STEM Readiness: Biology Readiness - Cell Biology: Membrane Transport - Osmosis, Diffusion, Active vs. Passive, Exocytosis/Endocytosis

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 store DNA in the nucleoid, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotic ribosomes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• 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 with embedded proteins.
• Eukaryotes have membrane-bound organelles (e.g., nucleus, mitochondria, lysosomes); prokaryotes do not.
• Endoplasmic reticulum (ER) is present only in eukaryotes: rough ER has ribosomes; smooth ER synthesizes lipids.
• Golgi apparatus modifies, sorts, and packages proteins for secretion—exclusive to eukaryotes.
• Mitochondria generate ATP via oxidative phosphorylation and have double membranes and circular DNA.
• Chloroplasts perform photosynthesis in plants and algae; contain thylakoids, chlorophyll, and circular DNA.
• Lysosomes contain hydrolytic enzymes for degradation—present in animal cells, rare in plants and absent in prokaryotes.
• Centrioles organize microtubules during cell division in animal cells; absent in most plants and prokaryotes.
• Flagella differ structurally: prokaryotic flagella are made of flagellin and rotate; eukaryotic flagella are microtubule-based (9+2) and undulate.
• Cilia are short, numerous motile structures in some eukaryotes (e.g., Paramecium); prokaryotes lack cilia.
• Plasmids are small, circular extrachromosomal DNA molecules common in bacteria; rare in eukaryotes.
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide via mitosis and cytokinesis.
• Mycoplasma species are bacteria that lack a cell wall—an exception among prokaryotes.
• Mature mammalian red blood cells lack a nucleus and mitochondria—no DNA or aerobic respiration.
• Nuclear envelope is a double membrane with nuclear pores regulating transport—exclusive to eukaryotes.
• Nucleolus within the nucleus synthesizes ribosomal RNA and assembles ribosome subunits.
• Peroxisomes break down fatty acids and detoxify alcohol—present in eukaryotes, absent in prokaryotes.
• Vacuoles in plant cells are large and central, maintaining turgor pressure; smaller and transient in animal cells.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having 70S ribosomes, circular DNA, and ability to replicate independently.

Difficulty Level

Intermediate – requires precise distinction between structural and functional features across domains and kingdoms.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (other polymers) do not.
Trap: Ribosome size is the same across all cells – Fact: Prokaryotes use 70S ribosomes; eukaryotes use 80S in cytoplasm, but 70S in mitochondria and chloroplasts.
Trap: Prokaryotes have no DNA packaging – Fact: Prokaryotic DNA is supercoiled and associated with nucleoid-associated proteins, though not histones.
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 (e.g., multicellularity, organelles) correlates with eukaryotic evolution.

Practice MCQs (5–7 questions)

Question: Which structure is found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nucleoid
D) Plasma membrane
Answer: C
Explanation: The nucleoid is a defined region in prokaryotes without a membrane, where DNA resides.
Why the top distractor is wrong: 70S ribosomes (B) are in prokaryotes but also in mitochondria and chloroplasts of eukaryotes.

Question: Which of the following is a correct comparison between plant and bacterial cell walls?
A) Both contain chitin
B) Both contain peptidoglycan
C) Both contain cellulose
D) Both provide structural support
Answer: D
Explanation: Both cell walls function in structural support and protection.
Why the top distractor is wrong: Peptidoglycan (B) is in bacteria only; plants use cellulose.

Question: Which organelle is responsible for modifying and packaging proteins for secretion in eukaryotic cells?
A) Nucleus
B) Mitochondria
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies, sorts, and packages proteins from the ER into vesicles.
Why the top distractor is wrong: Lysosome (D) degrades materials, not packages for secretion.

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 using 80S ribosomes
C) Linear DNA attached to histones
D) Independent replication and 70S ribosomes
Answer: D
Explanation: Mitochondria replicate independently and have 70S ribosomes and circular DNA, like bacteria.
Why the top distractor is wrong: Double membrane (A) is supportive but not unique; other organelles also have double membranes.

Question: Which cell type lacks a nucleus and mitochondria?
A) Yeast cell
B) Escherichia coli
C) Mature human red blood cell
D) Onion root cell
Answer: C
Explanation: Mature mammalian red blood cells lose their nucleus and organelles to maximize hemoglobin capacity.
Why the top distractor is wrong: E. coli (B) lacks a nucleus but is prokaryotic and never had mitochondria.

Question: Where is ribosomal RNA synthesized in eukaryotic cells?
A) Rough ER
B) Nucleolus
C) Cytoplasm
D) Mitochondria
Answer: B
Explanation: The nucleolus within the nucleus synthesizes rRNA and assembles ribosomal subunits.
Why the top distractor is wrong: Rough ER (A) is where ribosomes translate proteins, not synthesize rRNA.

Question: Which of the following structures is present in both prokaryotes and eukaryotes?
A) Mitochondria
B) Plasma membrane
C) Nuclear envelope
D) Golgi apparatus
Answer: B
Explanation: All cells have a plasma membrane made of a phospholipid bilayer.
Why the top distractor is wrong: Mitochondria (A) are only in eukaryotes.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• Bacterial cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Both domains have phospholipid bilayer plasma membranes.
• Eukaryotes have membrane-bound organelles; prokaryotes do not.
• Rough ER has ribosomes; smooth ER makes lipids and detoxifies.
• Golgi apparatus: modifies, sorts, packages proteins.
• Mitochondria produce ATP via oxidative phosphorylation.
• Chloroplasts have thylakoids and perform photosynthesis.
• Lysosomes: contain hydrolytic enzymes; mainly in animal cells.
• Centrioles: in animal cells; organize spindle fibers.
• Prokaryotic flagella = flagellin, rotary motion; eukaryotic = microtubules (9+2), bending motion.
• Cilia: eukaryotic only; prokaryotes lack them.
• Plasmids: small circular DNA; common in bacteria.
• Prokaryotes reproduce by binary fission; eukaryotes by mitosis.
• Mycoplasma has no cell wall—only prokaryote without one.
• Mature red blood cell: no nucleus, no mitochondria, no DNA.
• Nuclear pores regulate molecular traffic between nucleus and cytoplasm.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Peroxisomes break down fatty acids and detoxify H?O?.
• Plant vacuole: large central, maintains turgor pressure.
• Endosymbiotic theory evidence: 70S ribosomes, circular DNA, binary fission in mitochondria/chloroplasts.