Biological Molecules MDCAT Notes, MCQs, Past Papers & Answers – Carbohydrates, Proteins, Lipids, Nucleic Acids

Q: What are biological molecules?

Biological molecules are organic compounds essential for life, containing mainly carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Major classes include carbohydrates, proteins, lipids, nucleic acids, and conjugated molecules.

Q: What are the 4 main types of biological molecules?

The four main types are carbohydrates, proteins, lipids, and nucleic acids (DNA and RNA). Conjugated molecules like glycolipids and glycoproteins are combinations.

Q: Why is water biologically important?

Water has high specific heat (thermal stability), is a polar solvent, participates in hydrolysis, has high heat of vaporization, shows density anomaly (ice floats), and exhibits cohesion/adhesion for transpiration.

Q: What is the difference between starch and cellulose?

Starch is made of α-1,4 linked glucose (digestible by humans), while cellulose is made of β-1,4 linked glucose (indigestible because humans lack cellulase).

Q: What are the 4 levels of protein structure?

Primary (amino acid sequence), secondary (α-helix/β-pleated sheet), tertiary (3D folding with disulfide/ionic/hydrophobic bonds), quaternary (multiple polypeptide subunits like haemoglobin).

Q: What is the difference between DNA and RNA?

DNA is double-stranded with deoxyribose and thymine; RNA is usually single-stranded with ribose and uracil. DNA stores genetic information; RNA helps in protein synthesis (mRNA, tRNA, rRNA).

Q: What is a gene?

A gene is a sequence of nucleotides in DNA that codes for a polypeptide or functional RNA. It is the basic unit of heredity.

Q: What are conjugated molecules?

Conjugated molecules are combinations of biological molecules: glycolipids (carbohydrate + lipid) and glycoproteins (carbohydrate + protein). They function in cell recognition, receptors, and membrane stability.

Q: What is the structure of DNA proposed by Watson and Crick?

DNA is a double helix with two antiparallel polynucleotide strands. Bases pair via hydrogen bonds: A=T (2 bonds), G≡C (3 bonds). Diameter 2 nm, pitch 3.4 nm, 10 base pairs per turn.

Q: Which sugar is non-reducing?

Sucrose is a non-reducing sugar because the glycosidic bond involves the anomeric carbons of both glucose and fructose, leaving no free reducing group.

🧬 Chapter 3 · MDCAT Biology

📅 Updated Jan 2026 ⏱️ 30 min read 🎯 High-yield

Biological Molecules — Complete MDCAT Notes & MCQs

Full MDCAT Biology Chapter 3 on biological molecules: water properties, carbohydrates, proteins, lipids, RNA, conjugated molecules, DNA double helix, and genes. Includes 185 solved MCQs with explanations and year-wise past papers.

⚡Quick Answers to Common Searches

Direct, snippet-optimized answers to the top questions students search about Biological Molecules.

What are biological molecules?

Biological molecules are organic compounds essential for life, containing mainly C, H, O, N, P, S. Classes: carbohydrates, proteins, lipids, nucleic acids, conjugated molecules.

What are the 4 main types?

Carbohydrates, proteins, lipids, and nucleic acids (DNA & RNA). Conjugated molecules like glycolipids and glycoproteins are combinations.

Why is water biologically important?

Water has high specific heat (thermal stability), is a polar solvent, participates in hydrolysis, and shows density anomaly (ice floats).

Starch vs cellulose?

Starch = α-1,4 linked glucose (digestible). Cellulose = β-1,4 linked glucose (indigestible by humans, lacks cellulase).

What are the 4 levels of protein structure?

Primary (sequence), secondary (α-helix/β-sheet), tertiary (3D folding), quaternary (multiple subunits like haemoglobin).

What is a gene?

A gene is a sequence of nucleotides in DNA that codes for a polypeptide or functional RNA. Basic unit of heredity.

What are conjugated molecules?

Glycolipids (carbohydrate + lipid) and glycoproteins (carbohydrate + protein) — function in cell recognition and membrane stability.

What is DNA structure by Watson & Crick?

DNA is a double helix with two antiparallel strands. A=T (2 H-bonds), G≡C (3 H-bonds). Diameter 2 nm, pitch 3.4 nm, 10 bp/turn.

Which sugar is non-reducing?

Sucrose is non-reducing because the glycosidic bond involves the anomeric carbons of both glucose and fructose.

What are phospholipids?

Amphipathic lipids (glycerol + 2 fatty acids + phosphate + base) that form the cell membrane bilayer.

What is the difference between DNA and RNA?

DNA: double-stranded, deoxyribose, thymine. RNA: usually single-stranded, ribose, uracil. DNA stores; RNA helps in protein synthesis.

What is the role of tRNA?

tRNA has a cloverleaf shape and carries specific amino acids to the ribosome during translation.

How many MCQs from Biological Molecules?

Biological Molecules typically contributes 3–5 MCQs out of 81 Biology MCQs in MDCAT.

What is cellulose?

Cellulose is a β-1,4 glucose polymer — the most abundant organic molecule on Earth, forming plant cell walls.

What is the specific heat of water?

4.18 J/g°C (1 cal/g°C) — provides thermal stability to cells and organisms.

❓People Also Ask — Biological Molecules

Questions students most frequently search about this chapter. Answers are direct and exam-focused.

QWhat is the difference between starch and glycogen?

Starch is plant storage (amylose + amylopectin). Glycogen is animal storage, similar to amylopectin but more highly branched.

QWhat is the difference between amylose and amylopectin?

Amylose is linear (α-1,4 bonds). Amylopectin is branched with α-1,4 and α-1,6 bonds at branch points.

QWhat are the types of RNA?

mRNA (carries code from DNA to ribosomes), tRNA (transfers amino acids, cloverleaf shape), rRNA (structural component of ribosomes).

QWhat is the difference between a nucleoside and a nucleotide?

Nucleoside = pentose sugar + nitrogenous base. Nucleotide = nucleoside + phosphate. Nucleotides are the monomers of nucleic acids.

QWhat are triglycerides?

Triglycerides (triacylglycerols) are neutral fats formed from glycerol + 3 fatty acids via ester bonds. Main storage form of lipids.

QWhat is the difference between saturated and unsaturated fats?

Saturated fats have no double bonds (solid at room temp, e.g., butter). Unsaturated fats have one or more double bonds (liquid oils).

QWhy is cellulose the most abundant organic molecule?

Cellulose is the primary structural component of plant cell walls and is produced in enormous quantities by plants worldwide.

QWhat is the function of phospholipids?

Phospholipids are amphipathic and form the lipid bilayer of cell membranes. They also participate in cell signaling.

QWhat is the importance of water in biology?

Water is a universal solvent, has high specific heat (thermal stability), participates in hydrolysis, shows cohesion/adhesion (transpiration), and has density anomaly (ice floats).

QWhat is a peptide bond?

A peptide bond is a covalent bond between the carboxyl group of one amino acid and the amino group of another, formed by condensation (water loss).

QWhat are the types of conjugated molecules?

Glycolipids (lipid + carbohydrate) and glycoproteins (protein + carbohydrate). Both are involved in cell recognition and membrane stability.

QWhat is the difference between DNA and RNA?

DNA: double-stranded, deoxyribose, thymine. RNA: usually single-stranded, ribose, uracil. DNA stores genetic info; RNA helps in protein synthesis.

QWhat are amino acids?

Amino acids are the monomers of proteins. Each has an amino group, carboxyl group, and a variable R group. There are 20 standard amino acids.

QWhat is the difference between primary and secondary protein structure?

Primary = linear sequence of amino acids. Secondary = local folding into α-helix or β-pleated sheet via H-bonds.

QWhat is the function of rRNA?

rRNA is a structural component of ribosomes and also acts as a ribozyme (catalyzes peptide bond formation during translation).

QHow many hydrogen bonds are between A and T?

Two hydrogen bonds between Adenine and Thymine. Three between Guanine and Cytosine.

QWhat is the difference between a fat and an oil?

Fats are solid at room temperature (saturated). Oils are liquid at room temperature (unsaturated).

QWhat is the role of cholesterol in membranes?

Cholesterol is a steroid lipid that modulates membrane fluidity — it decreases fluidity at high temperatures and increases it at low temperatures.

QWhat are glycoproteins?

Glycoproteins are proteins with attached carbohydrate chains. They act as receptors, antigens, and mucus components.

QWhat is the difference between a codon and an anticodon?

A codon is a triplet of nucleotides on mRNA that codes for an amino acid. An anticodon is the complementary triplet on tRNA.

📋Syllabus & Learning Outcomes

According to PMDC/PMC MDCAT syllabus, Biological Molecules covers:

3.1 Define and classify biological molecules.
3.2 Discuss the importance of biological molecules.
3.3 Describe biologically important properties of water (polarity, hydrolysis, specific heat, water as solvent and reagent, density, cohesion/ionization).
3.4 Discuss carbohydrates: monosaccharides (glucose), oligosaccharides (cane sugar, sucrose, lactose), polysaccharides (starches, cellulose, glycogen).
3.5 Describe proteins: amino acids, structure of proteins.
3.6 Describe lipids: phospholipids, triglycerides, alcohol and esters (acylglycerol).
3.7 Give an account of structure and function RNA.
3.8 Discuss conjugated molecules (glycolipids, glycoproteins).
3.9 Explain the double helical structure of DNA as proposed by Watson and Crick.
3.10 Define gene is a sequence of nucleotides as part of DNA, which codes for the formation of a polypeptide.

📝Full Notes — Biological Molecules

🧬 Biological Molecules – Syllabus‑Based Schematic Notes

3.1–3.10 | Classification, importance, water, carbohydrates, proteins, lipids, RNA, conjugated molecules, DNA, gene

📌 3.1 & 3.2 Definition, Classification & Importance

Biological molecules = organic compounds essential for life, containing mainly C, H, O, N, P, S.

Classes: Carbohydrates, Proteins, Lipids, Nucleic acids, Conjugated molecules.
Importance: energy storage, structure, catalysis, genetic information, transport, signalling.

🔑 Note: All biomolecules are polymers built from monomers, except lipids.

💧 3.3 Biologically Important Properties of Water

Property	Description & Importance
Polarity	Partial charges ⇒ excellent solvent for polar/ionic substances.
Hydrolysis	Breaks polymers (e.g., digestion of starch, proteins).
Specific Heat	High (4.18 J/g°C) → thermal stability of cells.
Solvent	Dissolves nutrients, gases, waste – medium for biochemical reactions.
Reagent	Participates in reactions like hydrolysis and photosynthesis.
Density anomaly	Ice less dense than liquid → insulates aquatic life.
Cohesion & adhesion	Transpiration pull, surface tension.
Ionisation	Forms H⁺ and OH⁻ → pH regulation; buffer systems.

🍬 3.4 Carbohydrates

Monosaccharides (glucose)

General formula: C₆H₁₂O₆ (aldo‑hexose).
Two forms: α‑glucose (OH on C1 down) and β‑glucose (OH up).
Importance: primary respiratory fuel; subunit of disaccharides and polysaccharides.

Oligosaccharides

Sucrose (cane sugar) = glucose + fructose (α‑1,2 glycosidic bond).
Lactose (milk sugar) = glucose + galactose (β‑1,4 glycosidic bond).
Both are reducing sugars? ⚠️ Sucrose is a non‑reducing sugar.

Polysaccharides

Starch (plant storage): mixture of amylose (α‑1,4 linear) and amylopectin (α‑1,4 + α‑1,6 branches).
Glycogen (animal storage): similar to amylopectin but more highly branched.
Cellulose (plant cell wall): β‑1,4 glucose chains; hydrogen‑bonded to form microfibrils.

🔑 MCQ: Humans can digest starch/glycogen but not cellulose (lack cellulase).

🧱 3.5 Proteins

Monomers: 20 amino acids (common structure: amino group, carboxyl group, R group).
Peptide bond: covalent bond between COOH and NH₂ (condensation reaction).

Structure Levels

Primary: Linear sequence of amino acids.
Secondary: α‑helix / β‑pleated sheet (H‑bonds between backbone atoms).
Tertiary: 3D folding (disulfide, hydrophobic, ionic, H‑bonds).
Quaternary: Multiple polypeptide chains (e.g., haemoglobin – 4 subunits).

⚠️ Trap: Peptide bond is NOT broken by heat – only denatures secondary/tertiary structure.

🧈 3.6 Lipids

Triglycerides (triacylglycerols): glycerol + 3 fatty acids (ester bonds).
Phospholipids: glycerol + 2 fatty acids + phosphate group + nitrogenous base → amphipathic (membrane component).
Alcohols & esters: Fatty acids form ester bonds with glycerol (acylglycerols).

🔑 Note: Phospholipids are the main structural component of cell membranes.

🧬 3.7 Structure and Function of RNA

Single‑stranded polynucleotide (ribose sugar, bases: A, U, G, C).
Types:
- mRNA – carries genetic code from DNA to ribosomes.
- tRNA – transfers amino acids during translation (cloverleaf shape).
- rRNA – structural component of ribosomes.
Contains uracil instead of thymine.

🔗 3.8 Conjugated Molecules

Glycolipids: lipid + carbohydrate chain → cell recognition, membrane stability.
Glycoproteins: protein + carbohydrate chain → receptors, antigens, mucus.

🧬 3.9 DNA Double Helix (Watson & Crick)

Two antiparallel polynucleotide strands (sugar‑phosphate backbones).
Bases pair via H‑bonds: A=T (2 H‑bonds), G≡C (3 H‑bonds).
Diameter 2 nm; pitch 3.4 nm (10 base pairs per turn).
Strands held by H‑bonds and hydrophobic interactions.

⚠️ Trap: A pairs with T in DNA (not U – that’s in RNA).

📄 3.10 Gene

A gene is a sequence of nucleotides in DNA that codes for a polypeptide (or functional RNA).
DNA → (transcription) → mRNA → (translation) → protein.

🔑 Scheme‑Wise High‑Yield MCQs & Traps

Water: High specific heat, cohesion for transpiration, solvent for reactions.
Sucrose: Non‑reducing sugar; glycosidic bond between glucose and fructose.
Starch vs Cellulose: α‑1,4 glucose vs β‑1,4 glucose.
Protein structure: Primary = sequence; secondary = α‑helix/β‑sheet; tertiary = 3D folding.
Phospholipids: Amphipathic; form bilayer.
DNA bases: A=T (2 H‑bonds), G≡C (3). RNA: A=U.
Gene: DNA segment that codes for a polypeptide.
Conjugated molecules: Glycolipids, glycoproteins = cell recognition.
tRNA: Cloverleaf structure; carries amino acid.

💡 MDCAT tip: Focus on differences between DNA & RNA, starch & cellulose, and levels of protein structure.

✅Practice MCQs — Test 1

🧪 Basic Concepts

50 MCQs

1Which of the following is the most abundant compound in a living cell?

A. Proteins
B. Water
C. Carbohydrates
D. Lipids

Answer: B. Water

Water makes up 70–90% of the cell's weight.

2The percentage of water in a bacterial cell is approximately:

A. 20%
B. 50%
C. 70%
D. 95%

Answer: C. 70%

Bacterial cells contain about 70% water.

3Which property of water allows it to act as a temperature buffer in biological systems?

A. Low density of ice
B. High specific heat capacity
C. High surface tension
D. Incompressibility

Answer: B

High specific heat capacity means water absorbs or releases large amounts of heat with little temperature change.

4The amount of heat required to change 1g of liquid water into vapor is known as:

A. Specific heat
B. Heat of fusion
C. Heat of vaporization
D. Heat of ionization

Answer: C

Heat of vaporization is the energy needed to convert liquid water to water vapor.

5Water acts as a universal solvent primarily because of its:

A. High boiling point
B. Non-polar nature
C. Polarity and hydrogen bonding
D. Low viscosity

Answer: C

Water's polarity and ability to form hydrogen bonds make it an excellent solvent for polar and ionic substances.

6Which of the following is an example of an aldo-pentose sugar?

A. Glucose
B. Fructose
C. Ribose
D. Galactose

Answer: C. Ribose

Ribose is a 5-carbon sugar with an aldehyde group.

7Glycosidic bonds are formed by which type of reaction?

A. Hydrolysis
B. Condensation
C. Oxidation
D. Reduction

Answer: B. Condensation

Glycosidic bonds are formed by condensation (removal of water).

8Which of the following is a non-reducing sugar?

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

Answer: D. Sucrose

In sucrose, the reducing groups of both glucose and fructose are involved in the bond, making it non-reducing.

9Starch gives which color when treated with iodine?

A. Red
B. Blue
C. Yellow
D. No color

Answer: B. Blue

Iodine forms a blue-black complex with the helical structure of amylose in starch.

10The most abundant organic molecule in nature is:

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

Answer: C. Cellulose

Cellulose is the primary component of plant cell walls and the most abundant organic polymer on Earth.

11Glycogen is primarily stored in which parts of the human body?

A. Brain and Heart
B. Liver and Muscles
C. Kidneys and Lungs
D. Stomach and Intestines

Answer: B

Glycogen is stored mainly in the liver and skeletal muscles.

12Amino acids are linked together by:

A. Ester bonds
B. Glycosidic bonds
C. Peptide bonds
D. Phosphodiester bonds

Answer: C. Peptide bonds

Peptide bonds form between the carboxyl group of one amino acid and the amino group of another.

13How many different types of amino acids are typically found in proteins?

A. 10
B. 20
C. 64
D. 100

Answer: B. 20

There are 20 standard amino acids that are used to build proteins.

14The primary structure of a protein is determined by the:

A. Number and sequence of amino acids
B. Folding into alpha-helices
C. Formation of disulfide bridges
D. Interaction between multiple subunits

Answer: A

Primary structure is the linear sequence of amino acids.

15Which level of protein structure is represented by the Alpha-helix?

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

Answer: B. Secondary

Alpha-helix is a common secondary structure stabilized by hydrogen bonds.

16Hemoglobin consists of how many polypeptide chains?

A. 1
B. 2
C. 3
D. 4

Answer: D. 4

Hemoglobin has four polypeptide chains: two alpha and two beta.

17Disulfide bridges are most important in stabilizing which protein structure?

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

Answer: C. Tertiary

Disulfide bridges form between cysteine residues and stabilize the tertiary structure.

18Lipids are insoluble in water but soluble in:

A. Organic solvents like ether
B. Dilute acids
C. Pure water
D. Salt solutions

Answer: A

Lipids are non-polar and dissolve in non-polar organic solvents.

19A triglyceride is formed by the condensation of:

A. One glycerol and one fatty acid
B. One glycerol and three fatty acids
C. Three glycerols and one fatty acid
D. Three glycerols and three fatty acids

Answer: B

Triglycerides consist of a glycerol backbone esterified to three fatty acids.

20Saturated fatty acids are generally ______ at room temperature.

A. Liquid
B. Solid
C. Gaseous
D. Plasma

Answer: B. Solid

Saturated fatty acids pack tightly and are solid at room temperature (e.g., butter).

21Phospholipids are major components of:

A. Cell walls
B. Plasma membranes
C. Ribosomes
D. Nucleoli

Answer: B. Plasma membranes

Phospholipids form the lipid bilayer of plasma membranes.

22Which of the following is a steroid?

A. Oleic acid
B. Cholesterol
C. Cellulose
D. Insulin

Answer: B. Cholesterol

Cholesterol is a steroid lipid with a characteristic four-ring structure.

23Waxes are esters of long-chain fatty acids with:

A. Glycerol
B. Long-chain monohydric alcohols
C. Ethylene glycol
D. Amino acids

Answer: B

Waxes are formed from fatty acids and long-chain alcohols.

24The pentose sugar found in DNA is:

A. Ribose
B. Deoxyribose
C. Glucose
D. Xylose

Answer: B. Deoxyribose

DNA contains deoxyribose, which lacks an oxygen atom at the 2' position.

25Which nitrogenous base is NOT found in DNA?

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

Answer: D. Uracil

Uracil is found in RNA; DNA uses thymine instead.

26In the Watson-Crick model of DNA, the two strands are:

A. Parallel
B. Antiparallel
C. Discontinuous
D. Single-layered

Answer: B. Antiparallel

DNA strands run in opposite directions (5'→3' and 3'→5').

27How many hydrogen bonds are between Adenine and Thymine?

A. 1
B. 2
C. 3
D. 4

Answer: B. 2

A-T base pairs form two hydrogen bonds.

28One complete turn of the DNA double helix is ______ long.

A. 0.34 nm
B. 2.0 nm
C. 3.4 nm
D. 34 nm

Answer: C. 3.4 nm

A full turn of B-DNA is 3.4 nm, containing 10 base pairs.

29A 'Gene' is a sequence of nucleotides that codes for:

A. A lipid
B. A polysaccharide
C. A polypeptide
D. A vitamin

Answer: C. A polypeptide

A gene encodes the information for a polypeptide chain.

30RNA molecules are usually:

A. Double-stranded
B. Single-stranded
C. Triple-stranded
D. Circular

Answer: B. Single-stranded

Most RNA molecules are single-stranded, though they can form secondary structures.

31Which type of RNA carries the genetic code from DNA to ribosomes?

A. tRNA
B. rRNA
C. mRNA
D. snRNA

Answer: C. mRNA

mRNA (messenger RNA) carries the genetic code for protein synthesis.

32Conjugated molecules formed by carbohydrates and lipids are:

A. Glycoproteins
B. Glycolipids
C. Lipoproteins
D. Nucleoproteins

Answer: B. Glycolipids

Glycolipids are composed of carbohydrates attached to lipids.

33Glycoproteins are often found in the cell membrane as:

A. Storage molecules
B. Receptors or markers
C. Enzymes
D. Hormones

Answer: B. Receptors or markers

Glycoproteins act as cell surface receptors and recognition markers.

34The basic repeating unit of nucleic acids is:

A. Nucleoside
B. Nucleotide
C. Amino acid
D. Fatty acid

Answer: B. Nucleotide

Nucleotides (sugar + base + phosphate) are the monomers of nucleic acids.

35A nucleoside is composed of:

A. Pentose sugar + Phosphate
B. Pentose sugar + Nitrogenous base
C. Nitrogenous base + Phosphate
D. Base + Sugar + Phosphate

Answer: B

A nucleoside is a sugar and a base; a nucleotide adds a phosphate.

36Acylglycerols are also known as:

A. Neutral fats
B. Waxes
C. Phospholipids
D. Proteins

Answer: A. Neutral fats

Acylglycerols (triglycerides) are neutral fats.

37Which of the following is a fibrous protein?

A. Hemoglobin
B. Keratin
C. Insulin
D. Enzymes

Answer: B. Keratin

Keratin is a structural fibrous protein found in hair and nails.

38The distance between two consecutive base pairs in DNA is:

A. 3.4 nm
B. 0.34 nm
C. 20 nm
D. 10 nm

Answer: B. 0.34 nm

Base pairs stack 0.34 nm apart.

39Hydrolysis of maltose yields:

A. Glucose + Fructose
B. Glucose + Galactose
C. Glucose + Glucose
D. Fructose + Galactose

Answer: C. Glucose + Glucose

Maltose is a disaccharide of two glucose units.

40Which property of water prevents the concentration of ions from changing rapidly in cells?

A. High specific heat
B. High heat of vaporization
C. High dielectric constant
D. Low density of ice

Answer: C

High dielectric constant reduces electrostatic forces between ions, keeping them dissolved and buffering concentration changes.

41The backbone of a DNA strand is composed of alternating:

A. Bases and phosphates
B. Sugars and bases
C. Sugars and phosphates
D. Amino acids and sugars

Answer: C. Sugars and phosphates

The sugar-phosphate backbone consists of alternating deoxyribose and phosphate groups.

42Which of the following lipids is most common in animal fats?

A. Phospholipids
B. Triglycerides
C. Terpenes
D. Prostaglandins

Answer: B. Triglycerides

Animal fats are primarily triglycerides (neutral fats).

43The ratio of H to O in carbohydrates is usually:

A. 1:1
B. 2:1
C. 1:2
D. 3:1

Answer: B. 2:1

Carbohydrates have the general formula (CH₂O)ₙ, so H:O = 2:1.

44Silk fiber is an example of which type of protein?

A. Globular
B. Fibrous
C. Conjugated
D. Tertiary

Answer: B. Fibrous

Silk fibroin is a fibrous protein with extensive β-pleated sheet structure.

45Which of the following is a keto-hexose?

A. Glucose
B. Galactose
C. Fructose
D. Mannose

Answer: C. Fructose

Fructose is a ketohexose (ketone group on carbon 2).

46Amylopectin is the ______ part of starch.

A. Soluble and unbranched
B. Insoluble and branched
C. Soluble and branched
D. Insoluble and unbranched

Answer: B. Insoluble and branched

Amylopectin is the branched, insoluble component of starch.

47Amino acids show ______ behavior due to the presence of both amino and carboxyl groups.

A. Acidic
B. Basic
C. Amphoteric
D. Neutral

Answer: C. Amphoteric

Amino acids can act as both acid and base (zwitterions).

48Lipoproteins are involved in the transport of:

A. Sugars
B. Ions
C. Lipids in blood
D. Oxygen

Answer: C. Lipids in blood

Lipoproteins (e.g., LDL, HDL) transport lipids through the bloodstream.

49How many water molecules are released during the formation of a tripeptide?

A. 1
B. 2
C. 3
D. 4

Answer: B. 2

Forming two peptide bonds releases two water molecules.

50The specific sequence of amino acids in a protein is determined by:

A. tRNA
B. mRNA
C. rRNA
D. DNA

Answer: D. DNA

The amino acid sequence is ultimately encoded in DNA.

✅Practice MCQs — Test 2

🧪 Advanced Concepts

50 MCQs

1If a DNA molecule contains 20% Adenine, what is the total percentage of Pyrimidines in that molecule?

A. 20%
B. 30%
C. 50%
D. 80%

Answer: C. 50%

By Chargaff's rule, purines = pyrimidines = 50%.

2Which property of water is most directly responsible for the maintenance of the tertiary structure of globular proteins?

A. High specific heat
B. Hydrophobic exclusion
C. High heat of vaporization
D. Adhesion

Answer: B

Hydrophobic exclusion drives non-polar side chains to the interior, stabilizing folding.

3In a polypeptide chain of 100 amino acids, how many peptide bonds and water molecules are involved in its complete hydrolysis?

A. 100 bonds, 100 waters
B. 99 bonds, 99 waters
C. 101 bonds, 100 waters
D. 99 bonds, 100 waters

Answer: B

Number of peptide bonds = n-1 = 99; each hydrolysis consumes one water.

4Which of the following describes the linkage in Cellulose?

A. α (1-4) glycosidic linkage
B. α (1-6) glycosidic linkage
C. β (1-4) glycosidic linkage
D. β (1-2) glycosidic linkage

Answer: C. β (1-4) glycosidic linkage

Cellulose has β(1→4) linkages, making it linear and indigestible by humans.

5At its isoelectric point, an amino acid exists as a Zwitterion. This means it has:

A. No charge
B. A net positive charge
C. A net negative charge
D. Equal positive and negative charges

Answer: D

Zwitterion has both +NH₃ and -COO⁻, net charge zero.

6Which of these lipids has the highest number of double bonds in its fatty acid chains?

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

Answer: D. Linolenic acid

Linolenic acid is polyunsaturated with three double bonds.

7If the temperature of a cell increases, how might the cell adjust its membrane phospholipids to maintain fluidity?

A. Increase saturated fatty acids
B. Increase unsaturated fatty acids
C. Decrease cholesterol
D. Increase the length of fatty acid chains

Answer: A

Saturated fats pack more tightly, reducing fluidity to compensate for high temperature.

8Which of the following is a characteristic of B-DNA as described by Watson and Crick?

A. The two strands are parallel and run 5' → 3'
B. The helix is left-handed
C. Each base pair is rotated by 36° relative to the next
D. It contains 11 base pairs per turn

Answer: C

One full turn (360°) has 10 base pairs → 36° per base pair.

9The conversion of a protein from its native state to a denatured state usually involves the breaking of all EXCEPT:

A. Hydrogen bonds
B. Ionic bonds
C. Peptide bonds
D. Hydrophobic interactions

Answer: C

Denaturation disrupts secondary, tertiary, and quaternary structures but not primary (peptide bonds).

10Which molecule acts as a 'second messenger' in many biological signaling pathways?

A. ATP
B. cAMP
C. DNA
D. rRNA

Answer: B. cAMP

Cyclic AMP (cAMP) is a common second messenger.

11During the formation of a phosphodiester bond, the phosphate group of one nucleotide attaches to:

A. The 5' carbon of its own sugar
B. The 2' carbon of the next sugar
C. The 3' carbon of the next sugar
D. The nitrogenous base of the next nucleotide

Answer: C

The 5' phosphate of one nucleotide bonds to the 3' OH of the next.

12Which of the following is NOT a conjugated protein?

A. Casein (Phosphoprotein)
B. Mucin (Glycoprotein)
C. Myoglobin (Metalloprotein)
D. Albumin

Answer: D. Albumin

Albumin is a simple protein without a prosthetic group.

13A fatty acid with the formula C₁₈H₃₄O₂ is most likely:

A. Saturated
B. Monounsaturated
C. Polyunsaturated
D. A wax

Answer: B

C₁₈H₃₆O₂ is saturated; C₁₈H₃₄O₂ has one double bond (monounsaturated).

14Sucrose is non-reducing because:

A. It is a polysaccharide
B. The glycosidic bond involves the anomeric carbons of both monomers
C. It contains a β linkage
D. It is insoluble in water

Answer: B

Both anomeric carbons are involved in the glycosidic bond, so no free reducing end.

15What is the main reason that human enzymes cannot digest cellulose?

A. It is too large
B. It is highly branched
C. Humans lack the enzyme that breaks β (1-4) glycosidic bonds
D. Cellulose is non-polar

Answer: C

Humans lack cellulase; β(1→4) bonds are not hydrolyzed by human digestive enzymes.

16Which of the following is an example of a 'Secondary Metabolite' derived from lipids?

A. Triglycerides
B. Terpenoids
C. Phospholipids
D. Prostaglandins

Answer: B

Terpenoids are plant secondary metabolites derived from isoprene units.

17If a protein contains 4 subunits (like Hemoglobin), the way these subunits fit together is its:

A. Secondary structure
B. Tertiary structure
C. Quaternary structure
D. Supra-molecular structure

Answer: C

Quaternary structure refers to the arrangement of multiple polypeptide subunits.

18Water has a high dielectric constant. This allows it to:

A. Form ice that floats
B. Absorb much heat with little temperature change
C. Reduce the attraction between oppositely charged ions
D. Move up through xylem via capillary action

Answer: C

High dielectric constant weakens electrostatic forces, making water a good solvent for ions.

19How many chiral centers (asymmetric carbons) does a molecule of glucose have in its open-chain form?

A. 1
B. 2
C. 4
D. 5

Answer: C. 4

Glucose has four chiral centers (carbons 2,3,4,5).

20Which nitrogenous base has a single-ring structure and is found in both DNA and RNA?

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

Answer: C. Cytosine

Cytosine is a pyrimidine found in both DNA and RNA.

21Acylglycerols with higher proportions of short-chain or unsaturated fatty acids tend to be:

A. Waxes
B. Solid fats
C. Liquid oils
D. Phospholipids

Answer: C

Unsaturated fatty acids introduce kinks, preventing tight packing → liquid oils.

22In the DNA double helix, the two strands are held together by:

A. Covalent bonds between bases
B. Hydrogen bonds between sugar and phosphate
C. Hydrogen bonds between complementary bases
D. Phosphodiester bonds

Answer: C

Complementary base pairing via hydrogen bonds (A-T, G-C) holds strands together.

23Which of the following describes the 'Gene' in modern molecular terms?

A. A unit of inheritance on a chromosome
B. A sequence of amino acids in a protein
C. A cistron; a segment of DNA coding for one polypeptide
D. A single nitrogenous base

Answer: C

A gene is a DNA segment that codes for a polypeptide or functional RNA.

24Which type of RNA acts as an enzyme (ribozyme) during protein synthesis?

A. mRNA
B. tRNA
C. rRNA
D. snRNA

Answer: C. rRNA

rRNA in the ribosome catalyzes peptide bond formation.

25The specific heat of water is 1.0 cal/g/°C. This is higher than most substances because:

A. Water is a small molecule
B. Much of the heat is used to break hydrogen bonds before the temperature can rise
C. Water is very dense
D. Water is a polar solvent

Answer: B

Heat energy is first used to break hydrogen bonds, delaying temperature increase.

26Which of the following is a storage polysaccharide in the liver of animals?

A. Amylose
B. Amylopectin
C. Glycogen
D. Chitin

Answer: C. Glycogen

Glycogen is the storage polysaccharide in animal liver and muscles.

27The tertiary structure of a protein is NOT typically held by:

A. Ionic bonds
B. Disulfide bridges
C. Peptide bonds
D. Van der Waals forces

Answer: C

Peptide bonds define primary structure, not tertiary.

28Phospholipids are 'amphipathic' molecules. This means they are:

A. Strictly hydrophobic
B. Strictly hydrophilic
C. Part hydrophobic and part hydrophilic
D. Soluble in both oil and vinegar

Answer: C

Amphipathic molecules have both polar (hydrophilic) and non-polar (hydrophobic) regions.

29In a nucleotide, the nitrogenous base is attached to the sugar's:

A. 1' carbon
B. 2' carbon
C. 3' carbon
D. 5' carbon

Answer: A

The base is attached to the 1' carbon of the pentose sugar.

30Which of the following would be the most stable DNA molecule?

A. One with 10% G-C content
B. One with 30% G-C content
C. One with 50% G-C content
D. One with 80% G-C content

Answer: D

G-C pairs have three hydrogen bonds (vs. two for A-T), increasing thermal stability.

31Prostaglandins are a class of lipids that function primarily as:

A. Energy storage
B. Structural components of the cell wall
C. Local hormones and signaling molecules
D. Pigments

Answer: C

Prostaglandins act as local signaling molecules (paracrine/autocrine).

32Which of the following sugars is a keto-pentose?

A. Ribose
B. Ribulose
C. Xylulose
D. Both B and C

Answer: D

Ribulose and xylulose are ketopentoses.

33The α-helix of a protein is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amino hydrogen of the amino acid that is:

A. Next to it
B. Two positions away
C. Four positions away
D. Ten positions away

Answer: C

Hydrogen bonds occur between residues i and i+4.

34Hydrolysis of one mole of a triglyceride requires how many moles of water?

A. 1
B. 2
C. 3
D. 4

Answer: C. 3

Three ester bonds → three water molecules.

35A 'Nucleoside' differs from a 'Nucleotide' by the absence of:

A. Sugar
B. Nitrogenous base
C. Phosphate group
D. Hydroxyl group

Answer: C

A nucleotide has a phosphate group; a nucleoside does not.

36The bonding in Amylopectin at branch points is:

A. α (1-4)
B. α (1-6)
C. β (1-4)
D. β (1-6)

Answer: B. α (1-6)

Amylopectin has α(1→4) linkages in chains and α(1→6) at branch points.

37Which of the following is the most reduced (and thus energy-rich) biological molecule?

A. Glucose
B. Amino acid
C. Fatty acid
D. RNA

Answer: C. Fatty acid

Fatty acids have the highest ratio of C-H bonds, making them highly reduced and energy-dense.

38The concentration of H⁺ ions in pure water at 25°C is:

A. 10⁻¹⁴ M
B. 10⁻⁷ M
C. 7.0 M
D. 1.0 M

Answer: B. 10⁻⁷ M

[H⁺] = 10⁻⁷ M for neutral water.

39Silk (Fibroin) is an example of which secondary structure?

A. α-helix
B. β-pleated sheet
C. Triple helix
D. Random coil

Answer: B. β-pleated sheet

Silk fibroin is rich in β-pleated sheets.

40The 3' end of a DNA strand has a free:

A. Phosphate group
B. Nitrogenous base
C. Hydroxyl group
D. Methyl group

Answer: C. Hydroxyl group

The 3' end has a free hydroxyl (-OH) group on the deoxyribose.

41Which of the following is an example of an 'Omega-3' fatty acid?

A. Stearic acid
B. Oleic acid
C. α-Linolenic acid
D. Arachidonic acid

Answer: C. α-Linolenic acid

α-Linolenic acid is an omega-3 fatty acid.

42Collagen is a fibrous protein characterized by:

A. A single α-helix
B. A triple helix
C. β-pleated sheets
D. Globular subunits

Answer: B. A triple helix

Collagen has a triple helix structure.

43Which molecule is considered the 'Universal Energy Currency'?

A. Glucose
B. ATP
C. Creatine Phosphate
D. NADH

Answer: B. ATP

ATP is the primary energy carrier in cells.

44The most common form of DNA found in living cells is:

A. A-DNA
B. B-DNA
C. Z-DNA
D. C-DNA

Answer: B. B-DNA

B-DNA is the standard right-handed double helix under physiological conditions.

45Which of the following is a conjugated molecule found on the surface of HIV that allows it to recognize T-cells?

A. Glycolipid
B. Lipoprotein
C. Glycoprotein
D. Nucleoprotein

Answer: C. Glycoprotein

HIV's gp120 is a glycoprotein that binds CD4 receptors.

46How many hydrogen bonds are formed in the DNA sequence 5'-ATGC-3' with its complementary strand?

A. 4
B. 8
C. 10
D. 12

Answer: C. 10

A-T=2, T-A=2, G-C=3, C-G=3 → total 10 hydrogen bonds.

47Chitin is a polymer of:

A. Glucose
B. N-acetylglucosamine
C. Galactose
D. Amino acids

Answer: B. N-acetylglucosamine

Chitin is composed of N-acetylglucosamine monomers.

48Which property of water allows insects like the water strider to walk on its surface?

A. High density
B. Cohesion and surface tension
C. Adhesion
D. Hydrolysis

Answer: B

Cohesion creates surface tension, supporting small insects.

49An unsaturated fat can be converted to a saturated fat by the process of:

A. Oxidation
B. Hydrolysis
C. Hydrogenation
D. Dehydration

Answer: C. Hydrogenation

Hydrogenation adds hydrogen to double bonds, making the fat more saturated.

50The 'Primary Structure' of a protein is most important because it:

A. Determines the final 3D shape and function
B. Contains all the peptide bonds
C. Is the only structure not destroyed by heat
D. Is the simplest structure

Answer: A

The sequence of amino acids (primary structure) dictates all higher-order folding.

✅Practice MCQs — Test 3

🧪 Exam-Level Questions

50 MCQs

1At 37°C, the ionic product of water (K_w) is slightly higher than at 25°C. What is the physiological consequence of this in a human cell?

A. The pH of pure water remains 7.0
B. The concentration of H⁺ and OH⁻ ions increases, making the neutral point lower than 7.0
C. Water becomes a non-polar solvent
D. Specific heat capacity decreases by 50%

Answer: B

Neutrality is [H⁺]=[OH⁻]; as K_w increases, neutral pH drops to ~6.8 at 37°C.

2Which molecular behavior of water prevents the 'Cold Shock' in aquatic organisms during rapid environmental temperature drops?

A. High heat of fusion
B. Maximum density at 4°C
C. High dielectric constant
D. Thermal conductivity

Answer: B

Water's maximum density at 4°C causes colder water to float, insulating deeper layers.

3In the hydration shell of a Sodium ion (Na⁺) in an aqueous solution, the water molecules are oriented such that:

A. The Hydrogen atoms point toward the ion
B. The Oxygen atoms point toward the ion
C. The molecules rotate randomly
D. They form covalent bonds with the ion

Answer: B

The partial negative charge on oxygen is attracted to the positive sodium ion.

4The 'Hydrophobic Effect,' which drives protein folding, is primarily an:

A. Enthalpy-driven process
B. Entropy-driven process
C. Ionic-driven process
D. Exothermic reaction

Answer: B

Water molecules become more disordered when hydrophobic surfaces aggregate, increasing entropy.

5During the mutarotation of Glucose in an aqueous solution, the equilibrium mixture consists primarily of:

A. 100% α-D-Glucopyranose
B. 50% α and 50% β forms
C. ~64% β-D-Glucopyranose and ~36% α-D-Glucopyranose
D. Mostly the open-chain aldehyde form

Answer: C

The β form is more stable due to equatorial OH at C1, so it predominates.

6Which of the following is a 'Heteropolysaccharide'?

A. Amylopectin
B. Peptidoglycan
C. Inulin
D. Cellulose

Answer: B

Peptidoglycan contains alternating NAG and NAM units.

7In a molecule of Glycogen, a single branch point involves which of the following?

A. One glucose residue participating in three glycosidic bonds
B. One glucose residue participating in one glycosidic bond
C. Two fructose residues
D. Dehydration at C3 and C5

Answer: A

The branching glucose is linked via C1, C4, and C6.

8The sweetness of honey increases over time because of the slow hydrolysis of sucrose into glucose and fructose. Fructose is sweeter because:

A. It is an aldose
B. It is a furanose in its most stable sweet-tasting form
C. It has more Carbon atoms than glucose
D. It cannot form hydrogen bonds

Answer: B

Fructose in its furanose form is perceived as sweeter.

9In a Ramachandran Plot, the values of φ (phi) and ψ (psi) angles are restricted primarily by:

A. The length of the peptide bond
B. Steric hindrance between side chains and the main chain
C. The number of amino acids
D. The presence of quaternary structure

Answer: B

Steric clashes limit allowed backbone conformations.

10Which amino acid disrupts the α-helix because its side chain cycles back onto the backbone nitrogen, limiting rotation?

A. Glycine
B. Proline
C. Histidine
D. Tryptophan

Answer: B. Proline

Proline's cyclic side chain rigidifies the backbone, breaking helices.

11At physiological pH (7.4), which amino acid is most likely to act as a buffer due to its pK_a being close to 6.0?

A. Lysine
B. Arginine
C. Histidine
D. Aspartic acid

Answer: C. Histidine

Histidine's imidazole side chain has pK_a ≈ 6.0, ideal for buffering near neutral pH.

12The peptide bond has a 'partial double bond character.' This implies that:

A. It is longer than a single bond
B. It is free to rotate
C. The Six atoms of the peptide group are coplanar
D. It can be easily broken by cold water

Answer: C

Resonance restricts rotation, making the six atoms (Cα, C, O, N, H, Cα) planar.

13A protein is treated with β-mercaptoethanol. Which level of structure is specifically targeted?

A. Primary
B. Secondary
C. Tertiary (Disulfide bonds)
D. Quaternary (only if subunits are linked by peptide bonds)

Answer: C

β-Mercaptoethanol reduces disulfide bonds, disrupting tertiary structure.

14Which lipid is a precursor for the synthesis of 'Prostaglandins' (local hormones)?

A. Cholesterol
B. Arachidonic acid
C. Palmitic acid
D. Phosphatidylcholine

Answer: B

Arachidonic acid (20:4ω6) is the precursor for prostaglandins.

15The 'Saponification Value' of a fat is inversely proportional to its:

A. Degree of unsaturation
B. Average molecular weight of fatty acids
C. Number of double bonds
D. Melting point

Answer: B

Smaller fatty acids → more molecules per gram → higher saponification value.

16Sphingomyelins differ from triglycerides because they contain:

A. Glycerol
B. Sphingosine instead of glycerol
C. Three fatty acids
D. No phosphate

Answer: B

Sphingomyelins have a sphingosine backbone, not glycerol.

17In the 'Fluid Mosaic Model,' the flip-flop movement of phospholipids (moving from one leaflet to another) is:

A. Frequent and rapid
B. Rare and requires enzymes (Flippases)
C. Impossible
D. Driven by osmosis

Answer: B

Transverse diffusion (flip-flop) is slow and often catalyzed by flippases.

18If a DNA sample has a melting temperature (T_m) of 85°C, and another has a T_m of 92°C, the second sample likely has:

A. More A-T pairs
B. Higher G-C content
C. Longer strands
D. Fewer phosphodiester bonds

Answer: B

Higher G-C content increases T_m due to three hydrogen bonds per pair.

19The 'Hyperchromic Effect' in DNA refers to:

A. DNA becoming darker in color when heated
B. An increase in UV light absorption (260 nm) when DNA denatures
C. The formation of a triple helix
D. The binding of Histones to DNA

Answer: B

Denatured single strands absorb more UV light at 260 nm than double-stranded DNA.

20Which tautomeric form of nitrogenous bases can lead to spontaneous mutations during replication?

A. Keto form
B. Amino form
C. Enol or Imino forms
D. Cis form

Answer: C

Rare enol/imino tautomers alter base pairing, causing mispairing and mutations.

21In a nucleoside, the bond connecting the base to the sugar is a:

A. α-N-Glycosidic bond
B. β-N-Glycosidic bond
C. Phosphodiester bond
D. Ester bond

Answer: B

The glycosidic bond in nucleosides is β-N-glycosidic.

22'Z-DNA' differs from 'B-DNA' because it is:

A. Right-handed and wider
B. Left-handed and zig-zag in appearance
C. Single-stranded
D. Found only in RNA

Answer: B

Z-DNA is a left-handed double helix with a zig-zag backbone.

23Which conjugated molecule is responsible for the ABO blood group specificity on the surface of RBCs?

A. Lipoproteins
B. Glycosphingolipids (Glycolipids)
C. Nucleoproteins
D. Phosphoproteins

Answer: B

ABO antigens are glycolipids (glycosphingolipids) on red blood cells.

24The protein part of a conjugated protein is called the apoprotein, while the non-protein part is the:

A. Co-enzyme
B. Prosthetic group
C. Activator
D. Apoenzyme

Answer: B

The non-protein component tightly bound is a prosthetic group.

25Which of the following is a 'High-Energy' intermediate but NOT a nucleotide?

A. ATP
B. 1,3-Bisphosphoglycerate
C. cAMP
D. GTP

Answer: B

1,3-BPG is a high-energy metabolic intermediate, not a nucleotide.

26Calculate the number of Phosphodiester bonds in a linear double-stranded DNA molecule of 1000 base pairs.

A. 1000
B. 1998
C. 1999
D. 2000

Answer: B. 1998

Each strand has 1000 nucleotides → bonds per strand = 999 → total = 1998.

27A sample of DNA contains 35% Cytosine. What is the percentage of Thymine?

A. 35%
B. 65%
C. 15%
D. 25%

Answer: C. 15%

C=35%, G=35%, total G+C=70%, remaining A+T=30%, so T=15%.

28Which vitamin is chemically a derivative of a steroid nucleus?

A. Vitamin A
B. Vitamin C
C. Vitamin D
D. Vitamin K

Answer: C. Vitamin D

Vitamin D is derived from 7-dehydrocholesterol, a steroid.

29The buffering capacity of a protein is maximum at a pH:

A. Equal to its pI (isoelectric point)
B. Equal to the pK_a of its side chains
C. At pH 7.0
D. In highly acidic conditions

Answer: B

Buffering is greatest near the pK_a of ionizable groups.

30Which of the following lipids does NOT contain a fatty acid?

A. Triglyceride
B. Cholesterol
C. Wax
D. Lecithin

Answer: B. Cholesterol

Cholesterol is a steroid with no fatty acid chains.

31The primary force stabilizing the DNA double helix is NOT hydrogen bonding, but:

A. Covalent bonds
B. Base stacking interactions (Van der Waals/Hydrophobic)
C. Ionic bonds
D. Disulfide bridges

Answer: B

Base stacking (hydrophobic and π-π interactions) provides major stability.

32In 'Alternative Splicing,' a single gene can code for multiple proteins. This challenges which classical definition?

A. The Cell Theory
B. One Gene-One Polypeptide hypothesis
C. Watson-Crick Model
D. Lock and Key Hypothesis

Answer: B

Alternative splicing shows one gene can produce multiple polypeptides.

33What is the net charge of the peptide Gly-Ala-Val at pH 7.0?

A. +1
B. -1
C. 0
D. +2

Answer: C. 0

All three have non-polar R-groups; only terminal amino and carboxyl groups are ionized, giving net zero.

34Which sugar is the most dominant in the human bloodstream?

A. α-D-Glucose
B. β-D-Glucose
C. L-Glucose
D. Fructose

Answer: B. β-D-Glucose

β-D-glucose is the predominant anomer in blood due to its stability.

35A 'Cistron' is:

A. A non-coding part of a gene
B. The smallest unit of genetic material that codes for a single polypeptide
C. A type of RNA
D. A protein found in the nucleus

Answer: B

Cistron is a DNA segment coding for one polypeptide (gene).

36The hydrolysis of ATP to ADP is highly exergonic (ΔG ≈ -30.5 kJ/mol) primarily due to:

A. Breaking a strong covalent bond
B. Electrostatic repulsion between negative charges on phosphate groups
C. The formation of water
D. Atmospheric pressure

Answer: B

Relief of electrostatic repulsion between adjacent phosphate groups drives the reaction.

37Which carbohydrate is used as a 'plasma volume expander' in medical emergencies?

A. Cellulose
B. Dextran
C. Glycogen
D. Agar

Answer: B. Dextran

Dextran is a polysaccharide used as a blood plasma substitute.

38Which of the following shows 'Inverted Repeat' sequences, often forming hairpins?

A. mRNA
B. tRNA
C. DNA
D. All of the above

Answer: D

Inverted repeats can form hairpin structures in all nucleic acids.

39During protein denaturation, which of the following remains intact?

A. 3D shape
B. Biological activity
C. Solubility
D. Optical activity of individual amino acids

Answer: D

Denaturation does not affect the chirality of amino acids.

40The linkage between the two glucose units in Trehalose is:

A. α(1-4)
B. α(1-1)α
C. β(1-4)
D. α(1-6)

Answer: B. α(1-1)α

Trehalose has an α,α-1,1 glycosidic bond, making it non-reducing.

41Which amino acid has a second chiral center?

A. Isoleucine
B. Leucine
C. Valine
D. Proline

Answer: A. Isoleucine

Isoleucine and threonine have two asymmetric carbons.

42Which lipid is predominantly found in the 'Myelin Sheath' of nerve cells?

A. Phosphatidylserine
B. Galactocerebroside
C. Triglyceride
D. Prostaglandin

Answer: B. Galactocerebroside

Galactocerebrosides are abundant in myelin.

43The pitch (height) of one turn in Z-DNA is approximately:

A. 3.4 nm
B. 4.5 nm
C. 2.8 nm
D. 1.9 nm

Answer: B. 4.5 nm

Z-DNA has a pitch of about 4.5 nm (more extended than B-DNA).

44Which of the following is an 'Essential' fatty acid that humans must obtain from diet?

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

Answer: B. Linoleic acid

Linoleic acid (omega-6) is essential; humans cannot synthesize it.

45Which bond in a nucleotide has the highest energy?

A. N-Glycosidic bond
B. Phosphoester bond
C. Phosphoanhydride bond
D. C-C bond

Answer: C. Phosphoanhydride bond

The phosphoanhydride bond between phosphate groups (as in ATP) is high-energy.

46Amylose forms a helical structure in water. This helix is stabilized by:

A. Intramolecular hydrogen bonds
B. Peptide bonds
C. Intermolecular ionic bonds
D. Van der Waals forces

Answer: A

Amylose helices are stabilized by intrachain hydrogen bonds.

47The term 'Protoplasm' is chemically defined as a:

A. True solution
B. Suspension
C. Complex Polyphasic Colloidal System
D. Solid

Answer: C

Protoplasm is a colloidal system with multiple phases.

48Which protein motif is commonly found in DNA-binding proteins?

A. Beta-barrel
B. Zinc finger
C. Alpha-helix alone
D. Random coil

Answer: B. Zinc finger

Zinc finger motifs are common in transcription factors.

49An 'Anomer' is a type of isomer that differs at:

A. Every carbon atom
B. The C₁ (aldehyde) or C₂ (ketone) carbon after ring closure
C. The penultimate carbon
D. Only the C₆ position

Answer: B

Anomers differ in configuration at the anomeric carbon (C1 for aldoses, C2 for ketoses).

50What is the molecular weight of a typical amino acid residue in a protein (accounting for the loss of water)?

A. 128 Da
B. 110 Da
C. 180 Da
D. 75 Da

Answer: B. 110 Da

Average amino acid weight is ~128 Da; subtracting water (18) gives ~110 Da.

📄Past Paper MCQs

📅 Actual MDCAT / NUMS Questions

35 MCQs

1The amount of heat required to raise the temperature of 1g of water from 15°C to 16°C is called:

MDCAT 2021

A. Heat of vaporization
B. Specific heat
C. Heat of fusion
D. Boiling point

Answer: B. Specific heat

Specific heat is the heat needed to raise 1g of a substance by 1°C.

2Which of the following is the most abundant compound in living organisms?

MDCAT 2017

A. Proteins
B. Water
C. Carbohydrates
D. Lipids

Answer: B. Water

Water constitutes 70-90% of cell mass.

3Large amount of energy is required to expand the water from liquid to gas because of:

NUMS 2020

A. Ionic bonds
B. Covalent bonds
C. Hydrogen bonds
D. Hydrophobic interactions

Answer: C. Hydrogen bonds

Hydrogen bonds must be broken for vaporization, requiring high energy.

4The specific heat of vaporization of water is:

MDCAT 2018/2021

A. 574 kcal/kg
B. 547 kcal/kg
C. 475 kcal/kg
D. 754 kcal/kg

Answer: A. 574 kcal/kg

The heat of vaporization of water is approximately 574 kcal/kg.

5Which of the following is a non-reducing sugar?

MDCAT 2018/2022

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

Answer: C. Sucrose

Sucrose has both anomeric carbons involved in the glycosidic bond.

6Glycogen is a polymer of:

NUMS 2021

A. α-D-glucose
B. β-D-glucose
C. α-D-fructose
D. β-D-galactose

Answer: A. α-D-glucose

Glycogen is made of α-D-glucose units with α(1→4) and α(1→6) linkages.

7Which of the following is an example of an aldo-pentose sugar?

MDCAT 2019

A. Ribose
B. Ribulose
C. Fructose
D. Erythrose

Answer: A. Ribose

Ribose is an aldopentose.

8Cellulose is a polymer of β-glucose linked by:

NUMS 2023

A. α-1,4 glycosidic bond
B. β-1,4 glycosidic bond
C. α-1,6 glycosidic bond
D. β-1,6 glycosidic bond

Answer: B. β-1,4 glycosidic bond

Cellulose has β(1→4) linkages.

9The number of amino acids in the A-chain of insulin is:

MDCAT 2016

A. 21
B. 30
C. 51
D. 141

Answer: A. 21

Insulin A-chain has 21 amino acids.

10Alpha-helix is a type of secondary protein structure which is stabilized by:

MDCAT 2021/2024

A. Disulfide bonds
B. Ionic bonds
C. Hydrogen bonds
D. Peptide bonds

Answer: C. Hydrogen bonds

Hydrogen bonds between backbone amide groups stabilize α-helices.

11Which level of protein structure provides information about the number and sequence of amino acids?

NUMS 2019

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

Answer: A. Primary structure

Primary structure is the linear sequence.

12Hemoglobin shows which level of protein organization?

MDCAT 2020

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

Answer: D. Quaternary

Hemoglobin has quaternary structure (4 subunits).

13Phospholipids are major components of:

MDCAT 2022

A. Cell wall
B. Plasma membrane
C. Nucleolus
D. Ribosomes

Answer: B. Plasma membrane

Phospholipids form the bilayer of plasma membranes.

14Waxes are esters of long-chain fatty acids with:

NUMS 2021

A. Glycerol
B. Long-chain alcohols
C. Amino acids
D. Glucose

Answer: B. Long-chain alcohols

Waxes are fatty acid + long-chain alcohol esters.

15A fatty acid is unsaturated if it contains:

MDCAT 2023

A. Only single bonds
B. At least one double bond
C. Only double bonds
D. Only triple bonds

Answer: B. At least one double bond

Unsaturated fatty acids have one or more double bonds.

16Uracil is found in RNA instead of:

MDCAT 2017/2021

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

Answer: C. Thymine

RNA uses uracil; DNA uses thymine.

17The diameter of the DNA double helix is:

MDCAT 2024

A. 2 nm
B. 3.4 nm
C. 0.34 nm
D. 10 nm

Answer: A. 2 nm

B-DNA diameter is approximately 2 nm.

18Glycoproteins are formed by the conjugation of:

NUMS 2022

A. Carbohydrates and Lipids
B. Carbohydrates and Proteins
C. Lipids and Proteins
D. Nucleic acids and Proteins

Answer: B. Carbohydrates and Proteins

Glycoproteins have carbohydrate attached to protein.

19One complete turn of DNA consists of:

MDCAT 2018

A. 10 base pairs
B. 10 nucleotides
C. 20 base pairs
D. 5 base pairs

Answer: A. 10 base pairs

B-DNA has 10 base pairs per turn (3.4 nm).

20Messenger RNA (mRNA) carries genetic information from:

MDCAT 2021

A. Ribosomes to Nucleus
B. Nucleus to Ribosomes
C. Cytoplasm to Nucleus
D. Mitochondria to Nucleus

Answer: B. Nucleus to Ribosomes

mRNA transcribes DNA in nucleus and travels to ribosomes.

21The number of amino acids in the Beta-chain of a Hemoglobin molecule is:

MDCAT 2017/2021

A. 141
B. 146
C. 574
D. 21

Answer: B. 146

Hemoglobin beta chain has 146 amino acids.

22Which of the following is a keto-sugar?

NUMS 2020

A. Glucose
B. Galactose
C. Fructose
D. Ribose

Answer: C. Fructose

Fructose is a ketohexose.

23In a DNA molecule, the distance between two adjacent nucleotides (base pairs) is:

MDCAT 2018/2023

A. 3.4 nm
B. 2.0 nm
C. 0.34 nm
D. 34 nm

Answer: C. 0.34 nm

Base pair stacking distance is 0.34 nm.

24The percentage of carbohydrates in a mammalian cell is approximately:

MDCAT 2022

A. 1%
B. 2%
C. 3%
D. 4%

Answer: D. 4%

Mammalian cells contain about 4% carbohydrates.

25Which bond is involved in the formation of a nucleoside?

NUMS 2021

A. Phosphodiester bond
B. Peptide bond
C. C-N (Glycosidic) linkage
D. Hydrogen bond

Answer: C. C-N (Glycosidic) linkage

Glycosidic bond links base to sugar.

26Silk is a fibrous protein formed by:

MDCAT 2019/2024

A. α-helix
B. β-pleated sheets
C. Triple helix
D. Globular folding

Answer: B. β-pleated sheets

Silk fibroin has extensive β-pleated sheet structure.

27How many water molecules are required to break down a polysaccharide of 100 glucose units into monomers?

MDCAT 2021 Conceptual

A. 100
B. 99
C. 101
D. 50

Answer: B. 99

Number of glycosidic bonds = 99; each hydrolysis consumes one water.

28Which of the following lipids is most abundant in the human brain?

NUMS 2019

A. Triglycerides
B. Waxes
C. Phospholipids
D. Terpenoids

Answer: C. Phospholipids

Brain is rich in phospholipids (membrane components).

29The unique properties of each amino acid are determined by its:

MDCAT 2023

A. Amino group
B. Carboxyl group
C. R-group (side chain)
D. Alpha-carbon

Answer: C. R-group (side chain)

Side chains confer unique chemical properties.

30Which nitrogenous base is double-ringed (Purine)?

MDCAT 2016/2021

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

Answer: D. Guanine

Purines (A,G) have two rings; pyrimidines (C,T,U) have one.

31The heat of vaporization of water is expressed as:

NUMS 2022

A. cal/g
B. kcal/kg
C. J/kg
D. Both A and B

Answer: D. Both A and B

It can be expressed as 540 cal/g or 540 kcal/kg.

32In the secondary structure of proteins, hydrogen bonds form between:

MDCAT 2024

A. Adjacent R-groups
B. -NH and -C=O groups of the backbone
C. Two sulfur atoms
D. Two carboxyl groups

Answer: B

Backbone amide hydrogen and carbonyl oxygen form H-bonds.

33A phospholipid molecule contains:

MDCAT 2020

A. 3 Fatty acids
B. 2 Fatty acids + 1 Phosphate group
C. 1 Fatty acid + 2 Phosphate groups
D. 3 Phosphate groups

Answer: B

Phospholipids have two fatty acids and a phosphate group.

34Which form of RNA is the most stable and makes up 80% of the total RNA in a cell?

NUMS 2021/2023

A. mRNA
B. tRNA
C. rRNA
D. snRNA

Answer: C. rRNA

Ribosomal RNA is the most abundant and stable.

35Adenine always pairs with Thymine in DNA via:

MDCAT 2018/2022

A. 3 Hydrogen bonds
B. 2 Hydrogen bonds
C. 1 Covalent bond
D. 2 Ionic bonds

Answer: B. 2 Hydrogen bonds

A-T base pair has two hydrogen bonds.

🔍Direct Search Queries — Biological Molecules

Exact search terms students use on Google for this chapter. Click any to jump to the relevant section:

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❔Frequently Asked Questions

What are biological molecules?

Biological molecules are organic compounds essential for life, containing mainly C, H, O, N, P, S. Classes include carbohydrates, proteins, lipids, nucleic acids, and conjugated molecules.

What are the 4 main types of biological molecules?

Carbohydrates, proteins, lipids, and nucleic acids (DNA & RNA). Conjugated molecules like glycolipids and glycoproteins are combinations.

Why is water biologically important?

Water has high specific heat (thermal stability), is a polar solvent, participates in hydrolysis, has high heat of vaporization, shows density anomaly (ice floats), and exhibits cohesion/adhesion for transpiration.

What is the difference between starch and cellulose?

Starch is made of α-1,4 linked glucose (digestible by humans), while cellulose is made of β-1,4 linked glucose (indigestible because humans lack cellulase).

What are the 4 levels of protein structure?

Primary (amino acid sequence), secondary (α-helix/β-pleated sheet), tertiary (3D folding with disulfide/ionic/hydrophobic bonds), quaternary (multiple polypeptide subunits like haemoglobin).

What is the difference between DNA and RNA?

DNA is double-stranded with deoxyribose and thymine; RNA is usually single-stranded with ribose and uracil. DNA stores genetic information; RNA helps in protein synthesis (mRNA, tRNA, rRNA).

What is a gene?

A gene is a sequence of nucleotides in DNA that codes for a polypeptide or functional RNA. It is the basic unit of heredity.

What are conjugated molecules?

Glycolipids (carbohydrate + lipid) and glycoproteins (carbohydrate + protein). They function in cell recognition, receptors, and membrane stability.

What is the structure of DNA proposed by Watson and Crick?

DNA is a double helix with two antiparallel polynucleotide strands. Bases pair via hydrogen bonds: A=T (2 bonds), G≡C (3 bonds). Diameter 2 nm, pitch 3.4 nm, 10 base pairs per turn.

Which sugar is non-reducing?

Sucrose is a non-reducing sugar because the glycosidic bond involves the anomeric carbons of both glucose and fructose, leaving no free reducing group.

What are phospholipids?

Phospholipids are amphipathic lipids with glycerol, two fatty acids, a phosphate group, and a nitrogenous base. They form the bilayer of cell membranes.

How many MCQs come from Biological Molecules in MDCAT?

Biological Molecules typically contributes 3–5 MCQs out of 81 Biology MCQs in MDCAT.

Which topics from Biological Molecules are most frequently asked in MDCAT?

Water properties (specific heat, solvent), starch vs cellulose (α vs β linkage), sucrose non-reducing, protein structure levels, DNA base pairing (A=T, G≡C), RNA types, gene definition, phospholipids, and conjugated molecules.

What is the difference between amylose and amylopectin?

Amylose is the linear, unbranched component of starch (α-1,4 bonds). Amylopectin is the branched component with α-1,4 and α-1,6 bonds at branch points.

What are triglycerides?

Triglycerides (triacylglycerols) are neutral fats formed from glycerol and three fatty acids linked by ester bonds. They are the main storage form of lipids.

What is the role of tRNA?

tRNA (transfer RNA) has a cloverleaf shape and carries specific amino acids to the ribosome during translation.

What is the difference between a nucleoside and a nucleotide?

A nucleoside is a pentose sugar + nitrogenous base. A nucleotide adds a phosphate group. Nucleotides are the monomers of nucleic acids.

Why is cellulose the most abundant organic molecule?

Cellulose is the primary structural component of plant cell walls and is produced in enormous quantities by plants worldwide.

What is the specific heat of water?

Water has a high specific heat of about 4.18 J/g°C (1 cal/g°C), providing thermal stability to cells and organisms.

What is an ester bond?

An ester bond forms between a fatty acid's carboxyl group and glycerol's hydroxyl group in lipids (triglycerides, phospholipids, waxes).

Biological Molecules — Complete MDCAT Notes & MCQs

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❓People Also Ask — Biological Molecules

📋Syllabus & Learning Outcomes

📝Full Notes — Biological Molecules

🧬 Biological Molecules – Syllabus‑Based Schematic Notes

📌 3.1 & 3.2 Definition, Classification & Importance

💧 3.3 Biologically Important Properties of Water

🍬 3.4 Carbohydrates

Monosaccharides (glucose)

Oligosaccharides

Polysaccharides

🧱 3.5 Proteins

Structure Levels

🧈 3.6 Lipids

🧬 3.7 Structure and Function of RNA

🔗 3.8 Conjugated Molecules

🧬 3.9 DNA Double Helix (Watson & Crick)

📄 3.10 Gene

🔑 Scheme‑Wise High‑Yield MCQs & Traps

✅Practice MCQs — Test 1

🧪 Basic Concepts

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🧪 Advanced Concepts

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