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Must‑Know

• Prokaryotic cells range from 0.1–5.0 μm in diameter; eukaryotic cells range from 10–100 μm.
• Prokaryotes include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.
• DNA in prokaryotes is located in the nucleoid, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain organelles such as mitochondria, lysosomes, and the Golgi apparatus.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport.
• Eukaryotes have internal membranes forming organelles (e.g., nuclear envelope, endoplasmic reticulum), prokaryotes do not.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and undulate.
• Plasmids are small, circular extrachromosomal DNA molecules common in prokaryotes, occasionally found in yeast (eukaryote).
• Nuclear envelope in eukaryotes is double-membraned and contains nuclear pores for RNA and protein transport.
• Nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis and ribosome subunit assembly.
• Endoplasmic reticulum (ER): Rough ER has ribosomes and synthesizes proteins; smooth ER lacks ribosomes and synthesizes lipids and detoxifies.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other organelles.
• Mitochondria are the site of aerobic respiration and contain their own circular DNA and 70S ribosomes.
• Chloroplasts (in plants and algae) perform photosynthesis, contain thylakoids, and have circular DNA and 70S ribosomes.
• Lysosomes (in animal cells) contain hydrolytic enzymes for intracellular digestion; vacuoles in plant cells store water and maintain turgor.
• Cytoskeleton in eukaryotes includes microfilaments (actin), intermediate filaments, and microtubules (tubulin); prokaryotes have analogous proteins (e.g., FtsZ) but no true cytoskeleton.
• Mycoplasma species are bacteria without a cell wall, making them resistant to antibiotics like penicillin.
• Mature mammalian red blood cells lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, circular DNA, and 70S ribosomes.

Difficulty Level

Intermediate – expected familiarity with basic cell components, but requires precise differentiation between prokaryotic and eukaryotic features.

Common Traps

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacterial cell walls contain peptidoglycan; plants (cellulose), fungi (chitin), and archaea (no peptidoglycan) differ.
Trap: Ribosome size correlates with cell complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and inclusions, but no membrane-bound organelles.
Trap: All eukaryotic cells have a cell wall – Fact: Animal cells lack cell walls; only plants, fungi, and some protists have them.

Practice MCQs

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nucleoid region
D) Phospholipid bilayer
Answer: C
Explanation: The nucleoid is a distinct, non-membrane-bound region in prokaryotes where DNA is located.
Why the top distractor is wrong: While 70S ribosomes (B) are common in prokaryotes, they are also present in mitochondria and chloroplasts of eukaryotes.

Question: Which structure is present in plant cells but not in animal cells?
A) Mitochondria
B) Golgi apparatus
C) Large central vacuole
D) Nucleus
Answer: C
Explanation: A large central vacuole is a defining feature of plant cells, maintaining turgor pressure.
Why the top distractor is wrong: Mitochondria (A) are present in both plant and animal cells.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Ability to synthesize proteins
C) Containing 70S ribosomes and circular DNA
D) Involvement in ATP production
Answer: C
Explanation: 70S ribosomes and circular DNA are prokaryotic traits, supporting mitochondrial origin from bacteria.
Why the top distractor is wrong: While double membranes (A) are consistent with endosymbiosis, they can arise by other mechanisms; molecular evidence is stronger.

Question: A cell is observed to lack a nucleus and has a cell wall containing peptidoglycan. Which domain does it most likely belong to?
A) Eukarya
B) Archaea
C) Bacteria
D) Protista
Answer: C
Explanation: Bacteria are prokaryotes with peptidoglycan-containing cell walls and no nucleus.
Why the top distractor is wrong: Archaea (B) are prokaryotes but lack peptidoglycan in their cell walls.

Question: Which organelle is responsible for modifying and packaging proteins for secretion?
A) Rough endoplasmic reticulum
B) Smooth endoplasmic reticulum
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies, sorts, and packages proteins into vesicles for transport.
Why the top distractor is wrong: The rough ER (A) synthesizes proteins but does not package them for secretion.

Question: Which of the following cells lacks mitochondria?
A) Yeast cell
B) Leaf cell
C) Mature human red blood cell
D) Neuron
Answer: C
Explanation: Mature human red blood cells lack mitochondria and generate ATP via glycolysis.
Why the top distractor is wrong: Yeast cells (A) are fungi and have mitochondria.

Question: What is the primary structural component of fungal cell walls?
A) Cellulose
B) Peptidoglycan
C) Chitin
D) Silica
Answer: C
Explanation: Chitin is the polysaccharide that provides structural support in fungal cell walls.
Why the top distractor is wrong: Cellulose (A) is found in plant cell walls, not fungal.

Last‑Minute Revision

• ⚠️ Prokaryotic cell size: 0.1–5.0 μm; eukaryotic: 10–100 μm.
• ⚠️ Prokaryotes have no nucleus; DNA in nucleoid.
• ⚠️ Eukaryotes have membrane-bound nucleus with nuclear pores.
• ⚠️ Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• ⚠️ Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• ⚠️ Bacterial cell wall contains peptidoglycan; archaea lack it.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Both cell types have phospholipid bilayer plasma membrane.
• Eukaryotes have internal membranes (ER, Golgi, etc.); prokaryotes do not.
• ⚠️ Mycoplasma lacks a cell wall – smallest known cells.
• ⚠️ Mature red blood cells lack nucleus and mitochondria.
• Nuclear envelope is double-membraned with pores.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Rough ER = protein synthesis (with ribosomes); smooth ER = lipid synthesis, detox.
• Golgi apparatus = modifies, sorts, packages proteins.
• Mitochondria = site of aerobic respiration, ATP production.
• Chloroplasts = site of photosynthesis, contain thylakoids.
• Lysosomes (animal cells) = contain hydrolytic enzymes.
• Plant vacuole = turgor pressure, storage.
• Cytoskeleton: microfilaments (actin), microtubules (tubulin), intermediate filaments.
• Prokaryotes have FtsZ protein (tubulin analog) for division.
• Flagella: prokaryotic = flagellin, rotating; eukaryotic = 9+2 microtubules, bending.
• Plasmids = extrachromosomal DNA, common in bacteria.
• ⚠️ Endosymbiotic theory supported by: double membrane, 70S ribosomes, circular DNA in mitochondria/chloroplasts.
• Verify from standard textbook: exact composition of archaeal cell walls.
• ⚠️ Binary fission = prokaryotic division; mitosis/meiosis = eukaryotic.