STEM Readiness: Biology Readiness - Evolution/Ecology: Natural Selection - Mechanisms, Types, Evidence, Common Misconceptions

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, endoplasmic reticulum, and Golgi apparatus.
• Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S).
• Mitochondria and chloroplasts in eukaryotes contain 70S ribosomes, supporting their origin via 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 walls made of chitin; animal cells lack cell walls.
• Mycoplasma is a bacterium that lacks a cell wall, making it resistant to antibiotics like penicillin that target peptidoglycan synthesis.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane that regulates transport and maintains integrity.
• Eukaryotes have internal membranes forming organelles (e.g., nuclear envelope, ER, lysosomes); prokaryotes do not.
• The nucleus in eukaryotes contains chromatin (DNA + histone proteins) and a nucleolus where ribosomal RNA is synthesized.
• Nuclear pores in the nuclear envelope allow selective transport of molecules like mRNA and proteins.
• Mitochondria are the site of aerobic respiration and have double membranes; the inner membrane is highly folded into cristae.
• Chloroplasts are found in plant and algal cells, contain thylakoids and chlorophyll, and are the site of photosynthesis.
• Endoplasmic reticulum (ER) exists as rough ER (with ribosomes) for protein synthesis and smooth ER (without ribosomes) for lipid synthesis and detoxification.
• Golgi apparatus modifies, sorts, and packages proteins and lipids into vesicles for secretion or delivery to other organelles.
• Lysosomes contain hydrolytic enzymes for intracellular digestion and are present in animal cells; vacuoles in plant cells perform similar functions.
• Peroxisomes break down fatty acids and detoxify hydrogen peroxide; they are present in both plant and animal eukaryotic cells.
• Cytoskeleton components (microtubules, microfilaments, intermediate filaments) provide structure and facilitate transport in eukaryotes; prokaryotes have analogous proteins (e.g., FtsZ, MreB).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are made of microtubules (9+2 arrangement) and move in a whip-like fashion.
• Plasmids are small, circular extrachromosomal DNA molecules commonly found in prokaryotes; rare in eukaryotes (e.g., 2-micron plasmid in yeast).
• Red blood cells in mammals lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having circular DNA, 70S ribosomes, and double membranes.
• Archaea are prokaryotes but share some molecular features with eukaryotes (e.g., RNA polymerase, histone-like proteins).

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and organelles, including exceptions and evolutionary evidence.

Common Traps (3–5 factual traps)

Trap: All prokaryotes have cell walls – Fact: Mycoplasma, a bacterium, lacks a cell wall entirely.
Trap: Ribosome size is the same across all organelles in eukaryotes – Fact: Mitochondria and chloroplasts have 70S ribosomes, unlike the 80S ribosomes in the eukaryotic cytoplasm.
Trap: Eukaryotes are always larger than prokaryotes – Fact: Some giant bacteria (e.g., Thiomargarita namibiensis, up to 750 ?m) exceed typical eukaryotic cell sizes.
Trap: The nucleus is the only organelle with a double membrane – Fact: Mitochondria and chloroplasts also have double membranes.
Trap: All cells with cell walls have peptidoglycan – Fact: Peptidoglycan is specific to bacteria; plants (cellulose) and fungi (chitin) have different wall compositions.

Practice MCQs (5–7 questions)

Question: Which of the following is a defining feature of prokaryotic cells?
A) Membrane-bound nucleus
B) 80S ribosomes
C) Presence of nucleoid
D) Mitochondria for ATP production
Answer: C
Explanation: Prokaryotes lack a nucleus and instead have a nucleoid region containing DNA.
Why the top distractor is wrong: A is incorrect because a membrane-bound nucleus is exclusive to eukaryotes.

Question: A cell is observed to have a cell wall made of cellulose, chloroplasts, and a large central vacuole. It is most likely:
A) Fungal cell
B) Animal cell
C) Bacterial cell
D) Plant cell
Answer: D
Explanation: Cellulose cell walls, chloroplasts, and large central vacuoles are characteristic of plant cells.
Why the top distractor is wrong: A is incorrect because fungal cells have chitin in their walls and lack chloroplasts.

Question: Which structure is present in eukaryotic cells but absent in prokaryotic cells?
A) Plasma membrane
B) Ribosomes
C) Golgi apparatus
D) Circular DNA
Answer: C
Explanation: The Golgi apparatus is a membrane-bound organelle found only in eukaryotes.
Why the top distractor is wrong: A is incorrect because both cell types have a plasma membrane.

Question: Mitochondria are thought to have originated from free-living prokaryotes due to which of the following?
A) Presence of 80S ribosomes
B) Linear chromosomes with histones
C) Double membrane and 70S ribosomes
D) Use of anaerobic respiration
Answer: C
Explanation: Mitochondria have double membranes and 70S ribosomes, consistent with bacterial ancestry.
Why the top distractor is wrong: A is incorrect because mitochondria have 70S, not 80S, ribosomes.

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: A is incorrect because yeast is a eukaryotic fungus with a nucleus.

Question: Which component is found in both prokaryotes and eukaryotes?
A) Nucleolus
B) Peptidoglycan
C) Phospholipid bilayer
D) Endoplasmic reticulum
Answer: C
Explanation: Both cell types have a plasma membrane made of a phospholipid bilayer.
Why the top distractor is wrong: B is incorrect because peptidoglycan is found only in bacteria.

Question: What distinguishes the flagellum of a eukaryotic cell from that of a prokaryotic cell?
A) Composed of flagellin
B) Rotates to generate movement
C) Contains a 9+2 microtubule arrangement
D) Anchored by basal body
Answer: C
Explanation: Eukaryotic flagella have a 9+2 arrangement of microtubules; prokaryotic flagella are solid and lack this structure.
Why the top distractor is wrong: A is incorrect because flagellin is the protein in prokaryotic, not eukaryotic, flagella.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• DNA in prokaryotes: nucleoid; in eukaryotes: nucleus.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea do not.
• Plant cell walls: cellulose; fungal: chitin; animal cells: no cell wall.
• Mycoplasma has no cell wall – smallest known bacterium.
• Red blood cells (mammals) lack nucleus and mitochondria.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Nuclear envelope has nuclear pores for regulated transport.
• Nucleolus: site of rRNA synthesis and ribosome assembly.
• Mitochondria: double membrane, cristae, site of ATP production.
• Chloroplasts: contain thylakoids, grana, and perform photosynthesis.
• Rough ER: studded with ribosomes; synthesizes proteins.
• Smooth ER: synthesizes lipids, detoxifies drugs, stores Ca²?.
• Golgi apparatus: modifies, sorts, packages proteins into vesicles.
• Lysosomes: contain acid hydrolases; break down macromolecules.
• Peroxisomes: break down fatty acids, detoxify H?O?.
• Cytoskeleton: microtubules (tubulin), microfilaments (actin), intermediate filaments.
• Prokaryotic flagellum: made of flagellin, rotates; eukaryotic: 9+2 microtubules, whips.
• Plasmids: small circular DNA; common in bacteria, rare in eukaryotes.
• Mitochondria and chloroplasts have own DNA and ribosomes – supports endosymbiotic theory.
• Archaea: prokaryotic structure but share transcription machinery with eukaryotes.
• Giant bacterium Thiomargarita namibiensis can be up to 750 ?m – larger than some eukaryotes.
• Verify from standard textbook: exact size ranges and exceptions in organelle presence across eukaryotic lineages.