STEM Readiness: Physics Readiness Fluids Pressure and Buoyancy Pascals Law Archimedes Principle Fluid Statics

Must‑Know (20–25 detailed bullets)

• 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 (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting the endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., in Escherichia coli); archaea lack peptidoglycan and have different wall chemistries.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Mycoplasma, a bacterium, lacks a cell wall, making it resistant to antibiotics like penicillin that target peptidoglycan.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport and maintains integrity.
• Eukaryotes have an endomembrane system including the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles; prokaryotes do not.
• The smooth ER synthesizes lipids and detoxifies drugs; the rough ER (with ribosomes) synthesizes proteins for secretion.
• The Golgi apparatus modifies, sorts, and packages proteins into vesicles for transport; absent in prokaryotes.
• Mitochondria generate ATP via aerobic respiration; they have double membranes, circular DNA, and 70S ribosomes.
• Chloroplasts (in plants and algae) perform photosynthesis; they have thylakoids, chlorophyll, circular DNA, and 70S ribosomes.
• Lysosomes contain hydrolytic enzymes for breaking down macromolecules; found in animal cells, rare in plant cells (vacuoles perform similar roles).
• Plant cells have a large central vacuole that maintains turgor pressure; animal cells have small or temporary vacuoles.
• Centrioles (involved in spindle formation during mitosis) are present in animal cells but absent in most plant and fungal cells.
• Red blood cells in mammals lack a nucleus and mitochondria, maximizing space for hemoglobin; they cannot divide or perform aerobic respiration.
• Nuclear pores in the nuclear envelope allow selective transport of RNA and proteins; prokaryotes lack such structures.
• Cilia and flagella differ structurally: prokaryotic flagella are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and bend.
• Pili (or fimbriae) in bacteria aid in attachment and conjugation; eukaryotes lack pili.
• Plasmids are small, circular DNA molecules found in many prokaryotes and used in genetic engineering; eukaryotes rarely have plasmids (e.g., in yeast).
• Endosymbiotic theory is supported by mitochondrial and chloroplast double membranes, independent division, circular DNA, and 70S ribosomes.
• Archaea resemble prokaryotes in size and lack of nucleus but have eukaryote-like transcription and translation machinery.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and recognizing exceptions common in introductory biology exams.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; archaea, plants (cellulose), and fungi (chitin) have different wall compositions.
Trap: Ribosome size is the same across all cells – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes in the cytoplasm, but mitochondria and chloroplasts retain 70S.
Trap: Eukaryotes are always larger because they are more complex – Fact: While generally true, some eukaryotic cells (e.g., yeast) are within the prokaryotic size range, and some bacteria (e.g., Thiomargarita namibiensis) exceed 100 μm.
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 membranes – Fact: Some prokaryotes (e.g., cyanobacteria) have thylakoid membranes for photosynthesis.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes
C) Presence of peptidoglycan in the cell wall
D) Endoplasmic reticulum
Answer: C
Explanation: Peptidoglycan is a key component of bacterial cell walls, a defining feature of most prokaryotes.
Why the top distractor is wrong: A is incorrect because prokaryotes lack a membrane-bound nucleus.

Question: Which structure is found in plant cells but not in animal cells?
A) Mitochondria
B) Plasma membrane
C) Central vacuole
D) Ribosomes
Answer: C
Explanation: The large central vacuole is a hallmark of plant cells, maintaining turgor pressure.
Why the top distractor is wrong: A is incorrect because both plant and animal cells have mitochondria.

Question: Mitochondria are thought to have evolved from free-living prokaryotes due to all of the following EXCEPT:
A) Presence of 70S ribosomes
B) Circular DNA
C) Ability to synthesize all their own proteins
D) Double membrane
Answer: C
Explanation: Mitochondria cannot synthesize all their own proteins; many are encoded by nuclear DNA and imported.
Why the top distractor is wrong: A is correct evidence, so it does not fit the EXCEPT question.

Question: Which of the following cells lacks a nucleus?
A) Neuron
B) Leukocyte
C) Mature human red blood cell
D) Hepatocyte
Answer: C
Explanation: Mature human red blood cells eject their nucleus to accommodate more hemoglobin.
Why the top distractor is wrong: A is incorrect because neurons are fully differentiated eukaryotic cells with nuclei.

Question: Which ribosome type would be found in the cytoplasm of a human liver cell?
A) 50S
B) 70S
C) 80S
D) 30S
Answer: C
Explanation: Eukaryotic cytoplasmic ribosomes are 80S.
Why the top distractor is wrong: B is incorrect because 70S ribosomes are found in prokaryotes and organelles, not in the eukaryotic cytoplasm.

Question: Which organism has a cell wall made of chitin?
A) Rose bush
B) Escherichia coli
C) Saccharomyces cerevisiae
D) Mycoplasma pneumoniae
Answer: C
Explanation: Saccharomyces cerevisiae (baker’s yeast) is a fungus and has a chitin-containing cell wall.
Why the top distractor is wrong: A is incorrect because plants have cellulose, not chitin, in their cell walls.

Question: Which structure is present in both prokaryotes and eukaryotes?
A) Golgi apparatus
B) Nucleoid
C) Plasma membrane
D) Mitochondria
Answer: C
Explanation: Both cell types have a phospholipid bilayer plasma membrane.
Why the top distractor is wrong: B is incorrect because only prokaryotes have a nucleoid; eukaryotes have a nucleus.

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

• ⚠️ Prokaryotic cells: 0.1–5.0 μm; eukaryotic cells: 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 – evidence for endosymbiosis.
• Bacteria: peptidoglycan cell wall; Archaea: no peptidoglycan.
• Plants: cellulose cell wall; Fungi: chitin; Animals: no cell wall.
• ⚠️ Mycoplasma lacks a cell wall – resistant to penicillin.
• Both domains have phospholipid bilayer plasma membrane.
• Eukaryotes have endomembrane system: ER, Golgi, lysosomes, vesicles.
• Rough ER has ribosomes; smooth ER synthesizes lipids.
• Golgi apparatus modifies and sorts proteins for secretion.
• Mitochondria produce ATP via oxidative phosphorylation.
• Chloroplasts perform photosynthesis; contain thylakoids and chlorophyll.
• Lysosomes contain hydrolytic enzymes; mainly in animal cells.
• Plant central vacuole maintains turgor pressure.
• Animal cells have centrioles; most plant cells do not.
• Mature mammalian red blood cells lack nucleus and mitochondria.
• Nuclear envelope has nuclear pores for RNA/protein transport.
• Prokaryotic flagella: rotate, made of flagellin.
• Eukaryotic flagella: bend, 9+2 microtubule arrangement.
• Pili are present in bacteria for attachment and conjugation.
• Plasmids: small circular DNA; common in bacteria, rare in eukaryotes.
• ⚠️ Endosymbiotic theory supported by: double membrane, independent division, 70S ribosomes, circular DNA in mitochondria/chloroplasts.
• Archaea: prokaryotic structure but eukaryotic-like transcription/translation.
• Verify from standard textbook: exact lipid composition of archaeal membranes.