STEM Readiness: Chemistry Readiness - Chemical Bonding: Polarity - Bond Polarity vs. Molecular Polarity, Dipole Moments

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.
• Prokaryotic ribosomes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, similar to prokaryotes, supporting endosymbiotic theory.
• Most prokaryotes have a cell wall made of peptidoglycan; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cells have a cell wall composed of cellulose; fungal cells have chitin; animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membrane systems including the endoplasmic reticulum (ER), Golgi apparatus, and nuclear envelope; prokaryotes do not.
• The nucleus in eukaryotes contains chromatin (DNA + histones) and a nucleolus where ribosomal RNA is synthesized.
• Nuclear pores regulate transport between the nucleus and cytoplasm in eukaryotes; prokaryotes lack such structures.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Eukaryotic mitochondria generate ATP via aerobic respiration and contain their own circular DNA.
• Chloroplasts are found in plant and algal cells, perform photosynthesis, and contain thylakoids and chlorophyll.
• Lysosomes in animal cells contain hydrolytic enzymes for digestion; absent in most plant cells.
• Centrioles are present in animal cells and assist in spindle formation during cell division; absent in most plant cells.
• Mycoplasma species are bacteria without a cell wall, making them resistant to antibiotics like penicillin.
• Mature mammalian red blood cells lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, 70S ribosomes, and independent replication.
• Archaea are prokaryotes but share some molecular features with eukaryotes, such as RNA polymerase and histone-like proteins.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Cilia are short, numerous projections in some eukaryotic cells (e.g., trachea) with 9+2 microtubule structure; prokaryotes lack cilia.
• Peroxisomes in eukaryotes break down fatty acids and detoxify alcohol; absent in prokaryotes.

Difficulty Level

Intermediate – requires precise differentiation of structural and molecular features across domains and kingdoms.

Common Traps (3–5 factual traps)

Trap: All cells with a nucleus are eukaryotic – Fact: Mature red blood cells in mammals lack a nucleus but are derived from eukaryotic cells and contain organelles during development.
Trap: Ribosome size correlates with cell complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells, due to prokaryotic ancestry.
Trap: Cell walls are present in all non-animal cells – Fact: Mycoplasma, a bacterium, lacks a cell wall despite being prokaryotic.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have cytoskeletal elements (e.g., FtsZ) and some have protein-bound compartments (e.g., carboxysomes).
Trap: Archaea are bacteria – Fact: Archaea are prokaryotes but form a separate domain with distinct membrane lipids and genetic machinery.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature shared by both prokaryotic and eukaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes
C) Phospholipid bilayer plasma membrane
D) Mitochondria
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D (mitochondria) are only found in eukaryotes.

Question: A cell is observed to have a nucleus, mitochondria, and a cell wall made of cellulose. From which organism is this cell most likely derived?
A) Fungus
B) Bacterium
C) Animal
D) Plant
Answer: D
Explanation: Plants have cellulose cell walls and contain mitochondria and a nucleus.
Why the top distractor is wrong: A (fungus) has a chitin cell wall, not cellulose.

Question: Which structure is evidence supporting the endosymbiotic theory?
A) Nuclear envelope
B) 70S ribosomes in mitochondria
C) 80S ribosomes in cytoplasm
D) Peptidoglycan in bacterial walls
Answer: B
Explanation: Mitochondria having 70S ribosomes like prokaryotes supports their origin from free-living bacteria.
Why the top distractor is wrong: A (nuclear envelope) evolved via infolding of the plasma membrane, not endosymbiosis.

Question: Which of the following cells lacks a nucleus at maturity?
A) Yeast cell
B) Escherichia coli
C) Human red blood cell
D) Onion root cell
Answer: C
Explanation: Mature mammalian red blood cells expel their nucleus to carry more hemoglobin.
Why the top distractor is wrong: B (E. coli) is prokaryotic and never has a nucleus, but the question refers to loss during maturation.

Question: What is the primary component of the cell wall in fungi?
A) Peptidoglycan
B) Cellulose
C) Chitin
D) Lipopolysaccharide
Answer: C
Explanation: Fungal cell walls are composed primarily of chitin, a nitrogen-containing polysaccharide.
Why the top distractor is wrong: A (peptidoglycan) is found in bacterial cell walls, not fungi.

Question: Which of the following is found in eukaryotic cells but not in prokaryotic cells?
A) DNA
B) Ribosomes
C) Golgi apparatus
D) Plasma membrane
Answer: C
Explanation: The Golgi apparatus is a membrane-bound organelle unique to eukaryotes.
Why the top distractor is wrong: A (DNA) is present in both, though organized differently.

Question: Which organism has 70S ribosomes and lacks a nucleus?
A) Homo sapiens
B) Saccharomyces cerevisiae
C) Cyanobacteria
D) Arabidopsis thaliana
Answer: C
Explanation: Cyanobacteria are prokaryotes with 70S ribosomes and no nucleus.
Why the top distractor is wrong: B (S. cerevisiae) is a yeast (eukaryote) with 80S cytoplasmic ribosomes.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have no nucleus; DNA in nucleoid.
• Eukaryotes have membrane-bound nucleus with nuclear pores.
• 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 but are still prokaryotes.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Both cell types have phospholipid bilayer membranes.
• Eukaryotes have internal membranes (ER, Golgi, lysosomes); prokaryotes do not.
• Mature human red blood cells lack a nucleus and mitochondria.
• Mycoplasma is a bacterium without a cell wall.
• Flagellin = prokaryotic flagella protein; tubulin = eukaryotic flagella (9+2).
• Plasmids are common in prokaryotes, rare in eukaryotes.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Lysosomes contain acid hydrolases; found in animal cells, not plants.
• Peroxisomes break down fatty acids and detoxify H?O?.
• Centrioles present in animal cells, absent in most plants.
• Endosymbiotic theory: mitochondria evolved from aerobic bacteria, chloroplasts from cyanobacteria.
• Double membrane of mitochondria supports endosymbiosis: outer from host, inner from symbiont.
• Archaea have histone-like proteins and complex RNA polymerase like eukaryotes.
• Cytoskeleton elements (e.g., FtsZ, MreB) exist in prokaryotes.
• 70S ribosomes in mitochondria = key evidence for endosymbiosis.
• Nuclear envelope is continuous with the rough ER.
• Smooth ER synthesizes lipids; rough ER has ribosomes for protein synthesis.