STEM Readiness: Biology Readiness - Physiology Preview: Immune System - Innate vs. Adaptive, Antigen-Antibody, T and B Cells, Memory

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 but may have pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls are made of chitin; animal cells lack a cell wall.
• All cells have a phospholipid bilayer plasma membrane; eukaryotes have internal membrane systems (e.g., ER, Golgi).
• Eukaryotes possess membrane-bound organelles including nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.
• Prokaryotes lack membrane-bound organelles; metabolic functions occur in the cytoplasm or at the plasma membrane.
• Bacteria and archaea are prokaryotes; animals, plants, fungi, and protists are eukaryotes.
• Archaea resemble eukaryotes in transcription and translation machinery but are structurally prokaryotic.
• Binary fission is the primary mode of reproduction in prokaryotes; mitosis occurs in eukaryotes.
• Plasmids are small, circular DNA molecules commonly found in prokaryotes; rare in eukaryotes.
• Eukaryotic chromosomes are linear and associated with histone proteins; prokaryotic chromosomes are circular and lack histones (though archaea have histone-like proteins).
• Cytoskeleton is present in eukaryotes (microtubules, microfilaments, intermediate filaments) and some prokaryotes (homologs like FtsZ).
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and bend.
• Endosymbiotic theory proposes mitochondria and chloroplasts originated from engulfed aerobic bacteria and cyanobacteria, respectively.
• Evidence for endosymbiosis includes double membranes, circular DNA, 70S ribosomes, and independent replication of mitochondria and chloroplasts.
• Mycoplasma species are bacteria without a cell wall, making them resistant to antibiotics like penicillin.
• Mature mammalian red blood cells lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Plant cells have large central vacuoles for turgor pressure and storage; animal cells have small or temporary vacuoles.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; rare in plant cells.
• Chloroplasts are photosynthetic organelles in plants and algae, absent in prokaryotes except in cyanobacteria (which perform photosynthesis in thylakoid membranes without chloroplasts).
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin peptidoglycan and an outer membrane with lipopolysaccharide.

Difficulty Level

Intermediate – frequently tested with emphasis on comparison and exceptions in first-year biology courses.

Common Traps

• Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (other polymers) do not.
• Trap: Eukaryotes are always larger and more complex than prokaryotes – Fact: While generally true, some prokaryotes (e.g., Thiomargarita namibiensis, up to 750 ?m) exceed typical eukaryotic size.
• Trap: Mitochondria and chloroplasts use 80S ribosomes like the host cell – Fact: They use 70S ribosomes, similar to bacteria, supporting endosymbiotic origin.
• Trap: Prokaryotes have no internal structure – Fact: Prokaryotes have nucleoids, ribosomes, inclusions, and some have protein scaffolds or membrane invaginations.

Practice MCQs

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Peptidoglycan in the cell wall
D) Plasma membrane with phospholipid bilayer
Answer: C
Explanation: Peptidoglycan is unique to bacterial cell walls and absent in eukaryotes.
Why the top distractor is wrong: While 70S ribosomes are in prokaryotes, they are also present in mitochondria and chloroplasts of eukaryotes.

Question: Which structure is present in plant cells but not in animal cells?
A) Mitochondria
B) Golgi apparatus
C) Central vacuole
D) Nucleus
Answer: C
Explanation: The large central vacuole is a defining feature of mature plant cells.
Why the top distractor is wrong: Mitochondria are present in both plant and animal cells.

Question: What is the primary evidence that mitochondria originated from free-living bacteria?
A) They are surrounded by a single membrane
B) They contain 80S ribosomes
C) They have circular DNA and 70S ribosomes
D) They replicate by mitosis
Answer: C
Explanation: Mitochondria have circular DNA and 70S ribosomes, similar to bacteria.
Why the top distractor is wrong: Mitochondria have double membranes and replicate by binary fission, not mitosis.

Question: Which of the following organisms lacks a nucleus and has 70S ribosomes?
A) Saccharomyces cerevisiae (yeast)
B) Escherichia coli
C) Homo sapiens
D) Arabidopsis thaliana
Answer: B
Explanation: E. coli is a prokaryote with no nucleus and 70S ribosomes.
Why the top distractor is wrong: Yeast is a eukaryote and has 80S ribosomes and a nucleus.

Question: Which cell type lacks mitochondria but can still perform aerobic respiration?
A) Mature red blood cells
B) Neurons
C) Bacteria
D) Liver cells
Answer: C
Explanation: Some prokaryotes perform aerobic respiration using the plasma membrane, not mitochondria.
Why the top distractor is wrong: Mature red blood cells lack mitochondria and rely on anaerobic glycolysis.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 80S and located in the cytoplasm
B) 70S ribosomes are found in the nucleus
C) 80S ribosomes are found in mitochondria
D) 70S ribosomes are found in mitochondria
Answer: D
Explanation: Mitochondria contain 70S ribosomes, distinct from the 80S ribosomes in the cytoplasm.
Why the top distractor is wrong: Not all ribosomes in eukaryotes are 80S; organelles have 70S.

Question: Which organism has a cell wall but does not contain peptidoglycan?
A) Streptococcus pneumoniae
B) Mycobacterium tuberculosis
C) Penicillium chrysogenum (fungus)
D) Bacillus subtilis
Answer: C
Explanation: Fungi have cell walls made of chitin, not peptidoglycan.
Why the top distractor is wrong: Mycobacterium has peptidoglycan, though with a waxy mycolic acid layer.

Last?Minute Revision

• Prokaryotic cell size: 0.1–5.0 ?m
• Eukaryotic cell size: 10–100 ?m
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA
• Bacterial cell walls contain peptidoglycan; archaea do not
• Plant cell wall = cellulose; fungal = chitin; animal = no cell wall
• All cells have phospholipid bilayer membranes
• Eukaryotes have internal membranes (ER, Golgi, nucleus)
• Prokaryotes lack membrane-bound organelles
• DNA in prokaryotes is in the nucleoid (no membrane)
• DNA in eukaryotes is in the nucleus (double membrane)
• Mycoplasma – bacteria without cell walls
• Mature red blood cells lack a nucleus and mitochondria
• Endosymbiotic theory explains origin of mitochondria and chloroplasts
• Evidence: double membrane, binary fission, 70S ribosomes, circular DNA
• Archaea are prokaryotes but share molecular machinery with eukaryotes
• Flagellin = prokaryotic flagella protein; microtubules (9+2) = eukaryotic
• Plasmids common in bacteria, rare in eukaryotes
• Histones in eukaryotes and archaea, not in bacteria
• Cytoskeleton in eukaryotes (actin, tubulin); prokaryotes have FtsZ, MreB
• Gram-positive = thick peptidoglycan; Gram-negative = thin peptidoglycan + outer membrane
• Thiomargarita namibiensis can be up to 750 ?m – larger than many eukaryotes
• Cyanobacteria perform photosynthesis without chloroplasts
• verify from standard textbook: exact composition of archaeal cell walls varies by species