Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 27.1 Structural and Functional Adaptations Contribute to Prokaryotic Success
Prokaryotes—organisms in the domains Bacteria and Archaea—are small, single-celled, and incredibly diverse. Their unique structures and survival strategies allow them to thrive in extreme and ordinary environments, making them the most abundant and adaptable life forms on Earth.
Cell Structure and Shape
- Most prokaryotes are unicellular and small (0.5–5 μm in diameter).
- Common shapes include:
- Cocci (spherical)
- Bacilli (rod-shaped)
- Spirals
- Despite their simplicity, prokaryotes carry out all life functions within a single cell.
Cell Walls and Capsules
- A cell wall provides shape, protection, and prevents bursting in hypotonic environments.
- Bacterial walls contain peptidoglycan (unlike eukaryotic cell walls).
- Gram staining distinguishes:
- Gram-positive bacteria: Thick peptidoglycan layer.
- Gram-negative bacteria: Thin peptidoglycan layer and outer membrane with toxic lipopolysaccharides.
- Capsules and slime layers help prokaryotes adhere to surfaces and avoid immune defenses.
Specialized Structures
- Fimbriae: Short hairlike appendages that help cells stick to surfaces or each other.
- Pili: Longer structures that pull cells together for DNA transfer.
- Endospores: Dormant, highly resistant cells formed under harsh conditions (e.g., Bacillus anthracis).
Motility and Flagella
- About half of prokaryotes exhibit taxis (movement toward/away from stimuli).
- Flagella are the main motility structures:
- Structurally and functionally different from eukaryotic flagella.
- Powered by a motor driven by proton flow.
- Evolved through exaptation: modified from proteins in other bacterial systems.
Internal Organization
- Prokaryotes lack membrane-bound organelles.
- Some have infolded membranes for photosynthesis or respiration.
- DNA is circular and concentrated in the nucleoid (no nuclear membrane).
- Plasmids: Small, independently replicating DNA rings.
Reproduction and Growth
- Prokaryotes reproduce via binary fission, often rapidly (as fast as every 20 minutes).
- Under optimal conditions, their populations can grow exponentially—but natural limits like resource depletion and waste accumulation prevent indefinite growth.
Rapid Evolution via Mutation
- Though mutation rates are low per gene, high reproduction rates and large populations generate vast genetic diversity.
- This leads to rapid evolutionary adaptation, as shown in long-term E. coli experiments where populations improved growth in low-glucose environments over 20,000 generations.
In a Nutshell
Prokaryotes are structurally simple but functionally sophisticated. Their cell walls, motility structures, and rapid reproduction enable them to thrive in diverse environments. Mutations and adaptations occur quickly, fueling evolutionary success and immense biodiversity.