STEM Readiness: Physics Readiness Momentum Center of Mass Location Motion of CM for System of Particles

Must‑Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 μm in diameter; eukaryotic cells range from 10–100 μm.
• Prokaryotes include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.
• Prokaryotes lack a nucleus; their DNA resides in a region called the nucleoid.
• Eukaryotes have a membrane-bound nucleus that houses linear DNA organized into chromosomes.
• Prokaryotic ribosomes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• Mitochondria and chloroplasts contain 70S ribosomes, supporting their origin via endosymbiosis.
• Prokaryotes have no membrane-bound organelles; eukaryotes have endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria, and (in plants) chloroplasts.
• Both prokaryotes and eukaryotes have a plasma membrane made of a phospholipid bilayer with embedded proteins.
• Eukaryotes have internal membranes forming organelles; prokaryotes do not.
• Bacterial cell walls contain peptidoglycan; archaea lack peptidoglycan but may have pseudopeptidoglycan or other polymers.
• Plant cell walls are made of cellulose; fungal cell walls are made of chitin.
• Animal cells lack a cell wall; they are surrounded only by a plasma membrane and extracellular matrix.
• Gram-positive bacteria have thick peptidoglycan layers; gram-negative bacteria have thin peptidoglycan and an outer membrane.
• Mycoplasma are bacteria that lack a cell wall, making them resistant to antibiotics like penicillin.
• Red blood cells in mammals lack a nucleus and most organelles, maximizing space for hemoglobin.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport.
• Nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
• Mitochondria have double membranes; the inner membrane is highly folded into cristae to increase surface area for ATP production.
• Chloroplasts in plant cells contain thylakoids stacked into grana, where photosynthesis occurs.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own circular DNA, 70S ribosomes, and the ability to replicate independently.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes for breaking down macromolecules.
• Peroxisomes contain enzymes for breaking down fatty acids and detoxifying hydrogen peroxide.
• Cytoskeleton components (microtubules, microfilaments, intermediate filaments) are present in eukaryotes but absent in prokaryotes.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; they can carry antibiotic resistance genes.

Difficulty Level

Intermediate – integrates structural, biochemical, and evolutionary concepts commonly tested in first-year biology courses.

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.
Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S (except in mitochondria and chloroplasts, which have 70S).
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Prokaryotes have no DNA packaging – Fact: Prokaryotic DNA is supercoiled and associated with nucleoid-associated proteins, though not histones (except in some archaea).
Trap: Eukaryotic cells always have a nucleus – Fact: Mature mammalian red blood cells lack a nucleus, a key exception.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Presence of mitochondria
B) 80S ribosomes
C) DNA enclosed in a nucleus
D) DNA located in a nucleoid
Answer: D
Explanation: Prokaryotes lack a nucleus; their DNA is in the nucleoid region.
Why the top distractor is wrong: Distractor C is incorrect because only eukaryotes have a membrane-bound nucleus.

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

Question: Which structure is found in both prokaryotes and eukaryotes?
A) Mitochondria
B) Golgi apparatus
C) Plasma membrane
D) Nucleus
Answer: C
Explanation: Both cell types have a plasma membrane composed of a phospholipid bilayer.
Why the top distractor is wrong: Distractor A (mitochondria) are only in eukaryotes.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Eukaryotic ribosomes are larger than prokaryotic ribosomes
B) Mitochondria have their own circular DNA and 70S ribosomes
C) Prokaryotes can perform aerobic respiration
D) Lysosomes contain digestive enzymes
Answer: B
Explanation: Mitochondria resemble bacteria in having circular DNA and 70S ribosomes, supporting bacterial origin.
Why the top distractor is wrong: Distractor A is a structural difference but not evidence of endosymbiosis.

Question: Which of the following cells lacks a nucleus?
A) Yeast cell
B) Human neuron
C) Escherichia coli
D) Onion root cell
Answer: C
Explanation: E. coli is a prokaryote and lacks a membrane-bound nucleus.
Why the top distractor is wrong: Distractor B (human neuron) is a eukaryotic cell and contains a nucleus.

Question: What is the primary component of fungal cell walls?
A) Peptidoglycan
B) Cellulose
C) Chitin
D) Lipopolysaccharide
Answer: C
Explanation: Fungi have cell walls made of chitin, a nitrogen-containing polysaccharide.
Why the top distractor is wrong: Distractor A (peptidoglycan) is found in bacterial, not fungal, cell walls.

Question: Which organelle is responsible for modifying and packaging proteins for secretion?
A) Nucleolus
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: Distractor B synthesizes lipids but does not package proteins for secretion.

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

• ⚠️ Prokaryotic cell size: 0.1–5.0 μm; eukaryotic: 10–100 μm.
• ⚠️ Prokaryotes have no nucleus; DNA in nucleoid.
• Eukaryotes have linear DNA in nucleus; prokaryotes have circular DNA.
• ⚠️ Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria: peptidoglycan cell wall; plants: cellulose; fungi: chitin; animals: no cell wall.
• ⚠️ Mycoplasma lack a cell wall – naturally resistant to penicillin.
• ⚠️ Mature mammalian red blood cells lack a nucleus and mitochondria.
• Nuclear envelope has double membrane and nuclear pores.
• Nucleolus makes rRNA and assembles ribosomal subunits.
• Mitochondria have double membrane; inner membrane forms cristae.
• Chloroplasts contain thylakoids and grana for photosynthesis.
• Lysosomes: contain hydrolytic enzymes, found in animal cells.
• Peroxisomes break down fatty acids and detoxify H₂O₂.
• Cytoskeleton (microtubules, actin filaments) is eukaryotic only.
• Prokaryotic flagellum: made of flagellin, rotates; eukaryotic: 9+2 microtubules, bends.
• Plasmids: small, circular DNA in prokaryotes, often carry antibiotic resistance.
• Archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Gram-positive bacteria: thick peptidoglycan; gram-negative: thin peptidoglycan + outer membrane.
• ⚠️ Both cell types have phospholipid bilayer plasma membrane.
• Endoplasmic reticulum (rough and smooth) is membrane-bound, eukaryotic only.
• Golgi apparatus: modifies proteins, eukaryotic only.
• ⚠️ Mitochondria replicate independently via binary fission, like bacteria.
• Verify from standard textbook: histone-like proteins in some archaea resemble eukaryotic histones.
• ⚠️ No membrane-bound organelles in prokaryotes – only in eukaryotes.