STEM Readiness: Chemistry Readiness - Chemical Bonding: Intermolecular Forces - H-bonding, Dipole-Dipole, LDF Ranking, Boiling Points

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 lack a membrane-bound nucleus; DNA resides in the nucleoid region (e.g., Escherichia coli).
• Eukaryotes have a true nucleus enclosed by a double membrane (nuclear envelope), as seen in human liver cells.
• Prokaryotic ribosomes are 70S (30S + 50S subunits); eukaryotic cytoplasmic ribosomes are 80S (40S + 60S).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Prokaryotes include bacteria (e.g., Streptococcus) and archaea (e.g., Methanogens); both lack organelles.
• Eukaryotes include animals (no cell wall), plants (cellulose wall), fungi (chitin wall), and protists.
• Peptidoglycan is present in bacterial cell walls (e.g., Bacillus subtilis); absent in archaea and eukaryotes.
• Archaea lack peptidoglycan but have pseudopeptidoglycan or other polymers in their cell walls.
• Plant cell walls are composed of cellulose; fungal cell walls are made of chitin.
• Animal cells lack a cell wall entirely; structural support comes from extracellular matrix.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membrane systems (e.g., endoplasmic reticulum, Golgi apparatus); prokaryotes do not.
• Nuclear pores regulate transport between nucleus and cytoplasm in eukaryotes; nucleoid has no such barrier in prokaryotes.
• Mitochondria are present in nearly all eukaryotic cells (e.g., neurons); absent in mature human red blood cells.
• Chloroplasts are found in plant and algal cells (e.g., Chlamydomonas); absent in animal and fungal cells.
• Endoplasmic reticulum (ER) is a eukaryotic organelle: rough ER has ribosomes; smooth ER synthesizes lipids.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery; absent in prokaryotes.
• Lysosomes (membrane-bound, contain hydrolytic enzymes) are present in animal cells; rare in plant cells (vacuole functions similarly).
• Mycoplasma species are bacteria that lack a cell wall; resistant to penicillin.
• Red blood cells in mammals lack a nucleus and organelles, maximizing hemoglobin capacity.
• Plasmids are small, circular DNA molecules found in prokaryotes (e.g., F plasmid in E. coli); rare in eukaryotes.
• Cytoskeleton (microtubules, microfilaments) is complex in eukaryotes (e.g., mitotic spindle); prokaryotes have simpler analogs (e.g., FtsZ).
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes use mitosis.
• Endosymbiotic theory is supported by: mitochondria/chloroplasts having own circular DNA, 70S ribosomes, and double membranes.

Difficulty Level

Intermediate – routinely tested in first-year biology with emphasis on structural and functional comparisons.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plant (cellulose) and fungal (chitin) walls differ chemically.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based cytoskeletal elements.
Trap: The nucleus is the only site of DNA in eukaryotes – Fact: Mitochondria and chloroplasts contain their own DNA.
Trap: Archaea are prokaryotes and therefore identical to bacteria – Fact: Archaea lack peptidoglycan, have unique membrane lipids, and differ genetically.

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.

Question: A cell is observed to have a nucleus, mitochondria, and a cell wall made of cellulose. This cell is most likely from a:
A) Fungus
B) Bacterium
C) Plant
D) Animal
Answer: C
Explanation: Cellulose cell walls and the presence of mitochondria and nucleus are characteristic of plant cells.
Why the top distractor is wrong: Fungi have chitin in their cell walls, not cellulose.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Nuclear envelope
B) 80S ribosomes
C) Phospholipid bilayer membrane
D) Golgi apparatus
Answer: C
Explanation: Both cell types have a plasma membrane composed of a phospholipid bilayer.
Why the top distractor is wrong: 80S ribosomes are only in eukaryotic cytoplasm; prokaryotes have 70S.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Eukaryotic cells are larger than prokaryotic cells
B) Mitochondria have 70S ribosomes and circular DNA
C) Prokaryotes reproduce by binary fission
D) Eukaryotes have linear chromosomes
Answer: B
Explanation: Mitochondria resemble bacteria in ribosome type and genome structure, supporting bacterial origin.
Why the top distractor is wrong: Cell size difference does not explain organelle origin.

Question: Which organism lacks a cell wall and a nucleus?
A) Saccharomyces cerevisiae
B) Mycoplasma pneumoniae
C) Homo sapiens red blood cell
D) Escherichia coli
Answer: C
Explanation: Mature mammalian red blood cells lack both a nucleus and a cell wall.
Why the top distractor is wrong: Mycoplasma lacks a cell wall but is a prokaryote with a nucleoid (has DNA).

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: Nuclei are exclusive to eukaryotes.

Question: Which of the following organelles is responsible for protein modification and sorting in eukaryotic cells?
A) Rough endoplasmic reticulum
B) Smooth endoplasmic reticulum
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies, sorts, and packages proteins from the ER.
Why the top distractor is wrong: Rough ER synthesizes proteins but does not modify or sort them for secretion.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have nucleus with nuclear envelope.
• 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 = no cell wall.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nuclear pores allow RNA/protein transport in eukaryotes; no equivalent in prokaryotes.
• Mycoplasma – bacteria without cell walls.
• Mature human red blood cells lack a nucleus and mitochondria.
• Plasmids are small, circular DNA in bacteria (e.g., R plasmid).
• Binary fission = prokaryotic division; mitosis = eukaryotic.
• Cytoskeleton in eukaryotes includes actin, tubulin; prokaryotes have FtsZ, MreB analogs.
• Lysosomes are membrane-bound, enzyme-filled; mainly in animal cells.
• Chloroplasts found in plants and algae; perform photosynthesis.
• Smooth ER synthesizes lipids and detoxifies; rough ER synthesizes proteins.
• Golgi apparatus: cis face receives, trans face ships vesicles.
• Archaea are prokaryotic in structure but differ biochemically from bacteria.
• Endosymbiotic theory: mitochondria from aerobic bacteria, chloroplasts from cyanobacteria.
• Mitochondria have double membranes – outer from host, inner from symbiont.
• No ribosomes are 90S or 100S – only 70S (prokaryotes, organelles) and 80S (eukaryotic cytoplasm).
• Nuclear envelope is continuous with the rough ER.
• Verify from standard textbook: exact size ranges may vary slightly by source.
• Not all eukaryotes have cell walls – animals and many protists lack them.