Cuestionario: Endoplasmic Reticulum Functions and Structure — 20 preguntas

1. Which description best characterizes the endoplasmic reticulum in eukaryotic cells?

A. A membrane network found only in prokaryotic cells

B. A membrane network made of cisternae, canaliculi, and vesicles that is continuous with the nuclear envelope

C. A double-membrane organelle specialized only for ATP production

D. A stack of flattened sacs that is independent of the nuclear envelope

Answer: (B)

Explicación

The endoplasmic reticulum is an internal membrane network of cisternae, canaliculi, and vesicles that is continuous with the nuclear envelope. The other options describe unrelated structures or incorrect cell types.

2. What is a key structural connection of the endoplasmic reticulum?

A. It is enclosed completely by the plasma membrane

B. It is continuous with lysosomal membranes only

C. It is separated from all other organelles by thick protein walls

D. It is connected to the nuclear envelope and Golgi vesicles

Answer: (D)

Explicación

The ER membrane network is continuous with the nuclear envelope and linked to Golgi vesicles. This connection supports its role in the secretory pathway.

3. What mainly distinguishes rough endoplasmic reticulum from smooth endoplasmic reticulum?

A. Rough endoplasmic reticulum has a double membrane, while smooth endoplasmic reticulum has a single membrane

B. Rough endoplasmic reticulum is found only in plant cells, while smooth endoplasmic reticulum is found only in animal cells

C. Rough endoplasmic reticulum contains more cholesterol, while smooth endoplasmic reticulum contains none

D. Rough endoplasmic reticulum has ribosomes on its surface, while smooth endoplasmic reticulum does not

Answer: (D)

Explicación

RER appears rough because ribosomes are attached to its membrane, whereas SER lacks ribosomes and appears smooth. The other choices confuse ER types with unrelated membrane features.

4. In which cell type is smooth endoplasmic reticulum typically especially abundant?

A. Cells that synthesize lipids and steroid hormones

B. Cells that perform photosynthesis

C. Cells that store large amounts of DNA

D. Cells that lack internal membranes

Answer: (A)

Explicación

SER is especially abundant in cells involved in lipid and steroid hormone synthesis. This contrasts with RER-rich cells, such as exocrine pancreatic cells.

5. Which membrane composition best fits rough endoplasmic reticulum?

A. Mostly carbohydrates with only trace lipids

B. About 70% proteins and 30% lipids, with low cholesterol and many unsaturated phospholipids

C. About 30% proteins and 70% lipids, with very high cholesterol

D. A rigid membrane composed mainly of cellulose

Answer: (B)

Explicación

RER membranes are described as roughly 70% protein and 30% lipid, with low cholesterol and unsaturated phospholipids that increase fluidity. The other options do not match the described composition.

6. What is typically found in the lumen of rough endoplasmic reticulum?

A. DNA, histones, and RNA polymerase

B. Synthesized proteins, BiP chaperones, and Ca2+ ions

C. Contractile filaments and microtubules

D. Digestive enzymes and acidic hydrolases

Answer: (B)

Explicación

The RER lumen can contain newly synthesized proteins, BiP, and Ca2+ ions. These contents support protein processing inside the ER.

7. Which statement best describes smooth endoplasmic reticulum membranes?

A. They are about 70% proteins and 30% lipids, with low cholesterol and high phospholipid content

B. They are mostly carbohydrates with a thick outer coat

C. They have a composition identical to bacterial cell walls

D. They are built primarily from DNA and RNA

Answer: (A)

Explicación

SER membranes are also about 70% protein and 30% lipid, with low cholesterol and abundant phospholipids that keep the membrane fluid. The other choices are incorrect.

8. Which content is characteristic of the smooth endoplasmic reticulum lumen in certain cells?

A. Large amounts of DNA in dividing cells or pigments in neurons

B. Large amounts of glucose in red blood cells or bile in osteocytes

C. Large amounts of ribosomes in secretory cells or cellulose in plant cells

D. Large amounts of Ca2+ in muscle cells or steroid hormones in luteal cells

Answer: (D)

Explicación

SER lumen contents vary by cell type and can include Ca2+ in muscle cells or steroid hormones in luteal cells. This reflects the specialized roles of SER in storage and synthesis.

9. How are proteins targeted to the rough endoplasmic reticulum for co-translational translocation?

A. A nuclear localization signal is recognized by importins at the Golgi

B. A glycosylphosphatidylinositol anchor sends the protein directly into mitochondria

C. A signal sequence is recognized by SRP, which pauses translation and docks the ribosome at the RER

D. A stop codon causes the ribosome to move into the RER lumen

Answer: (C)

Explicación

A nascent protein’s signal sequence is bound by SRP, translation pauses, and the complex docks at the RER via the SRP receptor. This begins co-translational translocation into the lumen.

10. What does N-glycosylation add to a nascent protein in the rough endoplasmic reticulum?

A. A methyl group attached to glycine in the nucleus

B. A 14-sugar block attached to asparagine in an Asn-X-Ser or Asn-X-Thr motif

C. A lipid tail attached to lysine after translation ends

D. A phosphate group attached to serine in the cytosol

Answer: (B)

Explicación

N-glycosylation is a co-translational RER modification that attaches a 14-sugar block to asparagine within the Asn-X-Ser or Asn-X-Thr motif. The other options describe different types of modifications.

11. What is the main role of BiP in the rough endoplasmic reticulum?

A. It forms the ribosomes attached to the ER membrane

B. It helps newly synthesized proteins fold correctly and protects hydrophobic domains

C. It cuts mature proteins into peptides for secretion

D. It transports calcium into the nucleus

Answer: (B)

Explicación

BiP is an ER chaperone that assists folding and shields hydrophobic regions of new proteins. Proper folding is then assessed during ER quality control.

12. What happens to a misfolded protein after ER quality control detects an error?

A. It is returned through the translocon to the cytosol and degraded by the proteasome

B. It is stored permanently in the ER lumen

C. It is sent to the Golgi for immediate secretion

D. It is converted into a lipid droplet

Answer: (A)

Explicación

Misfolded proteins can be moved back through the translocon to the cytosol, where the proteasome degrades them. Correctly folded proteins instead detach from BiP and exit the ER.

13. What is a major function of the smooth endoplasmic reticulum in membrane renewal?

A. It breaks down membrane phospholipids into amino acids

B. It packages ribosomes for export

C. It synthesizes DNA for membrane repair

D. It synthesizes phospholipids that help build new cellular membranes

Answer: (D)

Explicación

SER synthesizes membrane phospholipids, which support renewal of the cytomembrane system. These lipids can be used for ER, plasma membrane, or other membrane systems.

14. Where are phospholipids progressively built during SER synthesis?

A. Only inside the mitochondrial matrix

B. Inside the Golgi lumen

C. On the outer face of the plasma membrane

D. On the hyaloplasmic face of the SER membrane

Answer: (D)

Explicación

Phospholipid synthesis begins on the cytosolic, or hyaloplasmic, face of the SER membrane. This supports later distribution and membrane asymmetry.

15. Which enzyme family is directly involved in hydroxylating pregnenolone during steroid hormone synthesis?

A. Ribosomal peptidyl transferases

B. Catalase enzymes

C. ATP synthase enzymes

D. Cytochrome P450 enzymes

Answer: (D)

Explicación

Steroid hormone synthesis in endocrine secretory cells involves cytochrome P450 enzymes that hydroxylate pregnenolone. This process works together with mitochondrial steps.

16. Which cell type is an example of a steroid-hormone-producing endocrine secretory cell?

A. Neuron of the cerebral cortex

B. Red blood cell

C. Hepatocyte packed with glycogen only

D. Adrenal cortex cell

Answer: (D)

Explicación

Adrenal cortex cells are among the endocrine secretory cells that produce steroid hormones. Other examples include theca interna, luteal, and Leydig cells.

17. What is the main role of ATP-dependent calcium pumps in the smooth endoplasmic reticulum?

A. They convert calcium into phosphate

B. They synthesize calcium-binding proteins

C. They move Ca2+ from the cytoplasm into the SER lumen

D. They export calcium directly out of the cell membrane

Answer: (C)

Explicación

ATP-dependent calcium pumps transport Ca2+ from the cytoplasm into the SER lumen for storage. This is central to calcium regulation in muscle and other cells.

18. How is calcium released from the smooth endoplasmic reticulum in muscle and related cells?

A. Through DNA pores in the nuclear envelope

B. Through ligand-gated IP3 channels or voltage-gated channels

C. Through ribosomes embedded in the membrane

D. Through passive diffusion only, without channels

Answer: (B)

Explicación

SER calcium release occurs through specific channels, including ligand-gated IP3 channels and voltage-gated channels. Pumps, binding proteins, and channels work together to control calcium levels.

19. How does cytochrome P450 help detoxify fat-soluble compounds in the smooth endoplasmic reticulum?

A. By converting them into proteins for storage

B. By binding them irreversibly to DNA

C. By hydroxylating them so they become more water-soluble

D. By pumping them into mitochondria for energy production

Answer: (C)

Explicación

Cytochrome P450 adds hydroxyl groups to fat-soluble toxins, increasing their water solubility. This change makes elimination possible.

20. What is the usual outcome of SER detoxification after hydroxylation of a toxin?

A. The toxin is converted into a ribosome and reused

B. The toxin is trapped permanently in the nucleus

C. The toxin becomes part of the cell wall

D. The toxin can be eliminated in urine and through the bloodstream

Answer: (D)

Explicación

After hydroxylation, toxins become water-soluble metabolites that can be removed via urine and the bloodstream. This is the end result of SER detoxification.