STEM Readiness: Biology Readiness Cell Biology Cell Signalling Signal Transduction Receptor Types Second Messengers Amplification

Must‑Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 μm in diameter; eukaryotic cells range from 10–100 μm.
• DNA in prokaryotes is located in the nucleoid, an unenclosed region lacking a membrane; eukaryotes house DNA 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; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cells have cell walls made of cellulose; fungal cells have chitin; animal cells lack a cell wall.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to antibiotics targeting peptidoglycan synthesis.
• 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 lack these.
• Mitochondria are present in nearly all eukaryotic cells but absent in mature human red blood cells.
• Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis and meiosis.
• Eukaryotic flagella are composed of microtubules (9+2 arrangement) and are covered by the plasma membrane; prokaryotic flagella are made of flagellin protein, lack membrane, and rotate.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes, but also in some yeasts (eukaryotes).
• Chloroplasts are organelles unique to plant cells and some protists, site of photosynthesis, contain thylakoids and chlorophyll.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; plant vacuoles perform similar degradative functions.
• Nuclear envelope in eukaryotes is a double membrane with nuclear pores regulating transport; prokaryotes lack this structure.
• 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 to lysosomes, plasma membrane, or secretion.
• Centrioles are found in animal cells and organize microtubules during cell division; absent in most plant cells.
• Cilia and flagella in eukaryotes are anchored by basal bodies derived from centrioles; prokaryotes lack basal bodies.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and double membranes.
• Gram-positive bacteria have thick peptidoglycan layers and retain crystal violet stain; Gram-negative have thin peptidoglycan and an outer membrane with lipopolysaccharide.
• Archaea lack peptidoglycan and have ether-linked lipids in their membranes, unlike ester-linked lipids in bacteria and eukaryotes.
• Plant cells have a large central vacuole that maintains turgor pressure; animal cells have smaller, multiple vacuoles.
• Nuclear DNA in eukaryotes is linear and associated with histone proteins; prokaryotic DNA is circular and not bound to histones (though archaea have histone-like proteins).

Difficulty Level

Intermediate – requires precise differentiation of structural and molecular features across domains and cell types.

Common Traps (3–5 factual traps)

• Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (pseudopeptidoglycan or none) differ.
• Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, like prokaryotes, due to endosymbiotic origin.
• Trap: Eukaryotes are always larger because they have more DNA – Fact: Genome size does not directly correlate with cell size or organism complexity (e.g., some protists have larger genomes than humans).
• Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, inclusions, and sometimes protein-based microcompartments (e.g., carboxysomes).

Practice MCQs (5–7 questions)

Question: Which structure is found in eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) 70S ribosome
C) Mitochondrion
D) Circular DNA
Answer: C
Explanation: Mitochondria are membrane-bound organelles unique to eukaryotes.
Why the top distractor is wrong: Circular DNA is present in prokaryotes and also in mitochondria and chloroplasts of eukaryotes.

Question: A cell is observed to have a nucleus, mitochondria, and a cell wall made of cellulose. This cell is most likely from:
A) Fungus
B) Bacterium
C) Plant
D) Animal
Answer: C
Explanation: Cellulose cell walls are characteristic of plant cells.
Why the top distractor is wrong: Fungi have chitin in their cell walls, not cellulose.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Ability to synthesize ATP
C) Containing 70S ribosomes and circular DNA
D) Dividing by binary fission
Answer: C
Explanation: 70S ribosomes and circular DNA are prokaryotic features retained in mitochondria.
Why the top distractor is wrong: Double membranes are present but not exclusive evidence; other organelles may have double membranes.

Question: Which cell type lacks a nucleus but contains ribosomes?
A) Human red blood cell
B) Escherichia coli
C) Mature sieve tube element
D) Both A and B
Answer: D
Explanation: Both prokaryotes (E. coli) and mature human red blood cells lack a nucleus but have ribosomes (though RBCs lose them during maturation).
Why the top distractor is wrong: Sieve tube elements lack nuclei but rely on companion cells for protein synthesis and lack functional ribosomes.

Question: Which component is common to all living cells?
A) Nucleus
B) Cell wall
C) Phospholipid bilayer
D) Mitochondria
Answer: C
Explanation: All cells have a plasma membrane made of phospholipid bilayer.
Why the top distractor is wrong: Not all cells have a nucleus (prokaryotes lack it) or mitochondria (prokaryotes and some anaerobic eukaryotes lack them).

Question: A new microorganism is discovered with no peptidoglycan, ether-linked lipids, and 70S ribosomes. It is most likely:
A) Gram-negative bacterium
B) Archaeon
C) Fungus
D) Virus
Answer: B
Explanation: Archaea lack peptidoglycan and have ether-linked lipids in their membranes.
Why the top distractor is wrong: Gram-negative bacteria have peptidoglycan and ester-linked lipids.

Question: Which organelle modifies proteins and packages them into vesicles for secretion?
A) Nucleus
B) Rough ER
C) Golgi apparatus
D) Lysosome
Answer: C
Explanation: The Golgi apparatus modifies and sorts proteins from the ER for secretion.
Why the top distractor is wrong: Rough ER synthesizes proteins but does not package them for secretion.

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

• ⚠️ Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• ⚠️ Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Prokaryote size: 0.1–5.0 μm; eukaryote size: 10–100 μm.
• DNA in prokaryotes is in the nucleoid; in eukaryotes, it’s enclosed in a nuclear envelope.
• Peptidoglycan is found only in bacterial cell walls.
• Archaea have ether-linked lipids; bacteria and eukaryotes have ester-linked lipids.
• Mycoplasma lacks a cell wall – naturally resistant to penicillin.
• Human red blood cells lack a nucleus and mitochondria.
• Plant cells have chloroplasts, large central vacuole, and cellulose cell wall.
• Fungal cell walls contain chitin; animal cells have no cell wall.
• Lysosomes are primarily in animal cells; plants use vacuoles for degradation.
• Smooth ER: lipid synthesis and detoxification; rough ER: protein synthesis.
• Golgi apparatus: modifies proteins, forms lysosomes, packages secretions.
• Nuclear pores regulate transport between nucleus and cytoplasm.
• Eukaryotic flagella: 9+2 microtubule arrangement; prokaryotic flagella: made of flagellin, rotary motion.
• Centrioles present in animal cells, absent in most plant cells.
• Plasmids are common in bacteria, rare in eukaryotes (e.g., yeast 2-micron plasmid).
• Binary fission occurs in prokaryotes and organelles (mitochondria, chloroplasts).
• ⚠️ Archaea are prokaryotes in structure (no nucleus) but differ biochemically from bacteria.
• Endosymbiotic theory: mitochondria evolved from aerobic bacteria, chloroplasts from cyanobacteria.
• Gram-positive: thick peptidoglycan, no outer membrane; Gram-negative: thin peptidoglycan, outer membrane with LPS.
• All cells have a plasma membrane composed of phospholipid bilayer.
• Histones are associated with eukaryotic nuclear DNA; bacterial DNA lacks histones (archaea have histone-like proteins).
• Basal bodies anchor eukaryotic cilia and flagella; derived from centrioles.
• Verify from standard textbook: exact ribosome subunit sizes (30S/50S in 70S; 40S/60S in 80S).