STEM Readiness: Chemistry Readiness - Stoichiometry: Mole Concept - Avogadro’s Number, Molar Mass, Mole Conversions

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, 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., Escherichia coli); archaea lack peptidoglycan and have different wall chemistry.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack cell walls.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport.
• Eukaryotes have internal membrane systems including the endoplasmic reticulum (ER) and Golgi apparatus; prokaryotes do not.
• The nucleus in eukaryotes is surrounded by a double membrane (nuclear envelope) with nuclear pores for RNA and protein transport.
• Nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis and ribosome subunit assembly.
• Rough ER is studded with ribosomes and synthesizes proteins for secretion or membranes; smooth ER lacks ribosomes and synthesizes lipids and detoxifies drugs.
• Golgi apparatus modifies, sorts, and packages proteins into vesicles for transport to lysosomes, plasma membrane, or secretion.
• Mitochondria are the site of aerobic respiration, produce ATP, and have cristae (inner membrane folds) and matrix (inner fluid).
• Chloroplasts (in plants and algae) perform photosynthesis, contain thylakoids with chlorophyll, and have stroma.
• Lysosomes (in animal cells) contain hydrolytic enzymes for breaking down macromolecules and cellular debris.
• Peroxisomes break down fatty acids via beta-oxidation and detoxify hydrogen peroxide (H?O?).
• Mycoplasma species are bacteria without a cell wall, making them resistant to antibiotics like penicillin that target peptidoglycan.
• Mature mammalian red blood cells lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and double membranes.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Cytoskeleton components (actin filaments, microtubules, intermediate filaments) are present in eukaryotes, absent in most prokaryotes.
• Plasmids are small, circular extrachromosomal DNA molecules common in prokaryotes, used in gene transfer.
• Plant cells contain large central vacuoles for storage, turgor pressure, and waste; animal cells have smaller, multiple vacuoles.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and organelles, with attention to exceptions and evolutionary evidence.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacterial cell walls contain peptidoglycan; plants (cellulose) and fungi (chitin) have different compositions.
Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotic cytoplasmic ribosomes are 80S, though mitochondria/chloroplasts have 70S.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes, consistent with endosymbiotic origin.
Trap: Eukaryotes are always larger because they are more complex – Fact: Size correlates with compartmentalization, but some prokaryotes (e.g., Thiomargarita namibiensis) can be larger than typical eukaryotic cells (up to 750 ?m).
Trap: All eukaryotic cells have a cell wall – Fact: Animal cells lack cell walls; only plants, fungi, and some protists have them.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nucleoid region
D) Phospholipid bilayer
Answer: C
Explanation: The nucleoid is a distinct, non-membrane-bound region in prokaryotes where DNA is located.
Why the top distractor is wrong: While 70S ribosomes (B) are common in prokaryotes, they are also present in mitochondria and chloroplasts of eukaryotes, so not exclusive.

Question: Which structure is present in plant cells but absent in animal cells?
A) Mitochondria
B) Golgi apparatus
C) Central vacuole
D) Plasma membrane
Answer: C
Explanation: The large central vacuole is a defining feature of mature plant cells, involved in storage and turgor.
Why the top distractor is wrong: Mitochondria (A) are present in both plant and animal cells for ATP production.

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
C) Containing 80S ribosomes
D) Being surrounded by the ER
Answer: A
Explanation: The double membrane and presence of 70S ribosomes and circular DNA support mitochondrial origin from engulfed prokaryotes.
Why the top distractor is wrong: 80S ribosomes (C) are found in the eukaryotic cytoplasm; mitochondria have 70S, not 80S.

Question: A cell is observed to lack a nucleus and has 70S ribosomes. It also contains peptidoglycan in its cell wall. This cell is most likely:
A) A plant cell
B) A fungal cell
C) A bacterium
D) An animal cell
Answer: C
Explanation: Peptidoglycan, 70S ribosomes, and no nucleus are diagnostic of bacteria.
Why the top distractor is wrong: Plant cells (A) have cellulose walls, 80S ribosomes, and a nucleus.

Question: Which organelle is responsible for modifying and packaging proteins into vesicles for secretion?
A) Nucleolus
B) Rough ER
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus receives proteins from the ER, modifies them (e.g., glycosylation), and sorts them for delivery.
Why the top distractor is wrong: The rough ER (B) synthesizes proteins but does not package them for secretion.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All eukaryotic ribosomes are 80S, including those in mitochondria
B) Cytoplasmic ribosomes are 80S, but mitochondrial ribosomes are 70S
C) Ribosomes are only found attached to the rough ER
D) Ribosomes are enclosed within a membrane
Answer: B
Explanation: Mitochondrial ribosomes are 70S, resembling prokaryotic ribosomes, while cytoplasmic ribosomes are 80S.
Why the top distractor is wrong: Mitochondrial ribosomes are 70S, not 80S (A), indicating their evolutionary origin.

Question: Which cell type lacks mitochondria?
A) Yeast cells
B) Leaf mesophyll cells
C) Mature human red blood cells
D) Amoeba
Answer: C
Explanation: Mature human red blood cells lack mitochondria and generate ATP via glycolysis.
Why the top distractor is wrong: Yeast cells (A) are fungi and have mitochondria for aerobic respiration.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• DNA in prokaryotes: nucleoid; eukaryotes: nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal cells = no cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nuclear envelope has nuclear pores for transport of RNA and proteins.
• Nucleolus makes rRNA and assembles ribosomal subunits.
• Rough ER: ribosome-studded, synthesizes proteins.
• Smooth ER: synthesizes lipids, detoxifies drugs, stores Ca²?.
• Golgi apparatus: modifies, sorts, packages proteins into vesicles.
• Mitochondria produce ATP via aerobic respiration; site = cristae and matrix.
• Chloroplasts perform photosynthesis; contain thylakoids and stroma.
• Lysosomes contain hydrolytic enzymes; found in animal cells.
• Peroxisomes break down fatty acids and detoxify H?O?.
• Mycoplasma – bacteria without a cell wall.
• Mature red blood cells lack a nucleus and mitochondria.
• Flagella: prokaryotes = flagellin, rotating; eukaryotes = microtubules (9+2), undulating.
• Cytoskeleton (actin, microtubules) present in eukaryotes, not in most prokaryotes.
• Plasmids: small circular DNA in prokaryotes, used in gene transfer.
• Mitochondria and chloroplasts have double membranes – supports endosymbiotic theory.
• Plant cells have large central vacuole; animal cells have small or multiple vacuoles.
• Verify from standard textbook: size of Thiomargarita namibiensis (giant bacterium, up to 750 ?m).