic solvents. They provide the highest energy yield among nutrients, supplying approximately 9 kcal/g. Lipids include triglycerides, phospholipids, steroids, and waxes. Triglycerides serve as energy reserves, phospholipids form the structural basis of cell membranes, and cholesterol acts as a precursor for steroid hormones, bile acids, and vitamin D. Essential fatty acids such as linoleic acid and α-linolenic acid must be obtained from the diet.

Amino Acids

Amino acids are the basic structural units of proteins and contain both amino (-NH₂) and carboxyl (-COOH) functional groups attached to the same carbon atom. Twenty standard amino acids are involved in protein synthesis. Amino acids are classified as essential, non-essential, acidic, basic, polar, and non-polar amino acids. Glycine is the only amino acid without an asymmetric carbon atom and is therefore optically inactive. Essential amino acids cannot be synthesised by the body and must be supplied through food.

Proteins

Proteins are polymers of amino acids linked together by peptide bonds. They perform numerous biological functions, including enzymatic catalysis, transport, immunity, muscle contraction, and hormone regulation. Proteins possess four levels of structure: primary, secondary, tertiary, and quaternary. Denaturation caused by heat, pH changes, or chemicals disrupts secondary and tertiary structures but generally leaves the primary structure intact. Most enzymes in the human body are proteins.

Nucleic Acids

Nucleic acids are polymers of nucleotides responsible for storing and transmitting genetic information. The two major types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA contains the sugar deoxyribose and the bases adenine, guanine, cytosine, and thymine, whereas RNA contains ribose and uracil instead of thymine. Nucleotides are connected by phosphodiester bonds. According to Chargaff's rule, adenine equals thymine and guanine equals cytosine in double-stranded DNA.

Points for Competitive Examinations

• Sucrose is a non-reducing sugar, whereas lactose and maltose are reducing sugars.
• Glycogen is the storage carbohydrate of animals; starch is the storage carbohydrate of plants.
• Phospholipids are amphipathic molecules and form biological membranes.
• Linoleic acid and α-linolenic acid are essential fatty acids.
• Glycine is optically inactive due to the absence of a chiral carbon atom.
• Methionine contains sulphur but cannot form disulphide bonds.
• Peptide bonds join amino acids, while phosphodiester bonds join nucleotides.
• DNA contains thymine, whereas RNA contains uracil.
• Denaturation usually does not affect the primary structure of proteins.

Exam Tips

1. Carbohydrates

Remember the Reducing Sugars Rule:

"All common disaccharides are reducing except sucrose."

Reducing sugars:

• Glucose

• Fructose

• Lactose

• Maltose

Non-reducing sugar:

• Sucrose

Storage Polysaccharides:

• Plants Store Starch

• Animals Gather Glycogen

Think: "Plants Store, Animals Gather."

Glycogen vs Amylopectin

• Glycogen → More branching (every 8–12 glucose units)

• Amylopectin → Less branching (every 24–30 glucose units)

Competitive exams frequently ask this comparison.

Structural Polysaccharides

• Cellulose → Plant cell wall

• Chitin → Fungal cell wall and insect exoskeleton

• Peptidoglycan → Bacterial cell wall

2. Lipids

Energy Yield Tip

• Carbohydrates = 4 kcal/g

• Proteins = 4 kcal/g

• Lipids = 9 kcal/g

Remember:
"Fat gives more than double the energy."

Essential Fatty Acids

Remember:
"LiLi are life essentials."

• Linoleic acid

• Linolenic acid

These cannot be synthesised by humans.

Saturated vs Unsaturated Fatty Acids

• Saturated fatty acids → No double bonds

• Unsaturated fatty acids → One or more double bonds

More double bonds = greater susceptibility to oxidation and rancidity.

3. Amino Acids

Optically Inactive Amino Acid

Remember:
"Glycine Goes Alone."

Glycine is the only amino acid without a chiral carbon and therefore is optically inactive.

Sulfur-Containing Amino Acids

• Methionine

• Cysteine

Important distinction:

• Cysteine forms disulfide bonds.

• Methionine does not form disulfide bonds.

Imino Acid

Only Proline contains a secondary amino group and is often called an imino acid.

4. Proteins

Levels of Protein Structure

Remember:
"Primary Students Take Questions"

• Primary structure

• Secondary structure

• Tertiary structure

• Quaternary structure

Denaturation Rule

Denaturation affects:

• Secondary structure

• Tertiary structure

• Quaternary structure

Denaturation usually does not affect:

• Primary structure

This is among the most frequently repeated GPAT and NIPER concepts.

Protein Bond Types

• Peptide bond → Joins amino acids

• Hydrogen bond → Stabilizes secondary structure

• Disulfide bond → Stabilizes tertiary structure

5. Nucleic Acids

DNA vs RNA Trick

• Feature         DNA                  RNA

• Sugar          Deoxyribose         Ribose

• Pyrimidine Base  Thymine              Uracil

• Structure  Double stranded    Usually, single stranded

Remember:
"DNA has Thymine, RNA has Uracil."

Chargaff's Rule Shortcut

In double-stranded DNA:

• A = T

• G = C

If Adenine is 30%:

• Thymine = 30%

• Guanine = 20%

• Cytosine = 20%

Nucleoside vs Nucleotide

• Nucleoside = Base + Sugar

• Nucleotide = Base + Sugar + Phosphate

Remember:
"Phosphate makes the difference."

Frequently Asked Previous-Year topics

• Reducing and non-reducing sugar

• Essential fatty acids

• Glycine as optically inactive amino acid

• Methionine versus cysteine

• Protein denaturation

• DNA-RNA differences

• Chargaff's rule

• Peptide and phosphodiester bonds

• Glycogen and amylopectin branching pattern

• Cholesterol is a precursor of steroid hormones, bile acids, and vitamin D

These facts are repeated because they are among the most frequently tested concepts in biochemistry, human anatomy, and physiology.

1. Which carbohydrate gives a positive Benedict's test but does NOT undergo mutarotation?

A. Glucose
B. Maltose
C. Lactose
D. Fructose

✅ Answer: D. Fructose

Explanation:
Fructose is a reducing sugar and therefore gives a positive Benedict's test. It exists mainly as a ketose and shows very limited mutarotation compared with aldoses, making it a favorite conceptual question in GPAT and NIPER examinations.

2. Which of the following carbohydrates is NOT a reducing sugar?

A. Lactose
B. Maltose
C. Sucrose
D. Cellobiose

✅ Answer: C. Sucrose

Explanation:
In sucrose, both anomeric carbons participate in glycosidic bond formation, leaving no free aldehyde or ketone group available for reduction.

3. Glycogen differs from amylopectin primarily because glycogen possesses:

A. α(1→4) linkages only
B. β(1→4) linkages
C. More frequent α(1→6) branching
D. Fewer glucose residues

✅ Answer: C. More frequent α(1→6) branching

Explanation:
Glycogen has branches every 8–12 glucose residues, whereas amylopectin branches every 24–30 residues.

4. Which polysaccharide forms the major structural component of the bacterial cell wall?

A. Cellulose
B. Chitin
C. Glycogen
D. Peptidoglycan

✅ Answer: D. Peptidoglycan

Explanation:
Peptidoglycan consists of alternating N-acetylglucosamine and N-acetylmuramic acid residues.

5. Which statement regarding cellulose is correct?

A. It contains α(1→4) glycosidic bonds.
B. Humans digest it using cellulase.
C. It consists of β-D-glucose units linked by β(1→4) bonds.
D. It is a branched polysaccharide.

✅ Answer: C. It consists of β-D-glucose units linked by β(1→4) bonds.

Explanation:
Humans lack cellulase enzyme and therefore cannot digest cellulose.

Lipids

6. Which lipid is amphipathic and forms the structural basis of biological membranes?

A. Triglycerides
B. Waxes
C. Phospholipids
D. Cholesterol esters

✅ Answer: C. Phospholipids

Explanation:
Phospholipids possess both hydrophilic and hydrophobic regions, allowing bilayer formation.

7. Which fatty acid contains the maximum number of double bonds?

A. Oleic acid
B. Linoleic acid
C. Linolenic acid
D. Stearic acid

✅ Answer: C. Linolenic acid

Explanation:
Oleic acid = 1 double bond, Linoleic acid = 2, Linolenic acid = 3.

8. Which of the following is an essential fatty acid?

A. Palmitic acid
B. Oleic acid
C. Linoleic acid
D. Stearic acid

✅ Answer: C. Linoleic acid

Explanation:
Humans cannot synthesize linoleic and α-linolenic acids and must obtain them from the diet.

9. Rancidity of fats occurs mainly due to:

A. Hydrolysis
B. Oxidation of unsaturated fatty acids
C. Reduction
D. Polymerization

✅ Answer: B. Oxidation of unsaturated fatty acids

Explanation:
Double bonds in unsaturated fatty acids are susceptible to oxidation, producing unpleasant odors.

10. Cholesterol serves as a precursor for all EXCEPT:

A. Steroid hormones
B. Bile salts
C. Vitamin D
D. Vitamin C

✅ Answer: D. Vitamin C

Explanation:
Cholesterol is the precursor of steroid hormones, bile acids, and vitamin D.

Amino Acids and Proteins

11. Which amino acid lacks an asymmetric carbon atom?

A. Alanine
B. Glycine
C. Valine
D. Serine

✅ Answer: B. Glycine

Explanation:
Glycine contains two hydrogen atoms attached to the α-carbon, making it optically inactive.

12. Which amino acid is classified as an imino acid rather than an amino acid?

A. Lysine
B. Proline
C. Methionine
D. Histidine

✅ Answer: B. Proline

Explanation:
Proline contains a secondary amino group incorporated into a pyrrolidine ring.

13. Which amino acid contains sulfur but cannot form disulfide bonds?

A. Cysteine
B. Methionine
C. Tyrosine
D. Histidine

✅ Answer: B. Methionine

Explanation:
Cysteine forms disulfide bonds due to its sulfhydryl group, whereas methionine contains sulfur in a thioether linkage.

14. Denaturation of proteins generally does NOT affect:

A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure

✅ Answer: A. Primary structure

Explanation:
Denaturation disrupts non-covalent interactions but usually leaves peptide bonds intact.

15. The peptide bond is best described as:

A. An ester bond
B. An amide bond
C. A hydrogen bond
D. A phosphodiester bond

✅ Answer: B. An amide bond

Explanation:
Peptide bonds are formed between the carboxyl group of one amino acid and the amino group of another.

16. Which level of protein structure is stabilized primarily by hydrogen bonding between peptide groups?

A. Primary
B. Secondary
C. Tertiary
D. Quaternary

✅ Answer: B. Secondary

Explanation:
α-Helix and β-pleated sheets are stabilized mainly by hydrogen bonds.

Nucleic Acids

17. Which nitrogenous base is present in RNA but absent in DNA?

A. Adenine
B. Guanine
C. Cytosine
D. Uracil

✅ Answer: D. Uracil

Explanation:
RNA contains uracil instead of thymine.

18. The sugar present in DNA differs from that in RNA by the absence of:

A. Carbon atom at 5' position
B. Hydroxyl group at 2' position
C. Oxygen atom at 3' position
D. Phosphate group

✅ Answer: B. Hydroxyl group at 2' position

Explanation:
DNA contains deoxyribose, lacking the 2'-OH group present in ribose.

19. Which bond links nucleotides together in nucleic acids?

A. Peptide bond
B. Glycosidic bond
C. Phosphodiester bond
D. Hydrogen bond

✅ Answer: C. Phosphodiester bond

Explanation:
The 3'-OH of one nucleotide links with the 5'-phosphate of the next nucleotide.

20. According to Chargaff's rule, if DNA contains 20% adenine, the percentage of cytosine will be:

A. 20%
B. 30%
C. 40%
D. 60%

✅ Answer: B. 30%

Explanation:
A = T = 20%, therefore A + T = 40%. The remaining 60% consists of G + C, so C = 30%.

21. Which nitrogenous base forms three hydrogen bonds with its complementary base?

A. Adenine
B. Thymine
C. Cytosine
D. Uracil

✅ Answer: C. Cytosine

Explanation:
Cytosine pairs with guanine through three hydrogen bonds, increasing DNA stability.

22. Which of the following is a nucleoside and not a nucleotide?

A. AMP
B. ATP
C. Adenosine
D. cAMP

✅ Answer: C. Adenosine

Explanation:
A nucleoside consists of a nitrogenous base plus sugar only, whereas nucleotides additionally contain phosphate groups.

23. Which biomolecule can act both as an energy source and as a structural component?

A. Cholesterol
B. Glycogen
C. Cellulose
D. Glucose

✅ Answer: D. Glucose

Explanation:
Glucose serves as the primary energy source and also acts as a precursor for structural polysaccharides.

24. Which statement is TRUE regarding biological macromolecules?

A. All proteins are enzymes.
B. All carbohydrates are reducing sugars.
C. All lipids are hydrophobic.
D. All enzymes are proteins in humans.

✅ Answer: D. All enzymes are proteins in humans.

Explanation:
Human enzymes are proteins, whereas some viral catalytic molecules are RNA (ribozymes).

25. Which of the following pairs is incorrectly matched?

A. Glycogen — Storage polysaccharide in animals
B. Cellulose — Structural polysaccharide in plants
C. Chitin — Storage polysaccharide in animals
D. Starch — Storage polysaccharide in plants

✅ Answer: C. Chitin — Storage polysaccharide in animals

Explanation:
Chitin is a structural polysaccharide found in fungal cell walls and arthropod exoskeletons.

Examination Points

• Sucrose is non-reducing; lactose and maltose are reducing sugars.

• Glycine is optically inactive.

• Methionine contains sulphur but does not form disulphide bonds.

• DNA contains thymine; RNA contains uracil.

• Phosphodiester bonds join nucleotides, while peptide bonds join amino acids.

• Linoleic and α-linolenic acids are essential fatty acids.

• Denaturation affects secondary, tertiary, and quaternary structures but usually spares primary structure.

Dr Alok Singh