AP Biology: Prokaryotic vs Eukaryotic Cells

Prokaryotic vs Eukaryotic Cells

Concept Summary

• Prokaryotic cells: Unicellular organisms lacking a nucleus and membrane-bound organelles; include Bacteria and Archaea.
• Eukaryotic cells: Cells with a nucleus and membrane-bound organelles; found in protists, fungi, plants, and animals.
• Nucleoid: Region in prokaryotes where DNA is localized (not membrane-bound); critical for genetic regulation.
• Endosymbiotic theory: Explains the origin of mitochondria and chloroplasts from engulfed prokaryotes in eukaryotic evolution.
• Peptidoglycan: Polymer in bacterial cell walls (absent in Archaea and eukaryotes); target for antibiotics like penicillin.

Core Questions

WHAT (definitional)

Q: What is the defining structural difference between prokaryotic and eukaryotic cells? A: Prokaryotes lack a nucleus and membrane-bound organelles, while eukaryotes have both. Trap/Clarification: All cells have ribosomes, but prokaryotic ribosomes (70S) differ in size from eukaryotic ribosomes (80S).

Q: What is a plasmid? A: Small, circular DNA molecules in prokaryotes (and some eukaryotes) that replicate independently and often carry antibiotic resistance genes. Trap/Clarification: Plasmids are not chromosomes—they are accessory genetic elements.

WHY (causal/explanatory)

Q: Why do prokaryotes lack mitochondria? A: Prokaryotes perform cellular respiration via enzymes embedded in their plasma membrane (no need for membrane-bound organelles). Trap/Clarification: Some prokaryotes do perform aerobic respiration, but they use the cell membrane, not mitochondria.

Q: Why is compartmentalization (e.g., organelles) important in eukaryotes? A: It allows specialization of functions (e.g., lysosomes for digestion, ER for protein synthesis) and increases metabolic efficiency. Trap/Clarification: Prokaryotes can have internal membranes (e.g., thylakoids in cyanobacteria), but these are not true organelles.

HOW (process/application)

Q: How do prokaryotes reproduce? A: Primarily via binary fission: DNA replicates, the cell elongates, and the plasma membrane pinches inward to divide the cell. Trap/Clarification: Binary fission is not mitosis—prokaryotes lack spindle fibers and a nucleus.

Q: How is genetic variation introduced in prokaryotes? A: Through horizontal gene transfer: conjugation (pilus-mediated DNA transfer), transformation (uptake of free DNA), or transduction (virus-mediated). Trap/Clarification: Prokaryotes do not undergo meiosis or sexual reproduction.

CAN (conditions/possibilities)

Q: Can prokaryotes perform photosynthesis? A: Yes—cyanobacteria use thylakoid membranes (not chloroplasts) to perform oxygenic photosynthesis. Trap/Clarification: Not all prokaryotes are photosynthetic; only specific groups (e.g., cyanobacteria) have this capability.

Q: Under what conditions can a eukaryotic cell lack a nucleus? A: Mature red blood cells (mammals) and sieve tube elements (plants) lose their nuclei during differentiation to maximize space for oxygen/fluid transport. Trap/Clarification: These are exceptions—most eukaryotic cells retain their nucleus.

Quick Facts & Traps

• Fact: Prokaryotes have a single, circular chromosome; eukaryotes have multiple linear chromosomes.
• Trap: "Prokaryotes are simpler and less evolved."-Reality: Prokaryotes are highly adapted to diverse environments (e.g., extremophiles) and have complex metabolic pathways.
• Fact: Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thin layers + an outer lipid membrane.
• Trap: "All prokaryotes have cell walls."-Reality: Mycoplasma (a bacterial genus) lack cell walls.
• Fact: Endospores (e.g., in Bacillus and Clostridium) allow prokaryotes to survive extreme conditions (heat, radiation, desiccation).
• Trap: "Eukaryotes are always larger than prokaryotes."-Reality: Some prokaryotes (e.g., Thiomargarita) are larger than small eukaryotes (e.g., Paramecium).

Rapid-Fire True/False

• Statement: Prokaryotes cannot perform aerobic respiration. Answer: FALSE Why the common mistake happens: Students confuse "lack of mitochondria" with "inability to respire aerobically."

• Statement: Eukaryotic cells always have a cell wall. Answer: FALSE Why the common mistake happens: Plant cells (eukaryotic) have cell walls, but animal cells (also eukaryotic) do not.

• Statement: The nucleoid in prokaryotes is equivalent to the nucleus in eukaryotes. Answer: FALSE Why the common mistake happens: Both contain DNA, but the nucleoid lacks a membrane and organized structure.