Biochemistry MCQs
1. Water acts as a universal solvent due to its:
A. High molecular weight
B. Ionic nature
C. Polarity
D. Neutral charge
Correct Answer: C. Polarity
Explanation: Water is a polar molecule because oxygen is more electronegative than hydrogen, creating partial charges ($\delta^-$ and $\delta^+$). This polarity allows water to disrupt the ionic bonds of salts and form hydrogen bonds with other polar substances, making it a “universal solvent.”
2. The pH of blood in most mammals is approximately:
A. 6.8
B. 7.0
C. 7.4
D. 7.8
Correct Answer: C. 7.4
Explanation: Mammalian blood is slightly alkaline. Maintaining a tight pH range (usually 7.35 to 7.45) is critical for enzyme function and oxygen transport. Deviations can lead to acidosis or alkalosis.
3. The major buffer system in blood is:
A. Phosphate buffer
B. Protein buffer
C. Bicarbonate buffer
D. Ammonia buffer
Correct Answer: C. Bicarbonate buffer
Explanation: The Bicarbonate buffer system ($\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+$) is the most important extracellular buffer. It is highly efficient because the lungs can regulate $\text{CO}_2$ levels and the kidneys can regulate $\text{HCO}_3^-$.
4. Which carbohydrate is the main blood sugar in animals?
A. Fructose
B. Galactose
C. Glucose
D. Lactose
Correct Answer: C. Glucose
Explanation: Glucose is the primary metabolic fuel for most animal tissues and the exclusive energy source for the brain and red blood cells under normal conditions.
5. Glycosidic bond is present in:
A. Lipids
B. Proteins
C. Carbohydrates
D. Nucleic acids
Correct Answer: C. Carbohydrates
Explanation: A glycosidic bond is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group or molecule, such as the bond between two monosaccharides to form a disaccharide.
6. The end product of glycolysis under aerobic conditions is:
A. Lactate
B. Acetyl-CoA
C. Pyruvate
D. Oxaloacetate
Correct Answer: C. Pyruvate
Explanation: In the presence of oxygen, glycolysis breaks down one molecule of glucose into two molecules of pyruvate. Under anaerobic conditions (like in straining muscles), pyruvate is converted into lactate.
7. ATP generation during glycolysis occurs by:
A. Oxidative phosphorylation
B. Substrate-level phosphorylation
C. Photophosphorylation
D. Chemiosmosis
Correct Answer: B. Substrate-level phosphorylation
Explanation: In glycolysis, ATP is produced by the direct transfer of a phosphate group from a high-energy intermediate (like phosphoenolpyruvate) to ADP. This is called substrate-level phosphorylation.
8. The TCA cycle (Krebs Cycle) occurs in the:
A. Cytoplasm
B. Nucleus
C. Mitochondria
D. Ribosomes
Correct Answer: C. Mitochondria
Explanation: The TCA cycle takes place in the mitochondrial matrix. It is the final common pathway for the oxidation of carbohydrates, lipids, and proteins.
9. The rate-limiting enzyme of glycolysis is:
A. Hexokinase
B. Phosphofructokinase-1 (PFK-1)
C. Pyruvate kinase
D. Glucose-6-phosphatase
Correct Answer: B. Phosphofructokinase-1
Explanation: PFK-1 is the key regulatory enzyme of glycolysis. It is allosterically inhibited by ATP and citrate, and activated by AMP and fructose-2,6-bisphosphate.
10. The primary function of gluconeogenesis is to:
A. Store glucose
B. Oxidize glucose
C. Form glucose from non-carbohydrates
D. Convert glucose to glycogen
Correct Answer: C. Form glucose from non-carbohydrates
Explanation: Gluconeogenesis is the synthesis of new glucose from non-carbohydrate precursors like lactate, glycerol, and glucogenic amino acids. It is vital for maintaining blood glucose during fasting.
11. Major site of gluconeogenesis is:
A. Muscle
B. Liver
C. Brain
D. Adipose tissue
Correct Answer: B. Liver
Explanation: The liver is the main site of gluconeogenesis (approx. 90%), with the kidneys contributing significantly during prolonged fasting.
12. Pentose phosphate pathway mainly provides:
A. ATP
B. NADH
C. NADPH
D. FADH₂
Correct Answer: C. NADPH
Explanation: The Pentose Phosphate Pathway (PPP) does not produce ATP. Its primary roles are to generate NADPH for fatty acid synthesis and ribose-5-phosphate for nucleotide synthesis.
13. Which lipid is a structural component of cell membranes?
A. Triglycerides
B. Phospholipids
C. Cholesterol ester
D. Fatty acids
Correct Answer: B. Phospholipids
Explanation: Phospholipids are amphipathic molecules that form the lipid bilayer of cell membranes, providing both structural integrity and a barrier to the passage of water-soluble substances.
14. Essential fatty acids cannot be synthesized because animals lack:
A. Elongase
B. Desaturase
C. Oxidase
D. Transferase
Correct Answer: B. Desaturase
Explanation: Animals lack the specific desaturase enzymes required to insert double bonds at the $\omega-3$ and $\omega-6$ positions of a fatty acid chain.
15. β-oxidation of fatty acids occurs in:
A. Cytosol
B. Nucleus
C. Mitochondria
D. Lysosomes
Correct Answer: C. Mitochondria
Explanation: After fatty acids are activated in the cytosol, they are transported into the mitochondria (via the carnitine shuttle) to undergo $\beta$-oxidation, which breaks them down into Acetyl-CoA.
16. Ketone bodies are mainly produced in the:
A. Kidney
B. Liver
C. Muscle
D. Brain
Correct Answer: B. Liver
Explanation: The liver produces ketone bodies (Acetoacetate, $\beta$-hydroxybutyrate, and Acetone) from Acetyl-CoA during periods of high fat mobilization and carbohydrate shortage.
17. Ketosis is common in:
A. Poultry
B. Dogs
C. High-yielding dairy cows
D. Horses
Correct Answer: C. High-yielding dairy cows
Explanation: In high-yielding dairy cows, the energy demand for milk production often exceeds dietary intake (negative energy balance), leading to excessive fat breakdown and the development of ketosis.
18. The basic unit of proteins is:
A. Fatty acid
B. Nucleotide
C. Amino acid
D. Monosaccharide
Correct Answer: C. Amino acid
Explanation: Amino acids are the monomers that polymerize to form polypeptide chains (proteins).
19. Peptide bond is formed between:
A. Two amino groups
B. Two carboxyl groups
C. Amino and carboxyl groups
D. Side chains
Correct Answer: C. Amino and carboxyl groups
Explanation: A peptide bond is a covalent amide bond formed between the $\alpha$-amino group of one amino acid and the $\alpha$-carboxyl group of another.
20. Primary structure of protein refers to:
A. $\alpha$-helix formation
B. Folding pattern
C. Amino acid sequence
D. Quaternary association
Correct Answer: C. Amino acid sequence
Explanation: The primary structure is the linear sequence of amino acids in the polypeptide chain, which is held together by covalent peptide bonds.
21. Transamination requires which vitamin?
A. Vitamin B₁₂
B. Vitamin B₆
C. Vitamin C
D. Vitamin K
Correct Answer: B. Vitamin B₆
Explanation: Pyridoxal phosphate (PLP), the active form of Vitamin $B_6$, is the essential coenzyme for all transaminase enzymes involved in amino acid metabolism.
22. Ammonia is converted to urea in the:
A. Kidney
B. Muscle
C. Liver
D. Brain
Correct Answer: C. Liver
Explanation: Ammonia is highly toxic, especially to the brain. The liver detoxifies it by converting it into urea, which is then transported to the kidneys for excretion.
23. Urea cycle occurs partly in:
A. Cytoplasm only
B. Mitochondria only
C. Cytoplasm and mitochondria
D. Nucleus
Correct Answer: C. Cytoplasm and mitochondria
Explanation: The Urea cycle is unique in that its first two reactions occur in the mitochondrial matrix, while the remaining reactions take place in the cytoplasm.
24. Enzymes are mostly:
A. Lipids
B. Carbohydrates
C. Proteins
D. Minerals
Correct Answer: C. Proteins
Explanation: With the exception of ribozymes (RNA), almost all enzymes are specialized proteins that act as biological catalysts.
25. Enzyme specificity is due to:
A. Coenzymes
B. Active site structure
C. pH
D. Temperature
Correct Answer: B. Active site structure
Explanation: The unique three-dimensional shape and chemical environment of the active site ensure that only a specific substrate can bind to the enzyme (often described by the “Lock and Key” model).