Endoplasmic Reticulum — RER vs SER, Luminal and Extra-Luminal Compartments
✅ Asked in NEET 2022Which of the following statements with respect to the endoplasmic reticulum is incorrect? NEET 2022 Phase 1
Which of the following statements with respect to the endoplasmic reticulum is incorrect? NEET 2022 Phase 1
Answer & NCERT explanation
Correct answer: B — In prokaryotes only RER are present
Statement b is incorrect because prokaryotes do NOT have endoplasmic reticulum at all - neither RER nor SER. According to NCERT, ER is found only in eukaryotic cells. Prokaryotes have ribosomes attached directly to their plasma membrane or freely floating in cytoplasm. Other statements are correct about SER lacking ribosomes, SER synthesizing lipids, and RER having attached ribosomes.
Read more NCERT concept on the PYQ
Electron microscopic studies of eukaryotic cells reveal the presence of a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER). Hence, ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
Electron microscopic studies of EUKARYOTIC cells reveal a network or RETICULUM of tiny TUBULAR STRUCTURES scattered in the cytoplasm — the ENDOPLASMIC RETICULUM (ER). The ER divides intracellular space into TWO COMPARTMENTS: LUMINAL (inside ER) and EXTRA-LUMINAL (cytoplasm). Two types exist: ROUGH ER (RER) — has RIBOSOMES attached to its surface, looks rough under EM; involved in PROTEIN SYNTHESIS and processing. SMOOTH ER (SER) — DEVOID of ribosomes, smooth appearance; involved in LIPID SYNTHESIS (steroids, phospholipids) and detoxification. IMPORTANT: ER is found ONLY in EUKARYOTIC CELLS — prokaryotes have NEITHER RER nor SER. Prokaryotic ribosomes are attached to plasma membrane or float freely in cytoplasm.
NEET 2022 Phase 1 trap: 'In prokaryotes only RER are present' — WRONG. Prokaryotes have NO endoplasmic reticulum at all (neither RER nor SER). ER is exclusively EUKARYOTIC. Other statements in the same question are correct: SER lacks ribosomes (true), SER synthesises lipids (true), RER has ribosomes attached (true). Memory: prokaryotes have NO membrane-bound organelles other than mesosomes (which are plasma membrane extensions, not ER). The 'extra-luminal' compartment is just the cytoplasm; 'luminal' is inside the ER tubules/cisternae.
In prokaryotes only the RER (rough endoplasmic reticulum) is present, while SER is absent.
Prokaryotes have NO endoplasmic reticulum at all — neither RER nor SER. ER is exclusively found in eukaryotic cells.
ER = EUKARYOTE ONLY. RER = ribosomes attached (protein synthesis). SER = smooth, no ribosomes (lipid synthesis). Prokaryotes = NO ER.
- ER divides intracellular space into LUMINAL (inside) and EXTRA-LUMINAL (cytoplasm) compartments.
- RER (rough ER) bears ribosomes on its cytoplasmic surface → site of PROTEIN synthesis.
- SER (smooth ER) DEVOID of ribosomes → site of LIPID synthesis (steroids, phospholipids) and detoxification.
- ER is EXCLUSIVE to EUKARYOTIC cells — prokaryotes have NO endoplasmic reticulum (NEET 2022 trap).
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