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Must?Know

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• 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, endoplasmic reticulum, and Golgi apparatus.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S).
• 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 and have different wall chemistry.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to penicillin.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane; eukaryotes also have internal membranes forming organelles.
• The nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate molecular transport.
• Chromosomes in prokaryotes are typically a single, circular DNA molecule; eukaryotes have multiple linear chromosomes.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rarely in eukaryotes (e.g., 2-micron plasmid in yeast).
• Binary fission is the method of asexual reproduction in prokaryotes; mitosis occurs in eukaryotes.
• Eukaryotic flagella are composed of microtubules (9+2 arrangement) and are covered by the plasma membrane; prokaryotic flagella are made of flagellin and are not membrane-bound.
• Cilia and flagella are absent in prokaryotes (except for archaeal flagella, structurally different).
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; rare in plant cells (vacuoles perform similar function).
• Plant cells contain large central vacuoles for turgor pressure; animal cells have small or temporary vacuoles.
• Chloroplasts are present in plants and algae, sites of photosynthesis; absent in animals and fungi.
• Red blood cells in mammals lack a nucleus and mitochondria, maximizing hemoglobin capacity.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own circular DNA, 70S ribosomes, and double membranes.
• Mitochondria are present in nearly all eukaryotic cells; absent in some protists like Giardia (anaerobic).
• Peroxisomes contain enzymes for beta-oxidation and detoxification of hydrogen peroxide; found in both plant and animal eukaryotic cells.
• Smooth endoplasmic reticulum (SER) synthesizes lipids and detoxifies drugs; rough ER (RER) has ribosomes and synthesizes proteins for secretion.
• Golgi apparatus modifies, sorts, and packages proteins into vesicles; absent in prokaryotes.

Difficulty Level

Intermediate – requires understanding of structural differences, functional implications, and exceptions commonly tested in first-year biology.

Common Traps

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (pseudopeptidoglycan or other) differ.
Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S (except mitochondria/chloroplasts with 70S).
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Eukaryotes are always larger because they are more complex – Fact: Size difference is consistent, but complexity is due to compartmentalization, not size alone.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and sometimes inclusions or microcompartments.

Practice MCQs

Question: Which of the following is a feature shared by both prokaryotic and eukaryotic cells?
A) Membrane-bound nucleus
B) Mitochondria
C) Phospholipid bilayer plasma membrane
D) 80S ribosomes
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: A is exclusive to eukaryotes; prokaryotes lack a nucleus.

Question: A cell is observed to contain 70S ribosomes, a peptidoglycan cell wall, and no nucleus. What type of cell is this?
A) Plant cell
B) Fungal cell
C) Bacterial cell
D) Animal cell
Answer: C
Explanation: Bacteria have 70S ribosomes, peptidoglycan walls, and no nucleus.
Why the top distractor is wrong: Plant cells have cellulose walls and 80S ribosomes, and are eukaryotic.

Question: Which structure is found in plant cells but not in animal cells?
A) Lysosome
B) Mitochondrion
C) Central vacuole
D) Golgi apparatus
Answer: C
Explanation: Mature plant cells have a large central vacuole; animal cells have only small vesicles.
Why the top distractor is wrong: Lysosomes are primarily in animal cells; plants use vacuoles for degradation.

Question: What evidence supports the endosymbiotic theory for mitochondria?
A) They have 80S ribosomes
B) They have circular DNA and 70S ribosomes
C) They are synthesized by the rough ER
D) They lack a membrane
Answer: B
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to bacteria.
Why the top distractor is wrong: 80S ribosomes are found in the eukaryotic cytoplasm, not in mitochondria.

Question: Which of the following cells lacks a nucleus?
A) Yeast cell
B) Human skin cell
C) Escherichia coli
D) Onion root cell
Answer: C
Explanation: E. coli is a prokaryote and lacks a membrane-bound nucleus.
Why the top distractor is wrong: Yeast is a eukaryotic fungus and has a nucleus.

Question: Which organelle is responsible for protein modification and packaging in eukaryotic cells?
A) Nucleolus
B) Smooth ER
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies and packages proteins into vesicles.
Why the top distractor is wrong: Smooth ER synthesizes lipids, not proteins for secretion.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 70S
B) Mitochondrial ribosomes are 70S
C) Ribosomes are enclosed in a membrane
D) Ribosomes are only found in the nucleus
Answer: B
Explanation: Mitochondria have 70S ribosomes, reflecting their prokaryotic origin.
Why the top distractor is wrong: Cytoplasmic ribosomes in eukaryotes are 80S, not 70S.

Last?Minute Revision

• Prokaryotic cell size: 0.1–5.0 ?m
• Eukaryotic cell size: 10–100 ?m
• Prokaryotes: no nucleus, DNA in nucleoid
• Eukaryotes: DNA in membrane-bound nucleus
• Prokaryotic ribosome = 70S (50S + 30S)
• Eukaryotic cytoplasmic ribosome = 80S (60S + 40S)
• Mitochondria and chloroplasts have 70S ribosomes
• Mitochondria have circular DNA – evidence for endosymbiosis
• Bacterial cell wall contains peptidoglycan
• Archaea lack peptidoglycan
• Plant cell wall = cellulose
• Fungal cell wall = chitin
• Animal cells = no cell wall
• Mycoplasma has no cell wall
• Both cell types have phospholipid bilayer
• Eukaryotes have internal membranes (organelles)
• Nuclear envelope has nuclear pores
• Prokaryotes have circular chromosome; eukaryotes have linear chromosomes
• Plasmids common in bacteria
• Eukaryotes divide by mitosis; prokaryotes by binary fission
• Eukaryotic flagella: 9+2 microtubules, membrane-covered
• Prokaryotic flagella: made of flagellin, not membrane-bound
• Red blood cells lack nucleus and mitochondria
• Lysosomes mainly in animal cells
• Central vacuole in plant cells
• Chloroplasts in plants and algae only
• Mitochondria have double membrane – inner folded into cristae
• Smooth ER: lipid synthesis, detoxification
• Rough ER: protein synthesis (with ribosomes)
• Golgi: modifies, sorts, packages proteins
• Peroxisomes: break down fatty acids, detoxify H?O?
• Verify from standard textbook: Archaeal flagella structure and composition