STEM Readiness: Chemistry Readiness Kinetics Arrhenius Equation Activation Energy Temperature Effect Catalysis

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.
• 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 contain 70S ribosomes, supporting their origin via endosymbiotic theory.
• Prokaryotic cell walls typically contain peptidoglycan (e.g., Escherichia coli); archaea lack peptidoglycan and have different wall chemistry.
• 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 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 sites of aerobic respiration; absent in prokaryotes.
• Chloroplasts are found in plant cells and some protists (e.g., Chlamydomonas), site of photosynthesis; absent in prokaryotes except in rare cyanobacteria analogs.
• Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis and cytokinesis.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and undulate.
• Eukaryotic cells contain linear chromosomes with histone proteins; prokaryotes have a single circular chromosome without histones (except some archaea).
• Plasmids are small, circular extrachromosomal DNA molecules common in prokaryotes; rare in eukaryotes (e.g., 2-micron plasmid in yeast).
• Mycoplasma species are bacteria without a cell wall, resistant to penicillin; exception among prokaryotes.
• Human red blood cells lack a nucleus and mitochondria in maturity; exception among eukaryotic cells.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, own DNA, and 70S ribosomes.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; absent in most plant cells (vacuole performs similar function).
• The nuclear envelope is a double membrane with nuclear pores regulating transport; absent in prokaryotes.
• Smooth ER synthesizes lipids and detoxifies drugs; rough ER has ribosomes and synthesizes proteins for secretion.
• Golgi apparatus modifies, sorts, and packages proteins into vesicles for transport; not present in prokaryotes.
• Peroxisomes contain enzymes for beta-oxidation of fatty acids and breakdown of hydrogen peroxide; found in eukaryotes.
• Cytoskeleton (microfilaments, intermediate filaments, microtubules) is present in eukaryotes; prokaryotes have analogous proteins (e.g., FtsZ, MreB) but no true cytoskeleton.
• Plant cells have a large central vacuole for turgor pressure and storage; animal cells have small or transient vacuoles.

Difficulty Level

Intermediate – typically covered in first-semester biology with emphasis on comparative structure and function.

Common Traps (3–5 factual traps)

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacterial cell walls contain peptidoglycan; plants use cellulose, fungi use chitin, and archaea lack peptidoglycan.

Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes like prokaryotes, despite being in complex eukaryotic cells.

Trap: Prokaryotes have no internal membranes – Fact: Some prokaryotes (e.g., photosynthetic bacteria) have invaginations of the plasma membrane (e.g., chromatophores) but lack membrane-bound organelles.

Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes, consistent with endosymbiotic origin.

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: Only eukaryotes have a membrane-bound nucleus.
Why the top distractor is wrong: 70S ribosomes are in prokaryotes and organelles, but not in the eukaryotic cytosol (which has 80S).

Question: A cell is observed to have a cell wall, chloroplasts, and a large central vacuole. It most likely belongs to which domain?
A) Bacteria
B) Archaea
C) Animalia
D) Plantae
Answer: D
Explanation: Chloroplasts, cellulose cell wall, and central vacuole are diagnostic of plant cells.
Why the top distractor is wrong: Bacteria may have cell walls and photosynthesis but lack membrane-bound organelles like chloroplasts.

Question: Which structure is evidence supporting the endosymbiotic theory?
A) Presence of a nucleoid
B) 80S ribosomes in the cytoplasm
C) Double membrane around mitochondria
D) Peptidoglycan in the cell wall
Answer: C
Explanation: The double membrane of mitochondria suggests engulfment by a host cell.
Why the top distractor is wrong: The nucleoid is a prokaryotic feature, not evidence of symbiosis.

Question: Which of the following cells lacks a nucleus?
A) Yeast cell
B) Human liver cell
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: Yeast is a eukaryotic fungus and has a nucleus.

Question: What is the primary component of fungal cell walls?
A) Cellulose
B) Peptidoglycan
C) Chitin
D) Silica
Answer: C
Explanation: Fungi use chitin, a nitrogen-containing polysaccharide, in their cell walls.
Why the top distractor is wrong: Cellulose is found in plant cell walls, not fungal.

Question: Which organelle is responsible for modifying and packaging proteins for secretion?
A) Nucleus
B) Mitochondria
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus processes and packages proteins into vesicles.
Why the top distractor is wrong: The nucleus houses DNA and directs protein synthesis but does not package proteins.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 70S
B) Cytoplasmic ribosomes are 80S
C) Ribosomes are enclosed in a membrane
D) Ribosomes are only found in the nucleus
Answer: B
Explanation: Eukaryotic cytoplasmic ribosomes are 80S; organelles have 70S.
Why the top distractor is wrong: 70S ribosomes are in prokaryotes and organelles, not in the eukaryotic cytosol.

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

• ⚠️ Prokaryotic cell size: 0.1–5.0 μm; eukaryotic: 10–100 μm.
• ⚠️ Prokaryotes lack a membrane-bound nucleus; DNA in nucleoid.
• ⚠️ Eukaryotes have 80S ribosomes in cytoplasm; 70S in mitochondria and chloroplasts.
• ⚠️ Prokaryotic ribosome = 70S (30S + 50S); eukaryotic = 80S (40S + 60S).
• ⚠️ Peptidoglycan is unique to bacterial cell walls.
• Archaea lack peptidoglycan; cell wall composition differs.
• Plant cell wall: cellulose; fungi: chitin; animals: no cell wall.
• Both cell types have phospholipid bilayer plasma membrane.
• Endoplasmic reticulum, Golgi, mitochondria – only in eukaryotes.
• Mitochondria perform aerobic respiration; absent in prokaryotes.
• Chloroplasts in plants and some protists; site of photosynthesis.
• Binary fission in prokaryotes; mitosis in eukaryotes.
• Prokaryotic flagellum made of flagellin, rotates; eukaryotic flagellum has 9+2 microtubules, bends.
• Eukaryotic chromosomes are linear with histones; prokaryotes have circular DNA without histones.
• Plasmids common in bacteria; rare in eukaryotes.
• ⚠️ Mycoplasma – no cell wall, smallest known cells.
• ⚠️ Human red blood cells – no nucleus, no mitochondria.
• ⚠️ Mitochondria have circular DNA, 70S ribosomes, double membrane – evidence for endosymbiosis.
• Lysosomes in animal cells; contain acid hydrolases.
• Nuclear envelope = double membrane with pores; regulates RNA/protein transport.
• Smooth ER – lipid synthesis, detoxification; rough ER – protein synthesis (with ribosomes).
• Golgi apparatus – modifies proteins, forms lysosomes, packages for secretion.
• Peroxisomes – break down fatty acids, neutralize H₂O₂.
• Cytoskeleton (microtubules, etc.) only in eukaryotes; prokaryotes have FtsZ, MreB analogs.
• Plant vacuole – maintains turgor pressure, stores ions; much larger than in animal cells.