STEM Readiness: Biology Readiness - Evolution/Ecology: Population Ecology - Exponential vs. Logistic Growth, r/K Selection, Carrying Capacity

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• DNA in prokaryotes is located in the nucleoid, a region without 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 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 penicillin.
• Human red blood cells lack a nucleus and organelles, maximizing space for hemoglobin.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have membrane-bound organelles (e.g., nucleus, mitochondria, endoplasmic reticulum); prokaryotes do not.
• The nucleus contains chromatin (DNA + histones) and the nucleolus, where ribosomal RNA is synthesized.
• Nuclear pores regulate transport between nucleus and cytoplasm; prokaryotes lack such structures.
• Endoplasmic reticulum (ER) is a eukaryotic organelle: rough ER has ribosomes; smooth ER synthesizes lipids and detoxifies.
• Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery; absent in prokaryotes.
• Mitochondria generate ATP via oxidative phosphorylation; have double membranes and their own circular DNA.
• Chloroplasts perform photosynthesis in plants and algae; contain thylakoids, chlorophyll, and circular DNA.
• Lysosomes contain hydrolytic enzymes for degradation; found in animal cells, rare in plant cells.
• Vacuoles in plant cells are large and central, maintaining turgor pressure; smaller and multiple in animal cells.
• Centrioles organize microtubules during cell division in animal cells; 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.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having 70S ribosomes, circular DNA, and double membranes.
• Archaea are prokaryotes but share some molecular features with eukaryotes (e.g., RNA polymerase).
• Binary fission is the method of asexual reproduction in prokaryotes; eukaryotes divide by mitosis.
• Plasmids are small, circular DNA molecules in prokaryotes; eukaryotes may have plasmids but rarely.
• Cytoskeleton (microfilaments, intermediate filaments, microtubules) is present in eukaryotes; prokaryotes have homologous proteins (e.g., FtsZ).

Difficulty Level

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

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 (no peptidoglycan) differ.

Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S; mitochondria and chloroplasts have 70S.

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

Trap: Eukaryotes are always larger and more complex than prokaryotes – Fact: Some bacteria (e.g., Thiomargarita namibiensis) can be larger than typical eukaryotic cells.

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: A membrane-bound nucleus is exclusive to eukaryotes.
Why the top distractor is wrong: Circular DNA is present in prokaryotes and also in mitochondria and chloroplasts.

Question: Which cell type lacks a nucleus and has 70S ribosomes?
A) Fungal cell
B) Plant cell
C) Bacterial cell
D) Animal cell
Answer: C
Explanation: Bacterial cells are prokaryotes with no nucleus and 70S ribosomes.
Why the top distractor is wrong: Fungal cells are eukaryotes and have 80S ribosomes and a nucleus.

Question: Which of the following provides evidence for the endosymbiotic theory?
A) Presence of peptidoglycan in mitochondria
B) Mitochondria have 80S ribosomes
C) Mitochondria have circular DNA and 70S ribosomes
D) Mitochondria are found in prokaryotes
Answer: C
Explanation: Mitochondria resemble bacteria with circular DNA and 70S ribosomes.
Why the top distractor is wrong: Peptidoglycan is found in bacterial cell walls, not in mitochondria.

Question: Which organism has a cell wall made of chitin?
A) Oak tree
B) Escherichia coli
C) Saccharomyces cerevisiae
D) Homo sapiens
Answer: C
Explanation: Saccharomyces cerevisiae (yeast) is a fungus with a chitin-based cell wall.
Why the top distractor is wrong: Oak trees are plants and have cellulose, not chitin, in their cell walls.

Question: Which structure is present in plant cells but typically absent in animal cells?
A) Lysosome
B) Centriole
C) Large central vacuole
D) Plasma membrane
Answer: C
Explanation: A large central vacuole is a defining feature of mature plant cells.
Why the top distractor is wrong: Lysosomes are common in animal cells and sometimes in plant cells, but not defining.

Question: What is the primary component of the bacterial cell wall?
A) Cellulose
B) Chitin
C) Peptidoglycan
D) Phospholipid bilayer
Answer: C
Explanation: Peptidoglycan is the defining structural polymer in bacterial cell walls.
Why the top distractor is wrong: Cellulose is found in plant cell walls, not bacterial.

Question: Which of the following is true of human red blood cells?
A) They contain mitochondria for ATP production
B) They divide by mitosis
C) They have a nucleus during maturity
D) They lack a nucleus and most organelles
Answer: D
Explanation: Mature human red blood cells lack a nucleus and organelles to maximize hemoglobin capacity.
Why the top distractor is wrong: They do not have mitochondria and rely on anaerobic metabolism.

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

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have nucleus with nuclear envelope.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA.
• Bacterial cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall.
• Mycoplasma has no cell wall – naturally penicillin-resistant.
• Mature red blood cells lack nucleus and organelles.
• Both domains have phospholipid bilayer plasma membrane.
• Eukaryotes have internal membranes (ER, Golgi, lysosomes); prokaryotes do not.
• Nuclear pores allow RNA/protein transport; absent in prokaryotes.
• Rough ER has ribosomes; smooth ER makes lipids and detoxifies.
• Golgi apparatus modifies and packages proteins; cis receives, trans ships.
• Mitochondria produce ATP; double membrane; matrix inside.
• Chloroplasts have thylakoids and grana; site of photosynthesis.
• Lysosomes contain acid hydrolases; active at low pH.
• Plant vacuole maintains turgor pressure; stores ions and pigments.
• Centrioles in animal cells form spindle fibers; absent in higher plants.
• Eukaryotic flagella: 9+2 microtubule array; bends for movement.
• Prokaryotic flagella: flagellin protein; rotates like a propeller.
• Endosymbiotic theory: mitochondria from alpha-proteobacteria; chloroplasts from cyanobacteria.
• Archaea are prokaryotes but share transcription machinery with eukaryotes.
• Binary fission = prokaryotic division; no mitosis.
• Plasmids = small, circular DNA; common in bacteria; rare in eukaryotes.
• Cytoskeleton: actin, tubulin, intermediate filaments – eukaryote-specific; FtsZ in prokaryotes.