STEM Readiness: Chemistry Readiness - Stoichiometry: Limiting Reagent - Identifying Theoretical, Yield Percent, Yield

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• DNA in prokaryotes is located in the nucleoid, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes have 70S ribosomes (30S + 50S subunits); eukaryotes have 80S ribosomes (40S + 60S subunits).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Prokaryotic cell walls typically contain peptidoglycan; archaea lack peptidoglycan but may have pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to antibiotics targeting peptidoglycan synthesis.
• 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 lack these.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores regulating transport; absent in prokaryotes.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Eukaryotic organelles include mitochondria, lysosomes, peroxisomes, endoplasmic reticulum, Golgi apparatus, and chloroplasts (in plants); absent in prokaryotes.
• Chloroplasts are found in plant cells and some protists; perform photosynthesis; contain thylakoids and chlorophyll.
• Lysosomes contain hydrolytic enzymes for intracellular digestion; present in animal cells, rare in plant cells.
• Prokaryotes may have plasmids (small circular DNA); eukaryotes typically do not, except in engineered systems.
• Cytoskeleton is present in both: prokaryotes have FtsZ, MreB, CreS; eukaryotes have microtubules, actin filaments, intermediate filaments.
• Flagella differ structurally: prokaryotic flagella are made of flagellin and rotate; eukaryotic flagella are microtubule-based (9+2) and undulate.
• Red blood cells in mammals lack a nucleus and mitochondria; cannot divide or perform aerobic respiration.
• Plant cells have large central vacuoles for turgor pressure and storage; animal cells have small or transient vacuoles.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and double membranes.
• Mitochondria are present in nearly all eukaryotic cells; absent in some protists like Giardia (mitosome instead).
• Gram-positive bacteria have thick peptidoglycan and no outer membrane; Gram-negative have thin peptidoglycan and an outer lipopolysaccharide (LPS) membrane.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes; more similar to eukaryotes in transcription and translation machinery.
• Nuclear DNA in eukaryotes is linear and associated with histones; prokaryotic DNA is circular and not bound to histones (though archaea have histone-like proteins).

Difficulty Level

Intermediate – requires distinguishing structural, genetic, and functional differences across domains and cell types, including exceptions.

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 and organelles use 70S ribosomes; eukaryotic cytoplasmic ribosomes are 80S.

Trap: Eukaryotes are always larger because they have more DNA – Fact: Genome size does not correlate directly with cell complexity (e.g., Amoeba dubia has larger genome than humans).

Trap: Mitochondria are found in all eukaryotic cells – Fact: Some anaerobic eukaryotes (e.g., Giardia, Trichomonas) lack mitochondria but have reduced forms (mitosomes or hydrogenosomes).

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) Circular DNA in nucleoid
D) Endoplasmic reticulum
Answer: C
Explanation: Prokaryotes have circular DNA located in the nucleoid, a non-membrane-bound region.
Why the top distractor is wrong: A is incorrect because a membrane-bound nucleus is exclusive to eukaryotes.

Question: Which structure is present 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 mature plant cells, maintaining turgor pressure.
Why the top distractor is wrong: A is incorrect because mitochondria are present in both plant and animal cells.

Question: What evidence best supports the endosymbiotic theory for mitochondria?
A) Presence of a single membrane
B) Linear DNA and 80S ribosomes
C) Circular DNA and 70S ribosomes
D) Synthesis of proteins using nuclear genes
Answer: C
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to bacteria, supporting bacterial origin.
Why the top distractor is wrong: A is incorrect because mitochondria have a double membrane, not single.

Question: Which organism lacks a cell wall entirely?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Mycoplasma pneumoniae
D) Bacillus subtilis
Answer: C
Explanation: Mycoplasma is a bacterium without a cell wall, making it resistant to penicillin.
Why the top distractor is wrong: A is incorrect because E. coli is a Gram-negative bacterium with a cell wall.

Question: Where is DNA located in a eukaryotic liver cell?
A) Nucleoid only
B) Nucleus and mitochondria
C) Cytoplasm and nucleus
D) Nucleus only
Answer: B
Explanation: Eukaryotic nuclear DNA is in the nucleus; mitochondrial DNA is in the mitochondria.
Why the top distractor is wrong: D is incorrect because mitochondria also contain their own DNA.

Question: Which of the following is true of both prokaryotes and eukaryotes?
A) Presence of lysosomes
B) Phospholipid bilayer membrane
C) Mitotic cell division
D) 80S ribosomes
Answer: B
Explanation: Both cell types have a plasma membrane composed of a phospholipid bilayer.
Why the top distractor is wrong: A is incorrect because lysosomes are only found in eukaryotic cells.

Question: Which cytoskeletal element is used in the movement of eukaryotic flagella?
A) Flagellin
B) Microfilaments
C) Intermediate filaments
D) Microtubules
Answer: D
Explanation: Eukaryotic flagella are composed of microtubules in a 9+2 arrangement.
Why the top distractor is wrong: A is incorrect because flagellin is the protein in prokaryotic flagella.

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

• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Prokaryotes: 0.1–5.0 ?m; eukaryotes: 10–100 ?m.
• Nucleoid = no membrane; nucleus = double membrane with pores.
• Peptidoglycan = only in bacteria; absent in archaea and eukaryotes.
• Cellulose in plant walls; chitin in fungi; no wall in animals.
• Mycoplasma = smallest known cell; no cell wall.
• Red blood cells lack nucleus and mitochondria; rely on glycolysis.
• Gram-positive = thick peptidoglycan, no outer membrane; Gram-negative = thin peptidoglycan, LPS outer membrane.
• Archaea have ether-linked lipids; bacteria and eukaryotes have ester-linked lipids.
• FtsZ protein in prokaryotes is homologous to eukaryotic tubulin.
• Endosymbiotic theory explains origin of mitochondria and chloroplasts.
• Lysosomes are acidic organelles with hydrolytic enzymes; found in animal cells.
• Peroxisomes break down fatty acids and detoxify alcohol via catalase.
• Smooth ER synthesizes lipids; rough ER has ribosomes and makes proteins.
• Golgi apparatus modifies, sorts, and packages proteins for secretion.
• Nuclear envelope is continuous with the rough ER.
• Chloroplasts have thylakoids and stroma; site of photosynthesis.
• Cytoskeleton components: microtubules (tubulin), microfilaments (actin), intermediate filaments (various proteins).
• Flagellum motion: prokaryotes rotate; eukaryotes bend (9+2 microtubule arrangement).
• Plasmids are small circular DNA in bacteria; used in genetic engineering.
• Histones package DNA in eukaryotes; prokaryotic DNA is not associated with histones (archaea have histone-like proteins).
• Mitochondria absent in Giardia; has mitosome instead.
• Verify from standard textbook: Exact lipid composition in archaeal membranes varies by species.
• All eukaryotes have a nucleus, but not all have mitochondria.