Prokaryotic Biology: Bacteria and Archaea Essentials

What Is a Prokaryote?

Prokaryotes are single-celled organisms that lack a nucleus; their DNA floats freely in the cytoplasm. There are two primary types: Bacteria and Archaea. These represent two of the three domains of life, the third being Eukarya.

Bacteria vs. Archaea

• Bacteria: These possess peptidoglycan in their cell walls.
• Archaea: These lack peptidoglycan. Their DNA is more similar to eukaryotes, and they often live in extreme environments such as hot springs, salty lakes, and deep mud.

Three Common Shapes

• Bacilli: Rod-shaped
• Cocci: Spherical
• Spirilla: Spiral-shaped

How Prokaryotes Capture Energy

• Heterotroph: Consumes other organisms for energy.
• Photoheterotroph: Consumes other organisms while also utilizing light.
• Photoautotroph: Uses light to produce its own food, similar to photosynthesis.
• Chemoautotroph: Uses chemicals to produce food; these often live in dark or harsh environments.

How Prokaryotes Release Energy

• Obligate aerobe: Requires oxygen to survive.
• Obligate anaerobe: Dies in the presence of oxygen; relies on fermentation.
• Facultative anaerobe: Can survive with or without oxygen, making them highly flexible.

Reproduction and Survival

• Binary fission: A process where a cell splits into two identical cells, occurring as often as every 20 minutes.
• Endospore: A thick protective shell formed during harsh conditions, allowing the organism to remain dormant for years.
• Conjugation: The process of sharing plasmid DNA through a hollow bridge between two cells.

Ecological Roles

• Decomposers: They break down dead organisms and recycle essential nutrients.
• Producers: They create food via photosynthesis (e.g., Prochlorococcus).
• Nitrogen fixers: They convert atmospheric nitrogen (N₂) into ammonia for plants (e.g., Rhizobium found in legume roots).

Additional Details on Producers

Prochlorococcus may account for more than half of the primary production in the open ocean. Cyanobacteria, such as Anabaena, form chains in ponds and other aquatic environments.

Additional Details on Nitrogen Fixers

Approximately 90% of the nitrogen used by living organisms comes from nitrogen-fixing bacteria and archaea. The remainder comes from weathering rocks or lightning combining oxygen and nitrogen in the atmosphere. Rhizobium grows in nodules or knobs on the roots of legumes like clover and soybean.

Human Uses of Prokaryotes

• Food: Used in the production of yogurt (e.g., Lactobacillus).
• Environment: Employed to clean up oil spills and remove toxins from water.
• Medicine: Used to manufacture drugs through genetic engineering.

Bacterial Cell Structure

Based on standard biological models, bacterial cell structures include:

• Outer membrane, peptidoglycan cell wall, cell membrane, ribosomes, DNA, pili, and flagellum.
• Some bacteria, such as E. coli, have a second outer membrane made of lipids, making them more resistant to damage.
• Pili: These help anchor bacteria to surfaces or to other bacteria.

Key Vocabulary

• Peptidoglycan: A polymer found in bacterial cell walls (but not in archaea).
• Plasmid: Small, circular DNA transferred during conjugation.
• Endospore: A survival structure for harsh conditions.
• Nitrogen fixation: The process of converting N₂ gas into ammonia.