STEM Readiness: Biology Readiness - Evolution/Ecology: Ecosystem Ecology - Energy Flow, 10% Rule, Nutrient Cycles, Primary Productivity

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 have endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria.
• Ribosomes in prokaryotes are 70S (composed of 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S and 40S).
• Mitochondria and chloroplasts in eukaryotes have 70S ribosomes, supporting their origin via endosymbiotic theory.
• Cell walls are present in most prokaryotes, made of peptidoglycan; archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Plant cells have cellulose in their cell walls; fungi have chitin; animal cells lack a cell wall entirely.
• Both prokaryotes and eukaryotes have a phospholipid bilayer plasma membrane with embedded proteins.
• Eukaryotes have internal membrane systems (e.g., nuclear envelope, ER, Golgi); prokaryotes do not.
• Prokaryotes reproduce by binary fission; eukaryotes undergo mitosis and meiosis.
• Eukaryotic chromosomes are linear and associated with histone proteins; prokaryotic DNA is circular and not bound by histones (though archaea have histone-like proteins).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and undulate.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Mycoplasma, a bacterium, lacks a cell wall—an exception among prokaryotes.
• Mature red blood cells in mammals lack a nucleus and most organelles—exception among eukaryotic cells.
• Chloroplasts are found in plant cells and some protists; absent in animal and fungal cells.
• Lysosomes are membrane-bound organelles containing hydrolytic enzymes; present in animal cells, rare in plant cells (vacuole performs similar function).
• The nuclear envelope in eukaryotes is a double membrane with nuclear pores that regulate transport.
• Nucleolus is located within the nucleus and is the site of ribosomal RNA synthesis and ribosome assembly.
• Endosymbiotic theory is supported by mitochondria and chloroplasts having their own circular DNA, 70S ribosomes, and double membranes.
• Prokaryotes can have capsules (e.g., Streptococcus pneumoniae) for protection; some eukaryotes have extracellular matrices (e.g., animal connective tissue).
• Cytoskeleton is present in both cell types: prokaryotes have FtsZ, MreB, CreS proteins; eukaryotes have microtubules, actin filaments, intermediate filaments.
• Peroxisomes are single-membrane organelles in eukaryotes that break down fatty acids and detoxify hydrogen peroxide; absent in prokaryotes.
• Plant cells have a large central vacuole for storage and turgor pressure; animal cells have small or transient vacuoles.

Difficulty Level

Intermediate – requires distinguishing structural and functional differences across domains and recognizing exceptions common in introductory biology exams.

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.
Trap: Ribosome size is the same across all organisms – Fact: Prokaryotes have 70S ribosomes; eukaryotes have 80S in cytoplasm (but 70S in mitochondria and chloroplasts).
Trap: The nucleus is the only organelle that contains DNA in eukaryotes – Fact: Mitochondria and chloroplasts also contain DNA.
Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, and some have protein-based cytoskeletal elements.
Trap: Eukaryotic cells are always larger because they are more complex – Fact: Size is not strictly correlated with complexity; some eukaryotes (e.g., Ostreococcus) are <1 ?m.

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) Linear chromosomes associated with histones
D) DNA located in a nucleoid region
Answer: D
Explanation: Prokaryotes lack a nucleus; their DNA is in the nucleoid.
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 containing 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 central vacuoles are characteristic of plant cells.
Why the top distractor is wrong: A is incorrect because fungal cell walls are made of chitin, not cellulose.

Question: Which structure is present in both prokaryotes and eukaryotes?
A) Mitochondria
B) Endoplasmic reticulum
C) Plasma membrane
D) Golgi apparatus
Answer: C
Explanation: Both cell types have a phospholipid bilayer plasma membrane.
Why the top distractor is wrong: A is incorrect because mitochondria are only in eukaryotes.

Question: Mitochondria are thought to have evolved from free-living prokaryotes due to all of the following EXCEPT:
A) Presence of 70S ribosomes
B) Circular DNA
C) Double membrane
D) Use of 80S ribosomes for protein synthesis
Answer: D
Explanation: Mitochondria use 70S ribosomes, not 80S.
Why the top distractor is wrong: D is incorrect because 80S ribosomes are found in the eukaryotic cytoplasm, not in mitochondria.

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: A is incorrect because yeast is a eukaryotic fungus and has a nucleus.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 80S, including those in mitochondria
B) Cytoplasmic ribosomes are 80S; mitochondrial ribosomes are 70S
C) Ribosomes are enclosed within the nucleus
D) Ribosomes are composed of DNA and protein
Answer: B
Explanation: Eukaryotic cytoplasmic ribosomes are 80S; mitochondrial and chloroplast ribosomes are 70S.
Why the top distractor is wrong: A is incorrect because mitochondrial ribosomes are 70S, not 80S.

Question: Which of the following is an exception to the typical eukaryotic cell structure?
A) Presence of lysosomes in animal cells
B) Cellulose in plant cell walls
C) Lack of a nucleus in mature mammalian red blood cells
D) Double membrane surrounding the nucleus
Answer: C
Explanation: Mature mammalian red blood cells lack a nucleus, unlike most eukaryotic cells.
Why the top distractor is wrong: A is incorrect because lysosomes are typical in animal cells.

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 cell wall contains peptidoglycan; archaea do not.
• Plant cell wall = cellulose; fungal cell wall = chitin; animal cells = no cell wall.
• Both cell types have phospholipid bilayer plasma membranes.
• Eukaryotes have internal membranes (ER, Golgi, nucleus); prokaryotes do not.
• Prokaryotes divide by binary fission; eukaryotes by mitosis/meiosis.
• Prokaryotic DNA is circular; eukaryotic chromosomes are linear.
• Flagellin = prokaryotic flagella protein; 9+2 microtubule array = eukaryotic flagella.
• Plasmids are common in prokaryotes, rare in eukaryotes.
• Mycoplasma has no cell wall – exception among bacteria.
• Mature human red blood cells lack a nucleus – exception among eukaryotes.
• Chloroplasts found in plants and some protists, not in animals or fungi.
• Lysosomes are common in animal cells, less so in plant cells.
• Nuclear envelope = double membrane with nuclear pores.
• Nucleolus = site of rRNA synthesis and ribosome assembly.
• Endosymbiotic theory supported by mitochondrial DNA, ribosomes, and double membrane.
• Cytoskeleton in prokaryotes: FtsZ, MreB, CreS; eukaryotes: actin, microtubules, intermediate filaments.
• Peroxisomes break down fatty acids and detoxify H?O?; only in eukaryotes.
• Plant vacuole = large and central; animal vacuoles = small and temporary.
• Archaea lack peptidoglycan but may have pseudopeptidoglycan.
• Histones are in eukaryotes and archaea, not in bacteria.
• Verify from standard textbook: exact protein composition of archaeal cell walls.