STEM Readiness: Chemistry Readiness - Chemical Bonding: VSEPR Theory - Electron Geometry vs. Molecular Geometry, Bond Angles

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, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes.
• 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.
• Mycoplasma species are bacteria that lack a cell wall—an exception among prokaryotes.
• 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, which prokaryotes lack.
• The nucleus in eukaryotes contains chromatin (DNA + histones) and a nucleolus where ribosomal RNA is synthesized.
• Prokaryotes have circular DNA as their main chromosome; eukaryotes have linear chromosomes.
• Plasmids are small, circular DNA molecules found in many prokaryotes and occasionally in yeast (a eukaryote).
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide by mitosis and meiosis.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and bend.
• Cilia are present in some eukaryotes (e.g., Paramecium, human trachea); prokaryotes lack cilia.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own DNA, 70S ribosomes, and double membranes.
• Red blood cells in mammals lack a nucleus and organelles—allowing more space for hemoglobin.
• Plant cells have chloroplasts, large central vacuoles, and plasmodesmata; animal cells do not.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; plant vacuoles perform similar functions.
• Peroxisomes in eukaryotes break down fatty acids and detoxify alcohol; absent in prokaryotes.
• Archaea resemble prokaryotes in size and structure but have eukaryote-like transcription and translation machinery.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin peptidoglycan and an outer membrane.
• Nuclear pores regulate transport between nucleus and cytoplasm in eukaryotes; prokaryotes lack such structures.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and recognizing 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 (pseudopeptidoglycan or other) do not.
Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes and eukaryotes have different ribosome sizes (70S vs 80S), with exceptions in mitochondria and chloroplasts.
Trap: The nucleus is the only place DNA is found in eukaryotic cells – Fact: Mitochondria and chloroplasts contain their own DNA.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based microcompartments (e.g., carboxysomes).
Trap: Eukaryotes evolved directly from archaea – Fact: Current models suggest eukaryotes share a common ancestor with archaea but evolved unique features like the nucleus and endomembrane system.

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 and chloroplasts, so it is not exclusive to prokaryotes.

Question: A cell is observed to have a nucleus, 80S ribosomes, and a cell wall made of chitin. This cell is most likely from which organism?
A) Bacterium
B) Plant
C) Fungus
D) Animal
Answer: C
Explanation: Fungi have 80S ribosomes, a nucleus, and cell walls made of chitin.
Why the top distractor is wrong: Plants have cellulose in their cell walls, not chitin.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Nucleolus
B) Golgi apparatus
C) 70S ribosome
D) Nuclear envelope
Answer: C
Explanation: 70S ribosomes are in prokaryotes and also in mitochondria and chloroplasts of eukaryotes.
Why the top distractor is wrong: The nucleolus is only present in eukaryotic nuclei.

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 using 80S ribosomes
C) Linear DNA structure
D) Connection to the endoplasmic reticulum
Answer: A
Explanation: Mitochondria have a double membrane and 70S ribosomes, supporting bacterial origin.
Why the top distractor is wrong: Mitochondria use 70S ribosomes, not 80S.

Question: Which organism lacks a cell wall entirely?
A) Mycoplasma
B) Saccharomyces cerevisiae
C) Escherichia coli
D) Arabidopsis thaliana
Answer: A
Explanation: Mycoplasma is a bacterium that naturally lacks a cell wall.
Why the top distractor is wrong: Saccharomyces cerevisiae (yeast) is a fungus with a chitin-containing cell wall.

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 membrane-bound nucleus.

Question: Which of the following is true about ribosomes in chloroplasts?
A) They are 80S and resemble eukaryotic cytoplasmic ribosomes
B) They are 70S and resemble prokaryotic ribosomes
C) They are synthesized in the Golgi apparatus
D) They are absent in plant cells
Answer: B
Explanation: Chloroplasts contain 70S ribosomes, similar to those in bacteria.
Why the top distractor is wrong: 80S ribosomes are found 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: DNA in nucleoid; eukaryotes: DNA in nucleus.
• 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 has no cell wall—only prokaryotic exception.
• Both domains have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nuclear envelope has nuclear pores for RNA/protein transport.
• Prokaryotes have circular chromosome; eukaryotes have linear chromosomes with telomeres.
• Plasmids occur in bacteria and yeast—not exclusive to prokaryotes.
• Prokaryotes reproduce by binary fission; eukaryotes by mitosis/meiosis.
• Prokaryotic flagella = flagellin, rotate; eukaryotic = microtubules (9+2), bend.
• Cilia are only in eukaryotes.
• Red blood cells lack a nucleus and mitochondria.
• Plant cells have chloroplasts, central vacuole, plasmodesmata.
• Lysosomes are in animal cells; plant vacuoles perform degradation.
• Peroxisomes break down fatty acids and detoxify; only in eukaryotes.
• Archaea lack peptidoglycan but share transcription machinery with eukaryotes.
• Gram-positive: thick peptidoglycan; Gram-negative: thin peptidoglycan + outer membrane.
• Mitochondria have double membrane, 70S ribosomes, own DNA—support endosymbiotic theory.
• Nuclear pores regulate mRNA export and protein import.
• Chromatin in nucleus = DNA + histone proteins.
• Verify from standard textbook: exact ribosomal subunit sizes (50S/30S for 70S; 60S/40S for 80S).