📌Definition Table

Term	Definition
Prokaryotic Cell	Simple cell without a nucleus or membrane-bound organelles.
Eukaryotic Cell	Cell with a nucleus and membrane-bound organelles.
Endosymbiotic Theory	Theory that eukaryotic organelles originated from engulfed prokaryotes.
Cyanobacteria	Photosynthetic bacteria that released oxygen into early Earth’s atmosphere.
Great Oxidation Event	Period when atmospheric oxygen levels rose sharply (~2.4 bya).

📌Introduction

Cellular evolution marks the transition from simple prokaryotes to complex eukaryotes. This shift was driven by environmental change, metabolic innovation, and symbiosis, ultimately enabling multicellular life.

📌 Early Prokaryotic Cells

• First life forms (~3.5 bya), anaerobic and simple in structure.
• Contained circular DNA and 70S ribosomes.
• Relied on fermentation or anaerobic respiration.
• Thrived in extreme environments.
• Played major roles in nutrient cycling.
• Formed the basis for all later life forms.

🧠 Examiner Tip: Mention ribosome type and DNA form when distinguishing prokaryotes from eukaryotes.

📌 Rise of Photosynthetic Prokaryotes

• Cyanobacteria evolved oxygenic photosynthesis.
• Released oxygen into oceans and atmosphere.
• Triggered Great Oxidation Event.
• Oxygen allowed aerobic respiration to evolve.
• Caused extinction of many anaerobic organisms.
• Set stage for complex life.

🧬 IA Tips & Guidance: Compare oxygen production in modern cyanobacteria under different light intensities.

📌 Endosymbiotic Theory

• Large prokaryotes engulfed smaller aerobic/phototrophic ones.
• Engulfed cells survived, forming symbiotic relationships.
• Mitochondria from aerobic bacteria; chloroplasts from cyanobacteria.
• Both have double membranes and circular DNA.
• Reproduce by binary fission.
• Provide strong evidence for symbiotic origins of eukaryotes.

🌐 EE Focus: Sequence mitochondrial DNA to compare with bacterial genomes.

📌 Transition to Eukaryotic Cells

• Development of nucleus and internal membranes.
• Cytoskeleton allowed structural support and movement.
• Compartmentalization improved efficiency.
• Endomembrane system evolved for transport and synthesis.
• Enabled greater cell size and complexity.
• Formed basis for multicellular life.

❤️ CAS Link: Create a classroom model showing prokaryote-to-eukaryote transition.

📌 Multicellularity and Specialization

• Evolved independently in plants, fungi, and animals.
• Cell adhesion proteins enabled tissue formation.
• Specialization increased efficiency and survival.
• Communication systems developed between cells.
• Allowed development of organs and body systems.
• Increased ecological diversity.

🔍 TOK Perspective: Endosymbiotic theory shows how scientific models evolve with new genetic evidence.

📝 Paper 2: Data Response Tips: Expect diagrams comparing prokaryotic and eukaryotic structures and identifying evolutionary evidence.