59 free MCQs on Biomolecules with worked answers and explanations. Carbohydrates, proteins, lipids, nucleic acids, and enzyme kinetics. Essential foundation for understanding metabolism.
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As substrate concentration rises, reaction rate increases steeply at first, then plateaus at Vmax once every enzyme molecule is working at full capacity; Km is the substrate concentration giving half-maximal rate, and a LOWER Km means the enzyme reaches that rate with less substrate (higher affinity).
Q1.
What are proteins made of?
Answer: B. Amino acids
Why: Proteins are polymers of amino acids linked by peptide bonds. There are 20 standard amino acids.
Q2.
Which carbohydrate is the energy storage molecule in animals?
Answer: C. Glycogen
Why: Glycogen is the storage carbohydrate in animals, found mainly in liver and muscle. Starch is the equivalent in plants.
Q3.
What is the primary function of enzymes?
Answer: B. Catalyze chemical reactions
Why: Enzymes are biological catalysts that speed up chemical reactions without being consumed. Most enzymes are proteins.
Q4.
Which biomolecule carries genetic information?
Answer: C. DNA
Why: DNA (deoxyribonucleic acid) carries genetic information. Its sequence of bases encodes instructions for making proteins.
Q5.
What type of bond links amino acids in a protein chain?
Answer: C. Peptide bond
Why: Amino acids are linked by peptide bonds, formed between the carboxyl group of one amino acid and the amino group of the next.
Q6.
Which molecule is called the energy currency of the cell?
Answer: C. ATP
Why: ATP (adenosine triphosphate) is the universal energy currency. Energy is released when the terminal phosphate bond is hydrolyzed.
Q7.
Cellulose is made up of which monomer?
Answer: D. Glucose
Why: Cellulose is a polysaccharide made of glucose monomers linked by beta-1,4-glycosidic bonds. It forms plant cell walls.
Q8.
Which type of fatty acid has one or more double bonds?
Answer: B. Unsaturated
Why: Unsaturated fatty acids have one or more C=C double bonds. Monounsaturated has one double bond; polyunsaturated has more than one.
Q9.
What is the basic structural unit of carbohydrates?
Answer: D. Monosaccharide
Why: Monosaccharides are the simplest carbohydrates. Examples include glucose, fructose, and galactose.
Q10.
Which lipid forms the structure of cell membranes?
Answer: C. Phospholipid
Why: Phospholipids form the lipid bilayer of cell membranes. Their hydrophilic head and hydrophobic tail arrange into a bilayer in water.
Q11.
Which sugar is found in RNA but not DNA?
Answer: C. Ribose
Why: RNA contains ribose sugar. DNA contains deoxyribose (lacks an oxygen at the 2' carbon). This difference makes DNA more stable.
Q12.
Starch is the storage carbohydrate found in which organisms?
Answer: D. Plants
Why: Starch is the storage polysaccharide in plants, found in amyloplasts. It exists as amylose (linear) and amylopectin (branched).
Q13.
Which nucleotide base in DNA pairs with Adenine?
Answer: C. Thymine
Why: Adenine pairs with Thymine in DNA (2 hydrogen bonds). Guanine pairs with Cytosine (3 hydrogen bonds). In RNA, Adenine pairs with Uracil.
Q14.
Fats provide how many kilocalories per gram?
Answer: C. 9 kcal
Why: Fats provide 9 kcal per gram, which is more than double that of carbohydrates or proteins (both 4 kcal per gram).
Q15.
The region on an enzyme where the substrate binds is called the
Answer: B. Active site
Why: The active site is the specific region of an enzyme where substrate binds and the reaction occurs. It has a complementary shape to the substrate.
Q16.
Which class of biomolecules includes fats, oils, and waxes?
Answer: C. Lipids
Why: Lipids include fats, oils, waxes, phospholipids, and steroids. They are insoluble in water and soluble in organic solvents.
Q17.
Which base is found in RNA but not DNA?
Answer: D. Uracil
Why: RNA contains Uracil instead of Thymine. Uracil pairs with Adenine. The lack of a methyl group makes it slightly less stable.
Q18.
Protein found in hair and nails is called
Answer: C. Keratin
Why: Keratin is a fibrous structural protein found in hair, nails, and skin. It is rich in the amino acid cysteine which forms disulfide bonds.
Q19.
Which vitamin is required as a coenzyme in many metabolic reactions?
Answer: B. Vitamin B complex
Why: B vitamins (thiamine, riboflavin, niacin, etc.) act as coenzymes. For example, NAD+ is derived from niacin (Vitamin B3).
Q20.
Which type of RNA carries amino acids to the ribosome?
Answer: C. tRNA
Why: tRNA (transfer RNA) carries specific amino acids to the ribosome during translation. Each tRNA has an anticodon that matches a codon on mRNA.
Q21.
According to Chargaff's rules, in a DNA molecule if Adenine is 30%, what is the percentage of Guanine?
Answer: B. 20%
Why: Chargaff's rule: A=T and G=C. If A=30%, then T=30%. Remaining 40% is split equally between G and C. So G=20%.
Q22.
The Michaelis constant (Km) of an enzyme represents
Answer: B. Substrate concentration at half Vmax
Why: Km is the substrate concentration at which the reaction velocity is half of Vmax. A lower Km means higher affinity of enzyme for substrate.
Q23.
A competitive inhibitor affects enzyme kinetics by
Answer: B. Increasing Km only
Why: Competitive inhibitors compete with substrate for the active site. They increase apparent Km (lower affinity) but do not affect Vmax (can be overcome by excess substrate).
Q24.
Denaturation of a protein involves
Answer: B. Loss of 3D structure without breaking peptide bonds
Why: Denaturation disrupts secondary and tertiary structure (breaking H bonds, hydrophobic interactions, disulfide bonds) but does NOT break peptide bonds of the primary structure.
Q25.
Which of the following is the most abundant organic compound on Earth?
Answer: C. Cellulose
Why: Cellulose is the most abundant organic compound on Earth. It is the main structural component of plant cell walls and is produced in enormous quantities.
Q26.
An enzyme that requires a metal ion cofactor for activity is called
Answer: B. Holoenzyme
Why: Holoenzyme = apoenzyme (protein part) + cofactor (metal ion or coenzyme). The apoenzyme alone is catalytically inactive.
Q27.
Lactose is composed of which two monosaccharides?
Answer: B. Glucose and galactose
Why: Lactose (milk sugar) is a disaccharide of glucose + galactose linked by a beta-1,4-glycosidic bond. Lactase enzyme breaks it down.
Q28.
The respiratory quotient (RQ) for fat oxidation is approximately
Answer: B. 0.7
Why: RQ for fats is about 0.7 (less O2 released relative to CO2 consumed). RQ for carbohydrates = 1.0, proteins ~0.8, organic acids > 1.
Q29.
Which levels of protein structure involve interactions between R groups of amino acids?
Answer: C. Tertiary and quaternary
Why: Tertiary structure involves interactions between R groups (hydrophobic, ionic, H bonds, disulfide bonds) in a single polypeptide. Quaternary involves interactions between multiple polypeptide subunits.
Q30.
What is a zymogen?
Answer: B. An inactive enzyme precursor
Why: Zymogens (proenzymes) are inactive enzyme precursors activated by cleavage of a peptide. Example: pepsinogen is converted to pepsin by HCl in the stomach.
Q31.
Sucrose is formed by joining glucose and fructose through which bond?
Answer: C. Alpha-1,2-glycosidic
Why: Sucrose is formed by an alpha-1,2-glycosidic bond between glucose (C1) and fructose (C2). This bond makes sucrose a non-reducing sugar.
Q32.
Fat-soluble vitamins are stored in body fat and liver. Which set is correct?
Answer: B. A, D, E, K
Why: Fat-soluble vitamins are A (vision), D (calcium), E (antioxidant), K (clotting). Water-soluble vitamins (B complex and C) are not stored and need regular intake.
Q33.
The induced fit model of enzyme action proposes that
Answer: B. The active site changes shape to better fit the substrate
Why: In the induced fit model (Koshland, 1958), the enzyme active site flexibly adjusts its shape when the substrate binds. This is more accurate than the rigid lock-and-key model.
Q34.
Which bond links nucleotides together in a DNA strand?
Answer: C. Phosphodiester bond
Why: Phosphodiester bonds link the 3' carbon of one nucleotide to the 5' phosphate of the next, forming the sugar-phosphate backbone of DNA and RNA.
Q35.
Which amino acid contains a sulfur atom and forms disulfide bonds?
Answer: C. Cysteine
Why: Cysteine contains a thiol (-SH) group. Two cysteine residues can form a disulfide bond (-S-S-), which is important for protein tertiary structure stability.
Q36.
What is the difference between alpha-glucose and beta-glucose?
Answer: B. Position of -OH group at C1
Why: Alpha-glucose has the -OH group at C1 pointing downward; beta-glucose has it pointing upward. This affects their polymerization: alpha forms starch, beta forms cellulose.
Q37.
The process of converting excess amino acids into glucose is called
Answer: D. Deamination followed by gluconeogenesis
Why: Amino acids are first deaminated (amino group removed as urea) and then the carbon skeleton enters gluconeogenesis to form glucose. Both steps are needed.
Q38.
Which class of enzymes transfers phosphate groups?
Answer: B. Kinases
Why: Kinases are a class of transferases that transfer phosphate groups from ATP to a substrate. Example: hexokinase phosphorylates glucose in glycolysis.
Q39.
Amylase breaks down which substrate?
Answer: C. Starch
Why: Amylase is a carbohydrate-digesting enzyme that breaks down starch (and glycogen) into maltose and dextrins. Salivary amylase begins digestion in the mouth.
Q40.
What is the secondary structure of a protein stabilized by?
Answer: C. Hydrogen bonds between backbone atoms
Why: Secondary structures (alpha helices and beta sheets) are stabilized by hydrogen bonds between the carbonyl oxygen and amide hydrogen of the peptide backbone.
Q41.
A non-competitive inhibitor affects enzyme kinetics by
Answer: B. Decreasing Vmax without changing Km
Why: Non-competitive inhibitors bind to the allosteric site (not active site) and reduce Vmax by decreasing enzyme efficiency. Km is unchanged because substrate can still bind with same affinity.
Q42.
The Bohr effect explains why hemoglobin releases O2 in metabolically active tissues. Which factors cause this?
Answer: C. High CO2 and low pH
Why: The Bohr effect: high CO2 (lowers pH by forming carbonic acid) and high H+ decrease hemoglobin affinity for O2. This promotes O2 release in active tissues where CO2 is produced.
Q43.
In enzyme kinetics, which plot linearizes the Michaelis-Menten equation?
Answer: B. Lineweaver-Burk plot
Why: The Lineweaver-Burk (double reciprocal) plot (1/v vs 1/[S]) linearizes the Michaelis-Menten equation, allowing accurate determination of Km and Vmax from the graph.
Q44.
Positive cooperativity in hemoglobin means
Answer: B. Binding of one O2 increases affinity for subsequent O2 molecules
Why: Positive cooperativity: binding of the first O2 causes conformational change that increases affinity for subsequent O2. This gives the sigmoidal O2 dissociation curve of hemoglobin.
Q45.
What is the role of ubiquitin in protein quality control?
Answer: B. It tags damaged proteins for proteasomal degradation
Why: Ubiquitin is a small protein that marks damaged or misfolded proteins for degradation by the 26S proteasome. Poly-ubiquitination signals rapid destruction.
Q46.
Which property explains why fats yield more energy per gram than carbohydrates?
Answer: B. Fats are more highly reduced (more C-H bonds)
Why: Fats have a higher degree of reduction (more C-H bonds, less C-O bonds) compared to carbohydrates. More reduced carbon means more electrons available for the ETC, yielding more ATP.
Q47.
The lock-and-key model of enzyme action was proposed by
Answer: B. Fischer
Why: Emil Fischer proposed the lock-and-key model in 1894, suggesting the enzyme active site is rigid and complementary to the substrate. Koshland later proposed the induced fit model in 1958.
Q48.
In protein synthesis, what is the role of the 5' cap and poly-A tail on mRNA?
Answer: B. They protect mRNA from degradation and aid ribosome binding
Why: The 5' 7-methylguanosine cap protects mRNA from nuclease degradation and helps ribosome binding. The 3' poly-A tail also protects mRNA and aids in nuclear export and translation.
Q49.
What distinguishes NAD+ from NADP+ functionally?
Answer: B. NADP+ is mainly used in anabolic reactions while NAD+ is in catabolic reactions
Why: NAD+ accepts electrons in catabolic reactions (glycolysis, Krebs cycle). NADPH (reduced NADP+) is mainly used in anabolic reactions like fatty acid synthesis and in the pentose phosphate pathway.
Q50.
Which enzyme catalyzes the rate-limiting step of glycolysis?
Answer: C. Phosphofructokinase-1
Why: Phosphofructokinase-1 (PFK-1) catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. It is the key regulatory enzyme of glycolysis, inhibited by ATP and citrate.
Q51.
What is the significance of the omega-3 and omega-6 fatty acids being called essential?
Answer: B. Humans cannot synthesize them and must obtain them from diet
Why: Essential fatty acids (omega-3 like ALA; omega-6 like linoleic acid) cannot be synthesized by humans because we lack the enzymes to introduce double bonds beyond C9. They must come from food.
Q52.
Feedback inhibition in a metabolic pathway means
Answer: B. The final product inhibits an early enzyme in its own synthesis
Why: End-product (feedback) inhibition: the final product of a pathway inhibits an early enzyme (often the first committed step). This is a key mechanism for controlling metabolic flux. Example: isoleucine inhibits threonine deaminase.
Q53.
In gel electrophoresis, DNA fragments are separated based on
Answer: C. Size (smaller fragments travel farther)
Why: In agarose gel electrophoresis, all DNA fragments have the same charge-to-mass ratio, so separation is based on SIZE. Smaller fragments migrate through the gel pores faster and travel farther from the well.
Q54.
Wobble base pairing in translation refers to
Answer: B. Flexible base pairing at the third codon position allowing one tRNA to recognize multiple codons
Why: Wobble hypothesis (Crick): the third position of the codon can form non-Watson-Crick base pairs. This allows one tRNA anticodon to recognize multiple codons differing at the third position, reducing the number of tRNAs needed.
Q55.
What makes telomerase important for cell immortality?
Answer: B. It adds telomeric repeats to chromosome ends, preventing shortening
Why: Telomerase is a reverse transcriptase that extends telomeres. Normal somatic cells lack telomerase, so chromosomes shorten with each division (Hayflick limit). Cancer cells and stem cells reactivate telomerase.
Q56.
What is the role of chaperone proteins?
Answer: B. Assist in proper protein folding and prevent aggregation
Why: Molecular chaperones (e.g., HSP70, HSP90) bind to exposed hydrophobic regions of newly synthesized or stress-denatured proteins, preventing misfolding and aggregation, and facilitate correct folding.
Q57.
The turnover number (kcat) of an enzyme represents
Answer: B. The number of substrate molecules converted per enzyme molecule per second
Why: kcat (catalytic constant or turnover number) is the maximum number of substrate molecules converted to product per enzyme molecule per second. Catalase has a very high kcat (~4 million/s).
Q58.
Protein denaturation by urea acts by
Answer: B. Disrupting hydrogen bonds and hydrophobic interactions
Why: Urea denatures proteins by forming hydrogen bonds with the backbone and disrupting internal H bonds and hydrophobic interactions that maintain tertiary structure. This unfolds the protein without cleaving peptide bonds.
Q59.
What is the key difference between Type I and Type II restriction enzymes?
Answer: B. Type II cuts within or near recognition site; Type I cuts away from it
Why: Type II restriction enzymes (used in genetic engineering) cut within or very close to their recognition sequence, producing predictable fragments. Type I enzymes cut far from their recognition site, making them impractical for cloning.