STEM Readiness: Biology Readiness - Cell Biology: Cell Membrane - Fluid Mosaic Model, Membrane Proteins, Selective Permeability

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 include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.
• DNA in prokaryotes is located in the nucleoid, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes possess organelles such as mitochondria, lysosomes, and peroxisomes.
• Ribosomes in prokaryotes are 70S (30S + 50S subunits); eukaryotic cytoplasmic ribosomes are 80S (40S + 60S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting the endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); 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 internal membrane systems including the endoplasmic reticulum (ER), Golgi apparatus, and nuclear envelope; prokaryotes do not.
• The nucleus in eukaryotes contains chromatin (DNA + histones) and a nucleolus where ribosomal RNA is synthesized.
• Prokaryotes have circular DNA in the nucleoid; eukaryotes have linear chromosomes associated with histone proteins.
• Plasmids are small, circular DNA molecules found in prokaryotes and some yeasts (eukaryotes); they often carry antibiotic resistance genes.
• Binary fission is the primary mode of reproduction in prokaryotes; mitosis and meiosis occur in eukaryotes.
• Mycoplasma species are prokaryotes without a cell wall, making them resistant to antibiotics like penicillin that target peptidoglycan.
• Red blood cells in mammals are eukaryotic cells without a nucleus, allowing more space for hemoglobin.
• Mitochondria have double membranes, cristae, and their own circular DNA; they generate ATP via oxidative phosphorylation.
• Chloroplasts are found in plant and algal cells, contain thylakoids, and perform photosynthesis using chlorophyll.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; plants use vacuoles for similar degradation.
• Peroxisomes contain enzymes like catalase that break down hydrogen peroxide; present in both plant and animal eukaryotic cells.
• The endosymbiotic theory proposes that mitochondria and chloroplasts originated from engulfed prokaryotes; evidence includes their 70S ribosomes, binary fission, and independent DNA.
• Cilia and flagella in eukaryotes have a 9+2 microtubule arrangement; prokaryotic flagella are composed of flagellin and lack microtubules.
• Gram-positive bacteria have a thick peptidoglycan layer and retain crystal violet stain; Gram-negative have a thin peptidoglycan layer and outer membrane with lipopolysaccharide.
• Archaea lack peptidoglycan, have unique membrane lipids with ether linkages, and often inhabit extreme environments.
• Plant cells contain a large central vacuole for turgor pressure and storage; animal cells have smaller, multiple vacuoles.

Difficulty Level

Intermediate – covered in introductory biology courses with emphasis on structural and functional comparisons across domains.

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 (pseudopeptidoglycan or none) have different compositions.
Trap: Eukaryotes are always larger and more complex than prokaryotes – Fact: Some prokaryotes (e.g., Thiomargarita namibiensis) can be up to 750 ?m, larger than typical eukaryotic cells.
Trap: Ribosome size correlates directly with cell complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, identical in size to prokaryotic ribosomes.
Trap: The nucleus is the only organelle that contains DNA in eukaryotes – Fact: Mitochondria and chloroplasts also contain their own DNA.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have cytoskeletal elements (e.g., FtsZ), ribosomes, and nucleoids, though not membrane-bound organelles.

Practice MCQs (5–7 questions)

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

Question: A cell is observed to contain a nucleus, mitochondria, and a cell wall made of cellulose. From which organism is this cell most likely derived?
A) Fungus
B) Bacterium
C) Animal
D) Plant
Answer: D
Explanation: Plants have cellulose cell walls, a nucleus, and mitochondria.
Why the top distractor is wrong: A (fungi) have chitin in their cell walls, not cellulose.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Ability to replicate independently
C) Containing 70S ribosomes and circular DNA
D) Involvement in ATP production
Answer: C
Explanation: 70S ribosomes and circular DNA are prokaryotic traits, supporting endosymbiosis.
Why the top distractor is wrong: B (independent replication) occurs in other organelles and does not uniquely support endosymbiosis.

Question: Which structure is found in prokaryotes but not in eukaryotes?
A) Nucleoid
B) Nucleolus
C) Endoplasmic reticulum
D) Golgi apparatus
Answer: A
Explanation: The nucleoid is the region in prokaryotes where DNA is located, lacking a membrane.
Why the top distractor is wrong: B (nucleolus) is found within the eukaryotic nucleus.

Question: Which of the following cells lacks a nucleus at maturity?
A) Yeast cell
B) Neuron
C) Human red blood cell
D) Leaf mesophyll cell
Answer: C
Explanation: Mature mammalian red blood cells expel their nucleus to carry more hemoglobin.
Why the top distractor is wrong: A (yeast) are fungi and have 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 (40S + 60S subunits).
Why the top distractor is wrong: B (70S) describes prokaryotic ribosomes.

Question: Which of the following organisms has a cell wall made of peptidoglycan?
A) Saccharomyces cerevisiae
B) Homo sapiens
C) Escherichia coli
D) Penicillium chrysogenum
Answer: C
Explanation: E. coli is a bacterium with a peptidoglycan cell wall.
Why the top distractor is wrong: D (Penicillium) is a fungus with chitin, not peptidoglycan.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: DNA in nucleoid; eukaryotes: DNA in nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Peptidoglycan is unique to bacterial cell walls.
• Archaea lack peptidoglycan and have ether-linked membrane lipids.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Both cell types have phospholipid bilayer membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nucleolus synthesizes rRNA and assembles ribosomal subunits.
• Prokaryotes have circular DNA; eukaryotes have linear chromosomes with histones.
• Plasmids are small, extrachromosomal DNA circles, common in bacteria.
• Binary fission = prokaryotic division; mitosis = eukaryotic nuclear division.
• Mycoplasma lacks a cell wall – smallest known cells.
• Mature mammalian RBCs lack a nucleus.
• Mitochondria have double membrane and cristae for ATP synthesis.
• Chloroplasts contain thylakoids and grana for photosynthesis.
• Lysosomes are in animal cells; plants use vacuoles for degradation.
• Peroxisomes break down fatty acids and detoxify H?O? with catalase.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria, chloroplasts from cyanobacteria.
• Eukaryotic flagella: 9+2 microtubule array; prokaryotic: flagellin filament, rotary motion.
• Gram-positive bacteria: thick peptidoglycan, no outer membrane.
• Gram-negative bacteria: thin peptidoglycan, outer membrane with LPS (endotoxin).
• FtsZ protein in prokaryotes is homologous to eukaryotic tubulin.
• Verify from standard textbook: exact size of Thiomargarita namibiensis (can exceed 750 ?m).