STEM Readiness: Chemistry Readiness - Atomic Structure: Periodic Trends - Atomic Radius, IE, EA, Electronegativity Direction, and Exceptions

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, a region without a membrane; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes lack membrane-bound organelles; eukaryotes contain organelles such as mitochondria, lysosomes, and peroxisomes.
• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S subunits).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, similar to prokaryotes, supporting endosymbiotic theory.
• Cell walls are present in most prokaryotes, composed of peptidoglycan in bacteria; archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cells have cell walls made of cellulose; fungal cells have chitin; animal cells lack cell walls.
• Mycoplasma, a bacterium, lacks a cell wall and is resistant to penicillin.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have extensive internal membranes forming the endomembrane system, including endoplasmic reticulum (ER), Golgi apparatus, and vesicles.
• Smooth ER synthesizes lipids and detoxifies drugs; rough ER, studded with ribosomes, synthesizes proteins for secretion.
• 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, found in plants and algae, perform photosynthesis, contain thylakoids, and have circular DNA and 70S ribosomes.
• Lysosomes contain hydrolytic enzymes for intracellular digestion; absent in most plant cells.
• Vacuoles in plant cells are large and central, maintaining turgor pressure; animal cells have small, temporary vacuoles.
• 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 subunit assembly.
• Cytoskeleton components (microtubules, microfilaments, intermediate filaments) are present in eukaryotes; prokaryotes have homologs like FtsZ but lack true cytoskeleton.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and move in whip-like motion.
• Red blood cells in mammals lack a nucleus and mitochondria, maximizing hemoglobin capacity.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having double membranes, circular DNA, 70S ribosomes, and ability to replicate independently.
• Archaea resemble prokaryotes in size and structure but have eukaryote-like transcription and translation machinery.
• Plant cells have plasmodesmata for intercellular communication; animal cells use gap junctions.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across diverse organisms, including exceptions and evolutionary evidence.

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, fungi of chitin, and archaea lack peptidoglycan.

Trap: Ribosome size correlates with cell complexity – Fact: Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, identical in size to prokaryotes, not 80S.

Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes (outer and inner).

Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, inclusions, and some have protein-based microcompartments (e.g., carboxysomes).

Practice MCQs (5–7 questions)

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 plasma membrane composed of a phospholipid bilayer.
Why the top distractor is wrong: A is exclusive to eukaryotes; prokaryotes lack a nucleus.

Question: A cell is observed to contain 70S ribosomes, a nucleoid, and peptidoglycan in its cell wall. Which organism is it most likely from?
A) Fungus
B) Flowering plant
C) Human
D) Escherichia coli
Answer: D
Explanation: E. coli is a bacterium with 70S ribosomes, nucleoid, and peptidoglycan.
Why the top distractor is wrong: Fungi are eukaryotes with 80S ribosomes and chitin cell walls.

Question: Which structure is found in plant cells but not in animal cells?
A) Lysosome
B) Centriole
C) Central vacuole
D) Plasma membrane
Answer: C
Explanation: The large central vacuole is a defining feature of mature plant cells.
Why the top distractor is wrong: Lysosomes are present in animal cells but rare in plants; plant cells may have lytic vacuoles.

Question: Which of the following provides the strongest evidence for the endosymbiotic origin of mitochondria?
A) Presence of a double membrane
B) Involvement in ATP production
C) Containing 70S ribosomes and circular DNA
D) Ability to divide by binary fission
Answer: C
Explanation: Circular DNA and 70S ribosomes are prokaryote-like traits retained in mitochondria.
Why the top distractor is wrong: Double membranes can form by invagination; circular DNA and ribosomes are more specific evidence.

Question: Which cell type lacks a nucleus and has 80S ribosomes?
A) Bacterial cell
B) Fungal cell
C) Mature human red blood cell
D) Liver cell
Answer: C
Explanation: Mature red blood cells lack a nucleus but are derived from eukaryotic cells with 80S ribosomes (though they lose ribosomes during maturation).
Why the top distractor is wrong: Bacterial cells have 70S ribosomes; fungal cells have nuclei.

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: peptidoglycan cell wall; archaea: no peptidoglycan.
• Plant cell wall: cellulose; fungal: chitin; animal: no cell wall.
• Mycoplasma has no cell wall – naturally penicillin-resistant.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus, etc.); prokaryotes do not.
• Rough ER has ribosomes; smooth ER does not.
• Golgi apparatus: modifies and sorts proteins; forms lysosomes and secretory vesicles.
• Mitochondria: site of Krebs cycle and oxidative phosphorylation.
• Chloroplasts: contain thylakoids and stroma; perform photosynthesis.
• Lysosomes: contain acid hydrolases; maintain acidic pH.
• Mature mammalian red blood cells lack a nucleus and mitochondria.
• Nuclear pores regulate nucleocytoplasmic transport.
• Nucleolus: synthesizes rRNA and assembles ribosomal subunits.
• Cytoskeleton: microtubules (tubulin), microfilaments (actin), intermediate filaments.
• Prokaryotic flagella: made of flagellin, rotary motion.
• Eukaryotic flagella: 9+2 microtubule arrangement, dynein-driven bending.
• Archaea have prokaryotic morphology but eukaryotic-like transcription.
• Plasmodesmata: plant cell junctions with cytoplasmic continuity.
• Gap junctions: animal cell communication channels.
• FtsZ protein in bacteria is homologous to eukaryotic tubulin.
• Verify from standard textbook: exact ribosomal subunit composition in mitochondria across species.