Biology MCQs with Answers and Explanations

The insulin receptor has intrinsic kinase activity. Upon insulin binding, which type of residue on the receptor is autophosphorylated?

A. Serine
B. Threonine
C. Tyrosine
D. Glycine

Explanation:
The insulin receptor is a tyrosine kinase. Upon ligand binding, it autophosphorylates tyrosine residues on its cytoplasmic domain.

Gap junction channels between animal cells are formed by which protein?

A. Connexin
B. Cadherin
C. Integrin
D. Actin

Explanation:
Connexins assemble into connexons that align between adjacent cells to form gap junction channels for direct cytoplasmic communication.

Which pH environment is required for optimal activity of lysosomal enzymes?

A. ~7.4
B. ~6.8
C. ~5.0
D. ~8.0

Explanation:
Lysosomal hydrolases function best at an acidic pH around 5.0, maintained by proton pumps in the lysosomal membrane.

The signal recognition particle (SRP) functions primarily to:

A. Direct ribosomes to the rough ER membrane for secretory proteins
B. Unfold misfolded proteins in the cytosol
C. Bind and degrade abnormal mRNA
D. Stimulate nuclear export of ribosomal subunits

Explanation:
SRP binds the signal peptide of a nascent polypeptide and pauses translation until the ribosome docks on the ER, targeting the protein to the secretory pathway.

One major difference between intermediate filaments and microtubules is that intermediate filaments:

A. Contain α- and β-tubulin
B. Are polar structures
C. Degrade in the absence of ATP
D. Lack structural polarity

Explanation:
Intermediate filaments are apolar (no distinct plus/minus ends) and do not require nucleotide hydrolysis for polymerization.

Lipid rafts in the plasma membrane are microdomains enriched in which of the following components?

A. Cholesterol and sphingolipids
B. Phosphatidylcholine
C. Triglycerides
D. Integral membrane proteins only

Explanation:
Lipid rafts are rich in cholesterol and sphingolipids, which pack tightly, creating ordered membrane regions.

An ER-resident protein contains a C-terminal KDEL sequence. If this sequence is missing, what would likely happen to the protein?

A. It would be transported to the Golgi
B. It would be secreted from the cell
C. It would remain in the cytosol
D. It would be targeted to mitochondria

Explanation:
KDEL is an ER retention signal. Without KDEL, the protein is not retrieved from the Golgi and is secreted.

Which cytoskeletal filament requires ATP hydrolysis for its polymerization?

A. Microtubules
B. Actin microfilaments
C. Intermediate filaments
D. Cellulose microfibrils

Explanation:
Actin polymerization uses ATP. Microtubules use GTP for tubulin assembly; intermediate filaments do not use nucleotide hydrolysis.

In the mitochondrion, where are the electron transport chain complexes located?

A. Outer mitochondrial membrane
B. Mitochondrial matrix
C. Inner mitochondrial membrane
D. Intermembrane space

Explanation:
The inner membrane houses complexes I–IV of the electron transport chain and ATP synthase, creating the proton gradient.

Which organelle is primarily responsible for lipid synthesis and detoxification of drugs in liver cells?

A. Rough endoplasmic reticulum
B. Smooth endoplasmic reticulum
C. Golgi apparatus
D. Mitochondrion

Explanation:
Smooth ER contains enzymes involved in lipid metabolism and drug detoxification (e.g., cytochrome P450s), especially in hepatocytes.