STEM Readiness: Physics Readiness - : Forces Newton’s, Laws Newton’s, Three Laws, Inertia, Fma Action-Reaction, Pairs

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.
• Prokaryotic DNA is located in the nucleoid, a region without a membrane; 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 in eukaryotes contain 70S ribosomes, similar to prokaryotes.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan and have different wall chemistry.
• Plant cell walls are made of cellulose; fungal cell walls contain 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 lysosomes; prokaryotes do not.
• Mitochondria are present in nearly all eukaryotic cells and are the site of aerobic cellular respiration; absent in prokaryotes.
• Chloroplasts are found in plant cells and some protists (e.g., Chlamydomonas), and perform photosynthesis; absent in prokaryotes (except in cyanobacteria, which have thylakoid membranes but no chloroplasts).
• Cyanobacteria are prokaryotes that perform oxygenic photosynthesis using thylakoid membranes, not chloroplasts.
• Eukaryotic nuclei contain linear chromosomes associated with histone proteins; prokaryotes have a single circular chromosome in the nucleoid.
• Plasmids are small, circular DNA molecules found in many prokaryotes and occasionally in yeast (a eukaryote).
• Nuclear envelope in eukaryotes consists of two lipid bilayers and contains nuclear pores for regulated transport.
• Endosymbiotic theory proposes that mitochondria and chloroplasts originated from engulfed prokaryotes; supported by their own DNA, 70S ribosomes, and double membranes.
• Mycoplasma species are bacteria that lack a cell wall; they are the smallest known cells capable of independent growth (~0.2 ?m).
• Human red blood cells (erythrocytes) lack a nucleus and most organelles in maturity; this is an exception among animal cells.
• Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis and cytokinesis.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and bend.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; absent in most plant cells (vacuoles perform similar function).
• Large central vacuoles are characteristic of mature plant cells and help maintain turgor pressure; not found in prokaryotes.
• Glycocalyx (capsule or slime layer) is present in many prokaryotes for protection and adhesion; eukaryotic cells may have a glycocalyx on epithelial cells but structurally different.
• Cytoskeleton is present in both domains: prokaryotes have FtsZ, MreB, CreS; eukaryotes have microtubules, actin filaments, and intermediate filaments.
• Archaea lack peptidoglycan, have ether-linked lipids in membranes, and their transcription and translation machinery resembles eukaryotes more than bacteria.

Difficulty Level

Intermediate – foundational for cell biology but requires precise differentiation of structures, components, and exceptions across domains.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacterial cell walls contain peptidoglycan; plant walls have cellulose, fungal walls have chitin, and archaeal walls lack peptidoglycan.

Trap: Mitochondria and chloroplasts are proof that eukaryotes evolved from prokaryotes – Fact: Endosymbiotic theory states these organelles originated from free-living prokaryotes engulfed by ancestral eukaryotes, not that eukaryotes evolved directly from them.

Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes (e.g., cyanobacteria, purple bacteria) have internal membrane systems like thylakoids or chromatophores, though not as complex as eukaryotic organelles.

Trap: Ribosome size correlates with cell complexity – Fact: Ribosome size is measured in Svedberg units (S), which reflect sedimentation rate, not physical size or complexity; mitochondrial 70S ribosomes are smaller than cytoplasmic 80S despite being in more complex cells.

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 prokaryotes and eukaryotes have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: D (mitochondria) are present only in eukaryotes and absent in prokaryotes.

Question: A cell is observed to contain a nucleoid, 70S ribosomes, and a cell wall containing peptidoglycan. Which of the following is the most likely identity of the cell?
A) Fungal cell
B) Plant cell
C) Animal cell
D) Bacterial cell
Answer: D
Explanation: The presence of a nucleoid, 70S ribosomes, and peptidoglycan indicates a bacterial cell.
Why the top distractor is wrong: A (fungal cells) are eukaryotes with a nucleus, 80S ribosomes, and chitin in their cell walls.

Question: Which structure is found in eukaryotic cells but NOT in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Golgi apparatus
D) DNA
Answer: C
Explanation: The Golgi apparatus is a membrane-bound organelle unique to eukaryotes.
Why the top distractor is wrong: A (plasma membrane) is present in both prokaryotes and eukaryotes.

Question: Mitochondria are thought to have originated from endosymbiotic bacteria. Which of the following supports this theory?
A) Mitochondria use 80S ribosomes for protein synthesis
B) Mitochondrial DNA is linear and histone-bound
C) Mitochondria have a single membrane
D) Mitochondria have circular DNA and 70S ribosomes
Answer: D
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to prokaryotes, supporting endosymbiotic origin.
Why the top distractor is wrong: A is incorrect because mitochondria use 70S ribosomes, not 80S.

Question: Which of the following cells lacks a nucleus at maturity?
A) Escherichia coli
B) Saccharomyces cerevisiae (yeast)
C) Human hepatocyte
D) Human erythrocyte
Answer: D
Explanation: Mature human red blood cells (erythrocytes) lack a nucleus to maximize hemoglobin space.
Why the top distractor is wrong: A (E. coli) is a prokaryote and never has a nucleus, but the question refers to cells that lose the nucleus during maturation.

Question: Which of the following organisms has a cell wall made of chitin?
A) Pine tree
B) Streptococcus pneumoniae
C) Agaricus bisporus (mushroom)
D) Homo sapiens
Answer: C
Explanation: Fungi, including mushrooms, have cell walls composed of chitin.
Why the top distractor is wrong: A (pine tree) is a plant and has cellulose in its cell wall, not chitin.

Question: What is the primary structural component of the plant cell wall?
A) Peptidoglycan
B) Chitin
C) Cellulose
D) Collagen
Answer: C
Explanation: The plant cell wall is primarily composed of cellulose, a polysaccharide.
Why the top distractor is wrong: A (peptidoglycan) is found in bacterial cell walls, not plants.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes: nucleoid; eukaryotes: membrane-bound nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• Bacteria: peptidoglycan cell wall; Archaea: no peptidoglycan.
• Plants: cellulose cell wall; fungi: chitin; animals: no cell wall.
• Both domains have phospholipid bilayer membranes.
• Eukaryotes have internal membranes (ER, Golgi, lysosomes); prokaryotes generally do not.
• Mitochondria perform aerobic respiration in eukaryotes; absent in prokaryotes.
• Chloroplasts in plants and algae; cyanobacteria have thylakoids but no chloroplasts.
• Cyanobacteria are photosynthetic prokaryotes with thylakoid membranes.
• Eukaryotic chromosomes are linear and histone-associated; prokaryotes have circular chromosome.
• Plasmids occur in bacteria and some yeasts.
• Nuclear envelope has two membranes and nuclear pores.
• Endosymbiotic theory: mitochondria and chloroplasts derived from prokaryotes.
• Evidence for endosymbiosis: double membrane, 70S ribosomes, circular DNA, binary fission.
• Mycoplasma: smallest known cells, lack cell wall.
• Human red blood cells lack nucleus and organelles at maturity.
• Prokaryotes divide by binary fission; eukaryotes by mitosis.
• Prokaryotic flagella: flagellin, rotate; eukaryotic: microtubules (9+2), bend.
• Lysosomes: in animal cells, contain hydrolytic enzymes.
• Plant vacuoles: large central vacuole maintains turgor.
• Glycocalyx in prokaryotes (protection); eukaryotes have glycoprotein layer but different.
• Cytoskeleton: prokaryotes have FtsZ (tubulin analog), MreB (actin analog).
• Archaea: ether-linked lipids, no peptidoglycan, transcription machinery resembles eukaryotes.