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STEM Readiness: Chemistry Readiness - Thermochemistry: Bond Enthalpies - Using Average, Bond Energies, to Estimate, ?H

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 endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); eukaryotes have 80S ribosomes (60S + 40S) in the cytoplasm.
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan; archaea lack peptidoglycan but may have pseudopeptidoglycan.
• 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 penicillin.
• Human red blood cells lack a nucleus and most organelles, maximizing space for hemoglobin.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have extensive internal membranes forming organelles; prokaryotes do not.
• Circular DNA is found in prokaryotic nucleoids, mitochondria, and chloroplasts; eukaryotic nuclear DNA is linear.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Binary fission is the method of reproduction in prokaryotes; eukaryotes divide by mitosis and meiosis.
• Peptidoglycan is a polymer of N-acetylglucosamine and N-acetylmuramic acid cross-linked by peptides.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin layers and an outer membrane.
• Endosymbiotic theory proposes mitochondria and chloroplasts originated from engulfed prokaryotes.
• Evidence for endosymbiosis includes double membranes, 70S ribosomes, and independent replication of mitochondria and chloroplasts.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; absent in most plant cells.
• Chloroplasts are found in plant cells and some protists; perform photosynthesis using chlorophyll.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores regulating transport.
• Nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome assembly.
• Cytoskeleton elements (microtubules, microfilaments) are present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and undulate.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and organelle roles.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose) and fungi (chitin) have different wall compositions.

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

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 nucleoids, ribosomes, and sometimes inclusions or protein scaffolds, but no membrane-bound organelles.

Practice MCQs (5–7 questions)

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

Question: A cell is observed to have a nucleus, chloroplasts, and a cell wall made of cellulose. What type of organism is this most likely?
A) Fungus
B) Bacterium
C) Animal
D) Plant
Answer: D
Explanation: Plants have nuclei, chloroplasts, and cellulose cell walls.
Why the top distractor is wrong: Fungi have nuclei and cell walls, but lack chloroplasts and use chitin, not cellulose.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Nucleoid
B) 80S ribosomes
C) Phospholipid bilayer
D) Golgi apparatus
Answer: C
Explanation: Both cell types have a plasma membrane made of phospholipid bilayer.
Why the top distractor is wrong: 80S ribosomes are only in eukaryotes; 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 divide independently
C) Prokaryotes lack a nucleus
D) Lysosomes contain digestive enzymes
Answer: B
Explanation: Mitochondria resemble bacteria in ribosome type and replication method.
Why the top distractor is wrong: Linear DNA is a eukaryotic nuclear feature but not evidence for 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 without 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) Within the nucleus
B) In mitochondria
C) In the nucleoid region
D) Bound to the Golgi apparatus
Answer: C
Explanation: Prokaryotes lack a nucleus; DNA is concentrated in the nucleoid.
Why the top distractor is wrong: Prokaryotes do not have a nucleus.

Question: Which of the following is true about ribosomes in chloroplasts?
A) They are 80S and identical to cytoplasmic ribosomes
B) They are 70S and similar to bacterial ribosomes
C) They are synthesized in the Golgi apparatus
D) They are absent in plant cells
Answer: B
Explanation: Chloroplast ribosomes are 70S, supporting their prokaryotic origin.
Why the top distractor is wrong: 80S ribosomes are in the eukaryotic cytoplasm, not in chloroplasts.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: nucleoid; eukaryotes: nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal cells = no cell wall.
• Mycoplasma lacks a cell wall – naturally resistant to penicillin.
• Human red blood cells lack a nucleus and mitochondria.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, etc.); prokaryotes do not.
• Prokaryotes reproduce by binary fission; eukaryotes by mitosis/meiosis.
• Plasmids are common in prokaryotes, rare in eukaryotes.
• Nuclear pores regulate mRNA and protein transport in eukaryotes.
• Nucleolus = site of rRNA synthesis and ribosome subunit assembly.
• Flagella: prokaryotes = flagellin, rotating; eukaryotes = microtubules (9+2), bending.
• Lysosomes are present in animal cells, not typically in plant cells.
• Chloroplasts perform photosynthesis and are found in plants and some protists.
• Mitochondria generate ATP via cellular respiration in eukaryotes.
• Cytoskeleton includes actin filaments, microtubules, intermediate filaments in eukaryotes.
• FtsZ protein in prokaryotes is homologous to eukaryotic tubulin.
• Gram-positive bacteria: thick peptidoglycan; Gram-negative: thin peptidoglycan + outer membrane.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria, chloroplasts from cyanobacteria.
• Mitochondria and chloroplasts have double membranes – outer from host, inner from original prokaryote.
• Organelles unique to eukaryotes: nucleus, ER, Golgi, lysosomes, mitochondria, chloroplasts.
• Verify from standard textbook: exact size ranges and ribosome subunit compositions.