STEM Readiness: Chemistry Readiness - Chemical Bonding: Lewis Structures - Formal Charge, Resonance, Exceptions to Octet Rule

Must?Know

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• 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 endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to antibiotics like penicillin that target peptidoglycan synthesis.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Only eukaryotes have membrane-bound organelles, including the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and mitochondria.
• Prokaryotes lack mitochondria and generate ATP via the plasma membrane and cytoplasm.
• Eukaryotic mitochondria are sites of aerobic respiration and contain their own circular DNA.
• Chloroplasts are found in plant and algal cells and are sites of photosynthesis; they also contain circular DNA and 70S ribosomes.
• 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.
• Smooth ER synthesizes lipids and detoxifies drugs; rough ER has ribosomes and synthesizes proteins for secretion or membranes.
• Golgi apparatus modifies, sorts, and packages proteins into vesicles for transport to lysosomes, plasma membrane, or secretion.
• Lysosomes contain hydrolytic enzymes active at acidic pH; absent in most plant cells (vacuole performs similar function).
• Plant cells have a large central vacuole that maintains turgor pressure and stores ions and metabolites.
• Centrioles are found in animal cells and organize microtubules during cell division; absent in most plant cells.
• Cilia and flagella in eukaryotes are composed of microtubules in a 9+2 arrangement; prokaryotic flagella are made of flagellin and rotate.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, circular DNA, and 70S ribosomes.
• Red blood cells in mammals lack a nucleus and organelles, maximizing space for hemoglobin.
• Archaea are prokaryotes but share some molecular features with eukaryotes (e.g., RNA polymerase, histones).
• Gram-positive bacteria have thick peptidoglycan; Gram-negative have thin peptidoglycan and an outer membrane with lipopolysaccharide.

Difficulty Level

Intermediate – requires precise differentiation of structures, components, and exceptions across domains and kingdoms.

Common Traps

Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (other polymers) have different wall compositions.
Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based microcompartments.
Trap: Eukaryotes evolved from prokaryotes directly – Fact: Eukaryotes likely arose from archaeal ancestors with endosymbiotic events.

Practice MCQs

Question: Which of the following is a feature shared by both prokaryotic and eukaryotic cells?
A) Membrane-bound nucleus
B) Mitochondria
C) Phospholipid bilayer plasma membrane
D) 80S ribosomes
Answer: C
Explanation: Both cell types have a phospholipid bilayer as the plasma membrane.
Why the top distractor is wrong: A is exclusive to eukaryotes; prokaryotes lack a nucleus.

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, cell walls (cellulose), and central vacuoles are characteristic of plant cells.
Why the top distractor is wrong: Bacteria may have cell walls but lack chloroplasts and membrane-bound organelles.

Question: Which structure is present in eukaryotic cells but absent in prokaryotic cells?
A) Ribosomes
B) Plasma membrane
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: Both cell types have ribosomes, though they differ in size.

Question: Mitochondria are thought to have originated from free-living bacteria due to which of the following?
A) Presence of 80S ribosomes
B) Linear chromosomes with histones
C) Circular DNA and 70S ribosomes
D) Peptidoglycan in the inner membrane
Answer: C
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to bacteria.
Why the top distractor is wrong: Mitochondria have 70S, not 80S, ribosomes.

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: Yeast is a eukaryotic fungus and has 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: Peptidoglycan is found in bacterial, not fungal, cell walls.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 70S
B) Ribosomes are only found in the cytoplasm
C) 80S ribosomes are found in the cytoplasm and on rough ER
D) Mitochondrial ribosomes are 80S
Answer: C
Explanation: 80S ribosomes are in the cytosol and on rough ER; mitochondria have 70S.
Why the top distractor is wrong: Eukaryotic cytoplasmic ribosomes are 80S, not 70S.

Last?Minute Revision

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have 70S ribosomes; eukaryotes have 80S in cytoplasm.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• Nuclear DNA in eukaryotes is linear; prokaryotic DNA is circular.
• Peptidoglycan is in bacterial cell walls only.
• Plant cell wall = cellulose; fungal = chitin; animal = no wall.
• Mycoplasma lacks a cell wall – smallest known cells.
• Plasma membrane = phospholipid bilayer in all cells.
• Only eukaryotes have membrane-bound organelles.
• Nucleoid = no membrane; nucleus = double membrane with pores.
• Nucleolus makes rRNA and assembles ribosome subunits.
• Rough ER has ribosomes; smooth ER makes lipids.
• Golgi modifies proteins and forms lysosomes or secretory vesicles.
• Lysosomes have acid hydrolases; optimal pH ~5.
• Plant vacuole maintains turgor pressure and stores metabolites.
• Red blood cells lack nucleus and mitochondria.
• Centrioles in animal cells only; organize spindle fibers.
• Eukaryotic flagella = 9+2 microtubules; prokaryotic = flagellin, rotary.
• Prokaryotes divide by binary fission; eukaryotes by mitosis.
• Endosymbiotic theory supported by double membranes, DNA, and ribosomes in mitochondria/chloroplasts.
• Archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Gram-negative bacteria have outer membrane with LPS.
• Mitochondria evolved from alpha-proteobacteria; chloroplasts from cyanobacteria.
• Histones are in archaea and eukaryotes, not bacteria.
• Verify from standard textbook: exact ribosomal subunit composition in organelles.