STEM Readiness: Chemistry Readiness - Atomic Structure: Atomic Theory - Protons, Neutrons, Electrons, Mass Number, Isotopes, Ions

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 contain 70S ribosomes, similar to prokaryotes, supporting the endosymbiotic theory.
• Cell walls are present in most prokaryotes, composed of peptidoglycan; archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cells have a cell wall made of cellulose; fungal cells have chitin; animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport.
• Eukaryotes have internal membranes forming organelles (e.g., endoplasmic reticulum, nucleus); prokaryotes do not.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Binary fission is the primary mode of reproduction in prokaryotes; eukaryotes divide by mitosis and meiosis.
• Prokaryotes lack a cytoskeleton; eukaryotes have a complex cytoskeleton (microtubules, actin filaments, intermediate filaments).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and undulate.
• Mycoplasma species are bacteria without a cell wall, an exception among prokaryotes.
• Human red blood cells lack a nucleus and mitochondria, an exception among eukaryotic cells.
• Chloroplasts are found in plant and algal cells, perform photosynthesis, and contain thylakoids and chlorophyll.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; rare in plant cells.
• The nucleus contains chromatin (DNA + histones) and a nucleolus where ribosomal RNA is synthesized.
• Nuclear pores regulate transport between the nucleus and cytoplasm; embedded in the double membrane nuclear envelope.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own circular DNA, 70S ribosomes, and double membranes.
• Eukaryotic cilia and flagella are covered by the plasma membrane; prokaryotic flagella are not.
• Gram-positive bacteria have a thick peptidoglycan layer; Gram-negative have a thin layer and outer membrane with lipopolysaccharides.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes, unlike bacteria and eukaryotes.
• Peroxisomes are single-membrane organelles in eukaryotes that break down fatty acids and detoxify hydrogen peroxide.

Difficulty Level

Intermediate – routinely tested in first-year biology with emphasis on comparative structure and functional implications.

Common Traps (3–5 factual traps)

Trap: All cells with a cell wall have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (no peptidoglycan) differ.

Trap: Ribosome size is the same across all domains – Fact: Prokaryotes and eukaryotes have different ribosome sizes (70S vs 80S), with mitochondrial/chloroplast ribosomes resembling 70S.

Trap: The nucleus is the only organelle that contains DNA in eukaryotes – Fact: Mitochondria and chloroplasts also contain DNA, inherited maternally in most cases.

Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have ribosomes, nucleoid, and sometimes inclusions or protein microcompartments, 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) Circular DNA
D) Membrane-bound nucleus
Answer: D
Explanation: Only eukaryotes have a membrane-bound nucleus enclosing their DNA.
Why the top distractor is wrong: Circular DNA is found in prokaryotes and also in mitochondria/chloroplasts, so it is not exclusive to prokaryotes.

Question: A cell is observed to have a cell wall, chloroplasts, and large central vacuoles. It is most likely:
A) Fungal cell
B) Animal cell
C) Bacterial cell
D) Plant cell
Answer: D
Explanation: Plant cells uniquely have chloroplasts, a cell wall (cellulose), and a large central vacuole.
Why the top distractor is wrong: Fungal cells have a cell wall but lack chloroplasts and large central vacuoles.

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Mitochondria
B) 80S ribosomes
C) Phospholipid bilayer membrane
D) Golgi apparatus
Answer: C
Explanation: Both cell types have a plasma membrane made 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 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 attached to histones
D) Division by mitosis
Answer: A
Explanation: Mitochondria have their own 70S ribosomes, circular DNA, and divide by binary fission—features shared with bacteria.
Why the top distractor is wrong: Mitochondria use 70S ribosomes, not 80S; 80S are cytoplasmic eukaryotic ribosomes.

Question: Which organism lacks a cell wall?
A) Escherichia coli
B) Saccharomyces cerevisiae
C) Mycoplasma pneumoniae
D) Arabidopsis thaliana
Answer: C
Explanation: Mycoplasma is a bacterium without a cell wall, making it resistant to penicillin.
Why the top distractor is wrong: E. coli has a Gram-negative cell wall with peptidoglycan.

Question: Where is ribosomal RNA synthesized in eukaryotic cells?
A) Rough endoplasmic reticulum
B) Nucleolus
C) Cytoplasm
D) Mitochondria
Answer: B
Explanation: The nucleolus within the nucleus is the site of rRNA transcription and ribosome assembly.
Why the top distractor is wrong: The rough ER is involved in protein synthesis, not rRNA production.

Question: Which of the following is true of both bacteria and archaea?
A) Presence of peptidoglycan in cell wall
B) 70S ribosomes
C) Membrane-bound nucleus
D) Ester-linked phospholipids
Answer: B
Explanation: Both bacteria and archaea have 70S ribosomes and lack a nucleus.
Why the top distractor is wrong: Archaea lack peptidoglycan and have ether-linked lipids, unlike bacteria.

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.
• Bacterial cell walls contain peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal cells = no cell wall.
• Mycoplasma is a bacterium without a cell wall.
• Human red blood cells lack a nucleus and mitochondria.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Plasmids are circular DNA molecules common in bacteria.
• Prokaryotes reproduce by binary fission; eukaryotes by mitosis/meiosis.
• Prokaryotic flagella made of flagellin; rotate.
• Eukaryotic flagella have 9+2 microtubule arrangement; covered by membrane.
• Lysosomes are primarily in animal cells; contain hydrolytic enzymes.
• Chloroplasts found in plants and algae; site of photosynthesis.
• Nuclear envelope is a double membrane with nuclear pores.
• Nucleolus synthesizes rRNA and assembles ribosomal subunits.
• Archaea have ether-linked lipids; bacteria and eukaryotes have ester-linked.
• Gram-positive bacteria: thick peptidoglycan; Gram-negative: thin peptidoglycan + outer membrane.
• Peroxisomes break down fatty acids and detoxify H?O?.
• Cytoskeleton (microtubules, actin) is present only in eukaryotes.
• Endosymbiotic theory supported by mitochondrial DNA being circular and maternally inherited.
• Verify from standard textbook: Exact lipid composition in archaeal membranes varies by species.