Q.77 Which of the following cellular structures are absent in eukaryotes and are present at least
in some prokaryotic organisms?
(i) Respiratory enzymes
(ii) Ribosomes
(iii) Nucleoid
(iv) Internal membranes
(v) Pili
(A) (iii) and (v) (B) (i) and (iv)
(C) (i), (iii) and (v) (D) (iii), (iv)
The correct answer is (A) (iii) and (v).
Option Analysis
Prokaryotes and eukaryotes differ in cellular organization, with prokaryotes lacking membrane-bound organelles. Structures absent in all eukaryotes but present in some prokaryotes include the nucleoid and pili.
(i) Respiratory enzymes: Present in both; in prokaryotes on plasma membrane or mesosomes, in eukaryotes in mitochondria.
(ii) Ribosomes: Present in both; prokaryotes have 70S, eukaryotes 80S in cytosol.
(iii) Nucleoid: Absent in eukaryotes (DNA in nucleus), present in prokaryotes as irregular DNA region.
(iv) Internal membranes: Present in both; prokaryotes have plasma membrane invaginations (mesosomes), eukaryotes have endoplasmic reticulum and others.
(v) Pili: Absent in eukaryotes, present in some prokaryotes (Gram-negative bacteria) for attachment and conjugation.
Options (B), (C), (D) include incorrect choices like (i) or (iv).
Prokaryotic and eukaryotic cells represent fundamental differences in cellular architecture, crucial for CSIR NET Life Sciences aspirants studying cell biology. Cellular structures absent in eukaryotes present in prokaryotes highlight prokaryotes’ simpler design without membrane-bound organelles. This guide breaks down key distinctions, focusing on respiratory enzymes, ribosomes, nucleoid, internal membranes, and pili.
Prokaryotic vs Eukaryotic Cell Features
Prokaryotes (bacteria, archaea) lack a nucleus and organelles, relying on cytoplasmic processes. Eukaryotes feature compartmentalization via membranes.
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Nucleus | Absent (nucleoid) | Present |
| Ribosomes | 70S, free | 80S, free/ER-bound |
| Pili | In some (attachment) | Absent |
| Respiratory Enzymes | Plasma membrane | Mitochondria |
| Internal Membranes | Invaginations (mesosomes) | ER, Golgi |
Key Structures Absent in Eukaryotes
-
Nucleoid: Prokaryote-specific DNA region without membrane; eukaryotes use nucleus.
-
Pili: Protein filaments in prokaryotes for adhesion/DNA transfer; no eukaryotic equivalent.
These prokaryotic-unique features enable rapid adaptation, vital for exams like CSIR NET.
