STEM Readiness: Chemistry Readiness Stoichiometry Gas Stoichiometry Ideal Gas Law STP Molar Volume Gas Mixtures

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, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• 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 pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to antibiotics like penicillin that target peptidoglycan synthesis.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membrane systems including the nuclear envelope, ER, and Golgi; prokaryotes do not.
• The nucleus in eukaryotes contains chromatin (DNA + histones) and a nucleolus where ribosomal RNA is synthesized.
• Nuclear pores regulate transport between nucleus and cytoplasm; prokaryotes lack nuclear pores.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Eukaryotic mitochondria generate ATP via oxidative phosphorylation; prokaryotes perform this at the plasma membrane.
• Chloroplasts, found in plants and algae, are sites of photosynthesis and contain thylakoids and chlorophyll.
• Lysosomes in animal cells contain hydrolytic enzymes for degradation; absent in most plant cells (vacuoles perform similar function).
• Plant cells have a large central vacuole that maintains turgor pressure; animal cells have small or transient vacuoles.
• Centrioles are present in animal cells and aid in spindle formation during mitosis; absent in most plant cells.
• 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 circular DNA, 70S ribosomes, and double membranes.
• Red blood cells in mammals lack a nucleus and organelles, maximizing space for hemoglobin.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes, unlike bacteria and eukaryotes.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin layers and an outer membrane with lipopolysaccharide.

Difficulty Level

Intermediate – Covered in foundational biology courses 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; plants (cellulose), fungi (chitin), and archaea (no peptidoglycan) differ.

Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S in the cytoplasm, but 70S in mitochondria and chloroplasts.

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, though not part of an endomembrane system.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of eukaryotic cells but absent in prokaryotes?
A) Plasma membrane
B) Ribosomes
C) Circular DNA
D) Membrane-bound nucleus
Answer: D
Explanation: The membrane-bound nucleus is exclusive to eukaryotes.
Why the top distractor is wrong: Circular DNA is found in prokaryotes and also in mitochondria and chloroplasts, so it is not exclusive to prokaryotes.

Question: A cell is observed to have a cell wall made of cellulose, chloroplasts, and a large central vacuole. What type of cell is this most likely?
A) Fungal
B) Bacterial
C) Animal
D) Plant
Answer: D
Explanation: Cellulose cell walls, chloroplasts, and large central vacuoles are characteristic of plant cells.
Why the top distractor is wrong: Fungal cells have chitin in their walls and lack chloroplasts.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Mitochondria
B) 80S ribosomes
C) Phospholipid bilayer membrane
D) Golgi apparatus
Answer: C
Explanation: Both cell types have a plasma membrane composed of a phospholipid bilayer.
Why the top distractor is wrong: 80S ribosomes are only in eukaryotic cytoplasm; prokaryotes have 70S.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Eukaryotes have linear DNA
B) Mitochondria have 70S ribosomes and circular DNA
C) Prokaryotes lack a nucleus
D) Lysosomes contain digestive enzymes
Answer: B
Explanation: Mitochondria resemble bacteria in ribosome type and DNA structure, supporting bacterial origin.
Why the top distractor is wrong: Linear DNA is not unique to eukaryotes and does not relate to endosymbiosis.

Question: Which organism lacks a cell wall entirely?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Mycoplasma pneumoniae
D) Arabidopsis thaliana
Answer: C
Explanation: Mycoplasma is a bacterium that naturally lacks a cell wall.
Why the top distractor is wrong: E. coli has a peptidoglycan cell wall and is Gram-negative.

Question: Where is DNA located in a prokaryotic cell?
A) Nucleus
B) Nucleolus
C) Nucleoid
D) Mitochondria
Answer: C
Explanation: Prokaryotes store DNA in the nucleoid, an unmembraned region.
Why the top distractor is wrong: A nucleus is only present in eukaryotes.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 70S
B) Cytoplasmic ribosomes are 80S
C) Ribosomes are only found in the nucleus
D) Ribosomes are membrane-bound
Answer: B
Explanation: Eukaryotic cytoplasmic ribosomes are 80S; 70S ribosomes are in mitochondria and chloroplasts.
Why the top distractor is wrong: 70S ribosomes are only in organelles, not throughout the cytoplasm.

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

• ⚠️ Prokaryotic cell size: 0.1–5.0 μm; eukaryotic: 10–100 μm.
• ⚠️ Prokaryotes have no nucleus; DNA in nucleoid.
• ⚠️ Eukaryotes have membrane-bound nucleus with nuclear pores.
• Prokaryotes: bacteria and archaea; eukaryotes: animals, plants, fungi, protists.
• ⚠️ Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• ⚠️ Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Peptidoglycan is found only in bacterial cell walls.
• Archaea lack peptidoglycan; have ether-linked membrane lipids.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• ⚠️ Mycoplasma has no cell wall – resistant to penicillin.
• Both cell types have phospholipid bilayer plasma membrane.
• Eukaryotes have internal membranes (ER, Golgi, nuclear envelope); prokaryotes do not.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Lysosomes are in animal cells; contain hydrolytic enzymes.
• Plant vacuole maintains turgor pressure; animal cells lack large vacuoles.
• Centrioles present in animal cells; absent in most plants.
• Eukaryotic flagella: 9+2 microtubule array; prokaryotic: flagellin-based, rotary motion.
• Binary fission = prokaryotic division; mitosis/meiosis = eukaryotic.
• Mitochondria = site of oxidative phosphorylation in eukaryotes.
• Chloroplasts = site of photosynthesis; contain thylakoids and chlorophyll.
• ⚠️ Red blood cells lack nucleus and organelles.
• Gram-positive: thick peptidoglycan; Gram-negative: thin peptidoglycan + outer LPS membrane.
• Endosymbiotic theory supported by organelle DNA, ribosomes, and double membranes.
• Verify from standard textbook: exact lipid composition in archaeal membranes.
• Verify from standard textbook: exceptions in protist cell structures.