STEM Readiness: Physics Readiness - Momentum: Conservation of Momentum - 1D and 2D Collisions, Elastic vs. Inelastic

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, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain organelles such as mitochondria, lysosomes, and peroxisomes.
• Ribosomes in prokaryotes are 70S (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, supporting their origin via endosymbiosis.
• 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 cell walls.
• Mycoplasma species are bacteria that lack a cell wall, making them resistant to antibiotics like penicillin.
• 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 nuclear envelope.
• The nuclear envelope is a double membrane with nuclear pores that regulate transport between nucleus and cytoplasm.
• Nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
• Rough ER is studded with ribosomes and synthesizes proteins for secretion or membranes; smooth ER lacks ribosomes and synthesizes lipids and detoxifies drugs.
• Golgi apparatus modifies, sorts, and packages proteins into vesicles for transport to lysosomes, plasma membrane, or secretion.
• Mitochondria are the site of aerobic respiration and contain their own circular DNA and 70S ribosomes.
• Chloroplasts (in plants and algae) perform photosynthesis, contain thylakoids, and have circular DNA and 70S ribosomes.
• Lysosomes (in animal cells) contain hydrolytic enzymes for breaking down macromolecules and cellular debris.
• Plant cells have a central vacuole that maintains turgor pressure and stores ions and metabolites; animal cells have small or no vacuoles.
• Centrioles are found in animal cells and organize microtubules during cell division; plants and fungi lack centrioles.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, circular DNA, 70S ribosomes, and the ability to replicate independently of the cell.
• Red blood cells in mammals lack a nucleus and organelles, maximizing space for hemoglobin; they cannot divide or repair proteins.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis for cell division and reproduction.
• Cilia and flagella differ in structure: prokaryotic flagella are made of flagellin and rotate; eukaryotic cilia/flagella have a 9+2 microtubule arrangement and bend.
• Peroxisomes contain catalase and break down fatty acids and hydrogen peroxide; present in both plant and animal cells.

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 is the same across all cells – Fact: Prokaryotes use 70S ribosomes; eukaryotes use 80S in cytoplasm, but mitochondria and chloroplasts retain 70S.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Eukaryotes are always larger and more complex than prokaryotes – Fact: While generally true, some bacteria (e.g., Thiomargarita namibiensis) can be larger than typical eukaryotic cells.
Trap: All eukaryotic cells have a cell wall – Fact: Only plants, fungi, and some protists have cell walls; animal cells do not.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes in the cytoplasm
C) Presence of peptidoglycan in the cell wall
D) Endoplasmic reticulum
Answer: C
Explanation: Peptidoglycan is a component of bacterial cell walls, a hallmark of prokaryotes.
Why the top distractor is wrong: A is incorrect because prokaryotes lack a membrane-bound nucleus.

Question: Where are 70S ribosomes found in eukaryotic cells?
A) Cytoplasm
B) Nucleus
C) Mitochondria
D) Golgi apparatus
Answer: C
Explanation: Mitochondria contain 70S ribosomes, similar to prokaryotes, supporting endosymbiotic theory.
Why the top distractor is wrong: A is incorrect because cytoplasmic ribosomes in eukaryotes are 80S.

Question: Which structure is present in plant cells but not in animal cells?
A) Lysosome
B) Central vacuole
C) Peroxisome
D) Smooth ER
Answer: B
Explanation: The large central vacuole is a key feature of mature plant cells, maintaining turgor pressure.
Why the top distractor is wrong: A is incorrect because lysosomes are primarily found in animal cells.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Mitochondria have linear DNA
B) Chloroplasts are surrounded by a single membrane
C) Mitochondria replicate by binary fission
D) Lysosomes contain hydrolytic enzymes
Answer: C
Explanation: Mitochondria divide independently via binary fission, similar to bacteria.
Why the top distractor is wrong: A is incorrect because mitochondria have circular, not linear, DNA.

Question: Which cell type lacks a nucleus but contains ribosomes?
A) Human red blood cell
B) Escherichia coli
C) Mature plant sieve tube element
D) Fungal hypha
Answer: B
Explanation: E. coli is a prokaryote with ribosomes but no nucleus.
Why the top distractor is wrong: A is incorrect because mature red blood cells lack both nucleus and ribosomes.

Question: What is the primary component of fungal cell walls?
A) Peptidoglycan
B) Cellulose
C) Chitin
D) Silica
Answer: C
Explanation: Fungi have cell walls composed of chitin, a nitrogen-containing polysaccharide.
Why the top distractor is wrong: A is incorrect because peptidoglycan is found in bacteria, not fungi.

Question: Which organelle is responsible for lipid synthesis and detoxification in liver cells?
A) Rough ER
B) Golgi apparatus
C) Lysosome
D) Smooth ER
Answer: D
Explanation: Smooth ER synthesizes lipids and detoxifies drugs, especially in hepatocytes.
Why the top distractor is wrong: A is incorrect because 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: DNA in nucleoid; eukaryotes: DNA in nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria have peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Mycoplasma lacks a cell wall – resistant to penicillin.
• Both cell types have phospholipid bilayer membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nuclear pores regulate molecule transport between nucleus and cytoplasm.
• Nucleolus = site of rRNA synthesis and ribosome subunit assembly.
• Rough ER = protein synthesis; smooth ER = lipid synthesis, detoxification.
• Golgi modifies, sorts, and packages proteins into vesicles.
• Mitochondria = site of ATP production via aerobic respiration.
• Chloroplasts = site of photosynthesis; contain thylakoids and stroma.
• Lysosomes = contain hydrolytic enzymes; found in animal cells.
• Plant central vacuole maintains turgor pressure.
• Centrioles present in animal cells only; organize spindle fibers.
• Peroxisomes break down fatty acids and neutralize H?O? with catalase.
• Prokaryotic flagella = made of flagellin, rotate; eukaryotic = 9+2 microtubules, bend.
• Red blood cells lack nucleus and organelles; cannot synthesize proteins.
• Endosymbiotic theory supported by double membranes, circular DNA, 70S ribosomes in mitochondria/chloroplasts.
• Binary fission = prokaryotic division; mitosis/meiosis = eukaryotic.
• Mitochondria replicate independently via binary fission.
• Verify from standard textbook: exact rRNA subunit sizes (50S/30S in 70S; 60S/40S in 80S).