STEM Readiness: Physics Readiness - Energy/Work: Conservation of Energy - Including Friction, Pendulums, Roller Coasters

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.
• Prokaryotic ribosomes 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—evidence for endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan; archaea lack peptidoglycan and have different cell wall chemistry.
• Plant cell walls are made of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• Mycoplasma, a bacterium, lacks a cell wall—making it resistant to antibiotics like penicillin that target peptidoglycan.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane; eukaryotes also have internal membrane systems (e.g., ER, Golgi).
• Eukaryotic organelles include the nucleus, endoplasmic reticulum (ER), Golgi apparatus, mitochondria, lysosomes, and (in plants) chloroplasts; prokaryotes lack these.
• The nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport; prokaryotes lack this structure.
• Chromosomes in prokaryotes are typically a single, circular DNA molecule; eukaryotes have multiple linear chromosomes.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; they can carry antibiotic resistance genes.
• Eukaryotic cells undergo mitosis for nuclear division; prokaryotes divide by binary fission.
• Cytoskeleton is present in both: prokaryotes have homologs of actin and tubulin (e.g., MreB, FtsZ); eukaryotes have microtubules, microfilaments, and intermediate filaments.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and bend.
• Endosymbiotic theory proposes mitochondria evolved from aerobic bacteria and chloroplasts from cyanobacteria engulfed by ancestral eukaryotic cells.
• Evidence for endosymbiosis: mitochondria and chloroplasts have own circular DNA, 70S ribosomes, double membranes, and reproduce independently via binary fission.
• Red blood cells in mammals lack a nucleus and most organelles, maximizing space for hemoglobin; they are an exception among eukaryotic cells.
• Plant cells have chloroplasts, large central vacuoles, and plasmodesmata; animal cells do not.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; they are rare or absent in plant cells (vacuoles perform similar functions).
• Peroxisomes are present in both plant and animal eukaryotic cells and are involved in fatty acid breakdown and detoxification of hydrogen peroxide.
• Gram-positive bacteria have thick peptidoglycan and no outer membrane; Gram-negative bacteria have thin peptidoglycan and an outer lipid membrane.
• Archaea lack peptidoglycan, have unique membrane lipids (ether linkages), and often live in extreme environments.
• Nuclear DNA in eukaryotes is associated with histone proteins to form chromatin; prokaryotic DNA is not packaged with histones (though some archaea have histone-like proteins).

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and recognizing exceptions common in introductory biology exams.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plant cell walls are made of cellulose, fungal walls of chitin, and archaeal walls lack peptidoglycan.
Trap: Ribosome size is the same across all cells – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes in the cytoplasm, but 70S in mitochondria and chloroplasts.
Trap: The nucleus is the only organelle that contains DNA in eukaryotes – Fact: Mitochondria and chloroplasts also contain their own DNA.
Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes (e.g., photosynthetic bacteria) have internal membrane systems like thylakoids, but lack membrane-bound organelles.
Trap: Eukaryotic cells are always larger and more complex than prokaryotic cells – Fact: While generally true, some giant bacteria (e.g., Thiomargarita namibiensis, up to 750 ?m) exceed typical eukaryotic cell size.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nucleoid region
D) Phospholipid bilayer
Answer: C
Explanation: The nucleoid is a membrane-free region in prokaryotes where DNA is located; eukaryotes have a membrane-bound nucleus.
Why the top distractor is wrong: 70S ribosomes (B) are also found in mitochondria and chloroplasts of eukaryotes, so not exclusive to prokaryotes.

Question: Which structure is present in plant cells but absent in animal cells?
A) Mitochondria
B) Golgi apparatus
C) Central vacuole
D) Plasma membrane
Answer: C
Explanation: The large central vacuole is a defining feature of plant cells, involved in storage and turgor pressure.
Why the top distractor is wrong: Mitochondria (A) are present in both plant and animal cells.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Mitochondria have a single membrane
B) Mitochondria contain 80S ribosomes
C) Mitochondria have circular DNA and 70S ribosomes
D) Mitochondria are smaller than bacteria
Answer: C
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to bacteria, supporting their origin from free-living prokaryotes.
Why the top distractor is wrong: Mitochondria have a double membrane (not single), and their ribosomes are 70S, not 80S.

Question: A bacterial cell is placed in a hypotonic solution. Which component prevents lysis due to osmotic pressure?
A) Capsule
B) Plasma membrane
C) Cell wall
D) Nucleoid
Answer: C
Explanation: The rigid peptidoglycan cell wall in bacteria resists osmotic pressure and prevents bursting in hypotonic environments.
Why the top distractor is wrong: The plasma membrane (B) regulates transport but does not provide structural resistance to lysis.

Question: Which of the following organisms lacks a nucleus and has 70S ribosomes?
A) Saccharomyces cerevisiae (yeast)
B) Homo sapiens (human)
C) Escherichia coli
D) Arabidopsis thaliana (plant)
Answer: C
Explanation: E. coli is a prokaryote with no nucleus and 70S ribosomes.
Why the top distractor is wrong: Yeast (A) is a eukaryote and has a nucleus and 80S ribosomes.

Question: Which of the following is true about the nuclear envelope?
A) It is a single phospholipid bilayer
B) It is continuous with the smooth ER
C) It contains pores that allow RNA export
D) It is found in both prokaryotes and eukaryotes
Answer: C
Explanation: Nuclear pores in the double-membrane nuclear envelope allow transport of mRNA and proteins.
Why the top distractor is wrong: The nuclear envelope is continuous with the rough ER, not the smooth ER (B), and is absent in prokaryotes (D).

Question: Which cell type lacks mitochondria but still performs aerobic respiration?
A) Mature human red blood cell
B) Paramecium
C) Nitrosomonas (a bacterium)
D) Liver cell
Answer: C
Explanation: Prokaryotes like Nitrosomonas perform aerobic respiration using plasma membrane enzymes, not mitochondria.
Why the top distractor is wrong: Mature human red blood cells (A) lack mitochondria and rely on anaerobic glycolysis.

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 = no cell wall.
• Mycoplasma has no cell wall – smallest known bacterium.
• Red blood cells lack a nucleus and mitochondria.
• Both domains have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, lysosomes); prokaryotes do not.
• Prokaryotes divide by binary fission; eukaryotes by mitosis/meiosis.
• Prokaryotic DNA is usually single, circular chromosome; eukaryotes have multiple linear chromosomes.
• Plasmids are small, circular DNA molecules common in bacteria.
• Flagellum in bacteria = flagellin, rotates; in eukaryotes = microtubules (9+2), bends.
• Cytoskeleton: eukaryotes have actin, tubulin; prokaryotes have MreB, FtsZ homologs.
• Gram-positive: thick peptidoglycan, no outer membrane; Gram-negative: thin peptidoglycan, outer membrane present.
• Archaea have ether linkages in membrane lipids; bacteria and eukaryotes have ester linkages.
• Endosymbiotic theory: mitochondria from aerobic bacteria, chloroplasts from cyanobacteria.
• Lysosomes are common in animal cells, rare in plant cells (vacuoles function similarly).
• Peroxisomes break down fatty acids and detoxify H?O? in both plant and animal cells.
• Nuclear DNA in eukaryotes is wrapped around histones; prokaryotic DNA is not.
• Chloroplasts are present only in plant and algal cells.
• Some prokaryotes (e.g., cyanobacteria) have thylakoid membranes for photosynthesis.
• Verify from standard textbook: exact size of Thiomargarita namibiensis (giant bacterium) exceeds many eukaryotic cells.