STEM Readiness: Physics Readiness - Electricity Foundations: DC Circuits - Ohm’s Law, Series and Parallel Resistors, Power Dissipation

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• Prokaryotic DNA is located in the nucleoid, an unenclosed region; eukaryotic DNA is enclosed within a membrane-bound nucleus.
• Prokaryotes have 70S ribosomes (30S + 50S subunits); eukaryotes have 80S ribosomes (40S + 60S subunits).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Prokaryotic cell walls typically contain peptidoglycan; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane; eukaryotes also have internal membrane systems (e.g., ER, Golgi).
• Eukaryotes possess membrane-bound organelles including the endoplasmic reticulum (ER), Golgi apparatus, mitochondria, lysosomes, and nucleus; prokaryotes lack these.
• Bacteria (e.g., Escherichia coli) are prokaryotes with peptidoglycan cell walls and circular DNA in the nucleoid.
• Archaea (e.g., Methanogens) are prokaryotes but differ in membrane lipid composition (ether linkages, branched hydrocarbons).
• Animal cells lack chloroplasts and cell walls; plant cells have both chloroplasts and a rigid cell wall.
• Fungi are eukaryotes with chitin in cell walls and absorb nutrients externally; protists are diverse eukaryotes including Amoeba and Paramecium.
• Mycoplasma species are bacteria that lack a cell wall, making them resistant to penicillin.
• Mature mammalian red blood cells lack a nucleus and most organelles, maximizing hemoglobin content.
• Plasmids are small, circular DNA molecules found in prokaryotes and some yeasts (eukaryotes), often carrying antibiotic resistance genes.
• Endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
• Evidence for endosymbiosis includes: mitochondria and chloroplasts have their own circular DNA, 70S ribosomes, and double membranes.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport; prokaryotes lack this structure.
• Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis (in somatic cells) and meiosis (in gametes).
• Cytoskeleton is present in both domains but more complex in eukaryotes; prokaryotes have homologs of actin (MreB) and tubulin (FtsZ).
• Flagella differ structurally: prokaryotic flagella are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 array) and undulate.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; absent in most plant cells (vacuoles perform similar function).
• Plant vacuoles are large, central, and maintain turgor pressure; animal cells may have small, multiple vacuoles.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery; absent in prokaryotes.
• Rough ER has ribosomes attached and synthesizes proteins; smooth ER lacks ribosomes and synthesizes lipids and detoxifies drugs.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and organelles, with attention to exceptions and evolutionary evidence.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; archaea, plants (cellulose), and fungi (chitin) have different wall compositions.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes (e.g., cyanobacteria) have thylakoid membranes for photosynthesis.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in eukaryotic cells but absent in prokaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Membrane-bound nucleus
D) Plasma membrane
Answer: C
Explanation: A membrane-bound nucleus is exclusive to eukaryotes.
Why the top distractor is wrong: The plasma membrane is present in both prokaryotes and eukaryotes.

Question: A cell is observed to have a cell wall, chloroplasts, and a large central vacuole. Which type of cell is it most likely to be?
A) Fungal cell
B) Animal cell
C) Bacterial cell
D) Plant cell
Answer: D
Explanation: Chloroplasts and a large central vacuole are characteristic of plant cells.
Why the top distractor is wrong: Fungal cells have cell walls but lack chloroplasts and large central vacuoles.

Question: Which structure is evidence supporting the endosymbiotic theory?
A) Presence of a nucleoid
B) 80S ribosomes in the cytoplasm
C) Circular DNA in mitochondria
D) Peptidoglycan in the cell wall
Answer: C
Explanation: Mitochondria have circular DNA similar to bacteria, supporting their prokaryotic origin.
Why the top distractor is wrong: The nucleoid is found in prokaryotes, not in mitochondria.

Question: Which of the following organisms lacks a cell wall?
A) Saccharomyces cerevisiae
B) Escherichia coli
C) Mycoplasma pneumoniae
D) Arabidopsis thaliana
Answer: C
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 are 70S ribosomes found in a eukaryotic cell?
A) Cytoplasm
B) Nucleus
C) Mitochondria
D) Golgi apparatus
Answer: C
Explanation: Mitochondria contain 70S ribosomes, similar to prokaryotes.
Why the top distractor is wrong: Cytoplasmic ribosomes in eukaryotes are 80S, not 70S.

Question: Which of the following is true about the nuclear envelope?
A) It is a single phospholipid bilayer
B) It is present in both prokaryotes and eukaryotes
C) It contains pores that regulate RNA export
D) It is continuous with the Golgi apparatus
Answer: C
Explanation: Nuclear pores control the movement of RNA and proteins between nucleus and cytoplasm.
Why the top distractor is wrong: The nuclear envelope is a double membrane, not single, and absent in prokaryotes.

Question: Which cellular structure is responsible for lipid synthesis and detoxification in eukaryotic cells?
A) Rough endoplasmic reticulum
B) Smooth endoplasmic reticulum
C) Lysosome
D) Nucleolus
Answer: B
Explanation: Smooth ER synthesizes lipids and detoxifies drugs and poisons.
Why the top distractor is wrong: Rough ER synthesizes proteins, not lipids.

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 double membrane.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria: peptidoglycan cell wall; Archaea: no peptidoglycan.
• Plant cell wall: cellulose; Fungal cell wall: chitin; Animal cells: no cell wall.
• Mycoplasma – only bacteria without cell wall.
• Mature red blood cells: no nucleus, no mitochondria.
• Plasma membrane: phospholipid bilayer in all cells.
• Eukaryotes have internal membranes (ER, Golgi, etc.); prokaryotes generally do not.
• Flagellin = prokaryotic flagella protein; 9+2 microtubule array = eukaryotic flagella.
• Lysosomes: hydrolytic enzymes, found in animal cells.
• Plant vacuole: turgor pressure, storage, large and central.
• Nuclear pores: regulate RNA and protein transport.
• Smooth ER: lipid synthesis, detoxification; Rough ER: protein synthesis.
• Golgi apparatus: modifies, sorts, packages proteins.
• Mitochondria: ATP production, double membrane, own DNA.
• Chloroplasts: photosynthesis, thylakoids, own DNA.
• Cytoskeleton: actin, microtubules, intermediate filaments in eukaryotes; MreB, FtsZ in prokaryotes.
• Binary fission: prokaryotic division; Mitosis: eukaryotic somatic cell division.
• Plasmids: small circular DNA, common in bacteria, sometimes in yeasts.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria, chloroplasts from cyanobacteria.
• Archaea membrane lipids: ether linkages, branched hydrocarbons.
• Nucleolus: site of ribosome assembly within nucleus.
• No lysosomes in most plant cells – vacuole performs degradation.
• Verify from standard textbook: exact lipid composition in archaeal membranes varies by species.