STEM Readiness: Biology Readiness - Cell Biology: Cell Structure - Prokaryotic vs. Eukaryotic, Organelles Membrane, Nucleus

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• Prokaryotes lack a membrane-bound nucleus; DNA resides in the nucleoid region (e.g., Escherichia coli).
• Eukaryotes have a membrane-bound nucleus housing linear DNA (e.g., human liver cell).
• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., bacterial cell walls in Staphylococcus aureus).
• Archaea lack peptidoglycan; their cell walls are made of pseudopeptidoglycan or other polysaccharides.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin.
• Animal cells lack a cell wall; structural support is provided by the extracellular matrix.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have extensive internal membrane systems, including the endoplasmic reticulum and Golgi apparatus; prokaryotes do not.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores regulating transport (e.g., import of transcription factors).
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Circular DNA is found in prokaryotes (nucleoid), mitochondria, and chloroplasts; eukaryotic nuclear DNA is linear.
• Plasmids are small, circular DNA molecules common in prokaryotes (e.g., F plasmid in conjugation); rare in eukaryotes.
• Eukaryotic organelles include mitochondria (ATP synthesis), lysosomes (hydrolysis), peroxisomes (detoxification), and Golgi apparatus (protein modification).
• Rough endoplasmic reticulum (RER) has bound ribosomes and synthesizes proteins for secretion (e.g., insulin in pancreatic beta cells).
• Smooth endoplasmic reticulum (SER) lacks ribosomes and synthesizes lipids (e.g., steroid hormones in adrenal cortex cells).
• Golgi apparatus modifies, sorts, and packages proteins into vesicles (e.g., glycosylation of lysosomal enzymes).
• Mycoplasma species are prokaryotes that lack a cell wall and are resistant to penicillin.
• Mature mammalian red blood cells lack a nucleus and most organelles, maximizing hemoglobin content.
• Chloroplasts are found in plant and algal cells; perform photosynthesis using thylakoid membranes.
• Cilia and flagella in eukaryotes have a 9+2 microtubule arrangement; prokaryotic flagella are composed of flagellin and lack microtubules.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, 70S ribosomes, and independent division.
• Lysosomes are membrane-bound organelles containing hydrolytic enzymes active at acidic pH (~4.5–5.0).

Difficulty Level

Intermediate – routinely tested in first-year biology with emphasis on comparative structure and functional implications.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; archaea, plants, and fungi use different structural polymers.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes (e.g., cyanobacteria) have thylakoid membranes for photosynthesis.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: DNA location is always the nucleus in eukaryotes – Fact: Mitochondria and chloroplasts contain their own DNA.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Presence of mitochondria
B) 80S ribosomes in the cytoplasm
C) DNA located in a nucleoid
D) Membrane-bound Golgi apparatus
Answer: C
Explanation: Prokaryotes lack a nucleus; DNA is in the nucleoid region.
Why the top distractor is wrong: D is incorrect because Golgi apparatus is exclusive to eukaryotes.

Question: A cell is observed to have a cell wall made of cellulose, chloroplasts, and a large central vacuole. This cell is most likely:
A) Fungal
B) Bacterial
C) Animal
D) Plant
Answer: D
Explanation: Cellulose cell walls, chloroplasts, and central vacuoles are characteristic of plant cells.
Why the top distractor is wrong: A is incorrect because fungal cell walls are made of chitin, not cellulose.

Question: Which structure is present in eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Nuclear envelope
D) Circular DNA
Answer: C
Explanation: The nuclear envelope is a double membrane enclosing the nucleus, found only in eukaryotes.
Why the top distractor is wrong: A is incorrect because both cell types have a plasma membrane.

Question: Mitochondria are thought to have originated from endosymbiotic bacteria. Which evidence best supports this?
A) They have a single membrane
B) They contain 80S ribosomes
C) They have 70S ribosomes and circular DNA
D) They synthesize proteins using the nucleus’s ribosomes
Answer: C
Explanation: Mitochondria have 70S ribosomes and circular DNA, similar to prokaryotes.
Why the top distractor is wrong: B is incorrect because 80S ribosomes are found in the eukaryotic cytoplasm, not mitochondria.

Question: Which of the following cells lacks a nucleus?
A) Yeast cell
B) Human neuron
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: A is incorrect because yeast is a eukaryotic fungus with a nucleus.

Question: What is the sedimentation coefficient of cytoplasmic ribosomes in eukaryotic cells?
A) 50S
B) 70S
C) 80S
D) 30S
Answer: C
Explanation: Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
Why the top distractor is wrong: B is incorrect because 70S ribosomes are found in prokaryotes and organelles.

Question: Which of the following is true about the nuclear envelope?
A) It is a single phospholipid bilayer
B) It is continuous with the smooth endoplasmic reticulum
C) It contains pores that regulate RNA export
D) It synthesizes ribosomal RNA
Answer: C
Explanation: Nuclear pores regulate transport, including mRNA export to the cytoplasm.
Why the top distractor is wrong: B is incorrect because the nuclear envelope is continuous with the rough ER, not the smooth ER.

Last?Minute Revision (20–25 one?liners)

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid, not nucleus.
• Eukaryotes have membrane-bound nucleus with nuclear pores.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea do not.
• Plant cell walls = cellulose; fungal = chitin; animal cells = no cell wall.
• Mycoplasma has no cell wall – naturally penicillin-resistant.
• Mature red blood cells lack nucleus and mitochondria.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, lysosomes); prokaryotes generally do not.
• Nuclear envelope is double membrane with nuclear pores.
• DNA in prokaryotes is circular and naked; eukaryotic nuclear DNA is linear and associated with histones.
• Plasmids are common in prokaryotes, rare in eukaryotes.
• RER has ribosomes and synthesizes proteins for export.
• SER synthesizes lipids and detoxifies drugs.
• Golgi apparatus modifies, sorts, packages proteins in vesicles.
• Lysosomes maintain acidic pH (~5) for hydrolytic enzyme activity.
• Peroxisomes break down fatty acids and detoxify hydrogen peroxide.
• Mitochondria produce ATP via oxidative phosphorylation.
• Chloroplasts perform photosynthesis in plants and algae.
• Eukaryotic flagella = 9+2 microtubule array; prokaryotic flagella = flagellin filament.
• Mitochondria and chloroplasts have double membranes – inner from endosymbiont, outer from host vesicle.
• Endosymbiotic organelles replicate independently of the cell cycle.
• Verify from standard textbook: exact composition of archaeal cell walls varies widely.
• Nuclear pores allow selective transport; small molecules diffuse, large molecules require signals.