Biology for AP Courses · Prokaryotes: Bacteria and Archaea
Prokaryotic Diversity
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In 30 seconds
Prokaryotes are single-celled organisms that lack a nucleus and other membrane-bound organelles; their DNA sits free in the cytoplasm in a region called the Nucleoid Region where the prokaryotic circular chromosome sits, without a membrane Full entry →. For roughly the first two-thirds of Earth's history, prokaryotes were the only life on the planet, and today they remain the most abundant and widespread organisms on Earth — in soil, water, ice, hot springs, deep-ocean vents, inside other organisms, and in environments that kill most other life. Only a small minority cause human disease; the vast majority are harmless or essential, cycling nutrients, producing oxygen, and living in and on our bodies.
The key structural idea: prokaryotes are not "simple eukaryotes" — they are a distinct organization of life. The key evolutionary idea: what we used to call "bacteria" actually comprises two separate domains, Bacteria and Archaea Domain of prokaryotes molecularly distinct from Bacteria; many extremophiles Full entry →. Though they look similar under a microscope, the two domains are as different from each other, genetically and biochemically, as either is from eukaryotes. Recognizing that division — and the ribosomal RNA evidence that established it — is the backbone of this topic.
Why this matters
Prokaryotes run the planet's biogeochemical cycles: Cyanobacteria Photosynthetic bacteria that release oxygen Full entry → produce much of the atmosphere's oxygen, bacteria and archaea drive the nitrogen cycle (including nitrogen fixation that plants depend on), and decomposers recycle carbon in every ecosystem. They shape human health both ways: pathogens such as Mycobacterium tuberculosis cause disease, while the trillions of prokaryotes in and on our bodies (the microbiome) help digest food, make vitamins, and train the immune system. Understanding prokaryotic diversity also sets up the Endosymbiotic theory Idea that mitochondria and chloroplasts originated from engulfed prokaryotes Full entry → — that mitochondria and chloroplasts descend from ancient prokaryotes engulfed by early eukaryotic cells — which AP exams test directly. Finally, Archaea's extremophiles (organisms thriving in boiling springs, hypersaline lakes, or acidic drainage) expand the definition of habitable environments, informing even the search for life beyond Earth.
The college version
Core Concepts
Prokaryotes versus eukaryotes: the organizational difference
Both cell types have plasma membranes, ribosomes, and DNA, but organization differs fundamentally. Prokaryotes have no nucleus (DNA is a usually-circular chromosome in the nucleoid), no membrane-bound organelles (metabolism runs in the cytoplasm or at the plasma membrane), and are generally much smaller. Their ribosomes are 70S (50S + 30S subunits) versus the 80S ribosomes of eukaryotic cytoplasm — a difference some antibiotics exploit by targeting the 70S ribosome, harming bacteria more than human cells. Most bacteria also have cell walls containing peptidoglycan, which eukaryotic cells lack. These differences are not deficiencies — the prokaryotic design has an extraordinary 3.5-billion-year track record.
Two domains: Bacteria and Archaea
In the 1970s, Carl Woese and colleagues compared small-subunit ribosomal RNA sequences across organisms and found that the prokaryotes split into two deeply separate lineages. This produced the three-domain system: Bacteria, Archaea, and Eukarya. In this tree, Archaea are molecularly closer to eukaryotes than to Bacteria in several respects (certain ribosomal proteins, DNA-replication machinery), even though archaeal cells look like bacterial cells.
Archaea are famous for extremophiles: thermophiles thrive at very high temperatures (Yellowstone hot springs, deep-sea vents); halophiles require very salty environments (Great Salt Lake, salt ponds, which they can color pink); methanogens live in oxygen-free environments (swamps, ruminant guts, the human colon) and produce methane; acidophiles tolerate very acidic conditions. But not all archaea are extremophiles — gut methanogens live in moderate environments too. Biochemically, archaea differ from bacteria in membrane lipids (ether-linked versus ester-linked, contributing to stability at extreme temperatures) and cell walls (many lack peptidoglycan, using other polymers or protein layers). This matters for antibiotics: many drugs targeting bacterial peptidoglycan do not affect archaea.
Where prokaryotes live and what they do
Prokaryotes occupy essentially every habitat with liquid water: photosynthetic bacteria are major ocean primary producers; soil prokaryotes decompose organic matter and recycle nutrients; the deep biosphere runs on chemical energy kilometers below the surface. Ecologically they play four great roles: decomposers (breaking down dead organic matter), producers (photosynthesis or chemosynthesis), nitrogen fixers (converting atmospheric N₂ into usable forms), and symbionts (mutualistic, neutral, or harmful relationships with other organisms). Many also form biofilms — dense, polymer-embedded communities attached to surfaces that resist treatment, responsible for everything from dental plaque to infections on medical implants.
The earliest life and the oxygen revolution
The fossil record of life begins with prokaryotes: stromatolites — layered structures built by photosynthetic Prokaryote Single-celled organism lacking a nucleus and membrane-bound organelles Full entry → communities — appear in rocks roughly 3.5 billion years old and are still built today in places like Shark Bay, Australia. Around 2.4 billion years ago, oxygenic photosynthesis by cyanobacteria triggered the Great Oxidation Event, transforming the atmosphere and making aerobic metabolism possible for everything that followed. The connection to our own cells is the endosymbiotic theory: mitochondria descend from an ancient alpha-proteobacterium engulfed by an early eukaryotic cell, and chloroplasts from an ancient cyanobacterium — which is why these organelles have their own DNA and 70S ribosomes, like bacteria. Prokaryotic diversity is not a side note in life's history; it is the opening chapter.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Bacteria | Archaea | Both are prokaryotes without nuclei, but they are separate domains with different membrane lipids and cell walls |
| "All prokaryotes are bacteria" | The two-domain reality | Prokaryotes include both Bacteria and Archaea; rRNA evidence split the old "bacteria" group in two |
| Prokaryote | Eukaryote | Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both (and 80S cytoplasmic ribosomes) |
| "No nucleus = no DNA" | Nucleoid organization | Prokaryotes have plenty of DNA — it is just not enclosed in a nuclear membrane |
| "Archaea only live in extreme environments" | Archaea in moderate habitats | Many archaea (e.g., gut methanogens) live in mild environments; extremophily is common but not universal |
| "All prokaryotes cause disease" | The small pathogenic minority | Most prokaryotes are harmless or beneficial; a small fraction are pathogens |
| Viruses | Prokaryotes | Viruses are acellular (no cells, no metabolism of their own); prokaryotes are fully cellular organisms |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Prokaryotes are like tiny single-room apartments — no walls inside, and the instructions (DNA) just sit loose in the middle. "Bacteria" and "archaea" are two big families that look almost the same under a microscope but are actually very different cousins: archaea include champions that love boiling water, giant salt lakes, or places with no oxygen. Most of these tiny creatures are not out to get you — many make oxygen, help plants get food, or live happily in your gut.
Worked example
Picture a shallow pond beside a hot spring. In the steaming water at the source, thermophilic archaea grow at temperatures that would denature most proteins. A few meters downstream, where the water has cooled, cyanobacteria form green mats bubbling oxygen — a layer may be building a stromatolite right now. At the muddy bottom, where oxygen is scarce, methanogenic archaea release methane bubbles. Now walk to a pasture beside the pond: inside a cow's rumen — an oxygen-free fermentation vat — methanogenic archaea produce methane the cow belches out, while bacteria ferment grass into compounds the cow can absorb. The same two domains that divide the pond cooperate inside the cow. Prokaryotic diversity is not a museum exhibit — it is the machinery of ecosystems, running in parallel everywhere from boiling springs to mammalian stomachs.
Key takeaways
- Prokaryote = no nucleus, no membrane-bound organelles; DNA in the nucleoid; 70S ribosomes; usually a cell wall (peptidoglycan in bacteria, not most archaea).
- The three-domain system (Bacteria, Archaea, Eukarya) is based on ribosomal RNA comparisons (Woese); Archaea are molecularly closer to Eukarya than to Bacteria in several respects.
- Archaea ≠ all extremophiles, but many are: thermophiles (hot springs, vents), halophiles (salt lakes), methanogens (oxygen-free environments, produce methane), acidophiles. Methanogens also live in the human gut.
- Bacteria and archaea differ biochemically: membrane lipids (ether vs ester linkages) and cell-wall composition (peptidoglycan vs other polymers).
- Ecological roles: decomposers, producers, nitrogen fixers, symbionts; biofilms are attached communities that resist treatment.
- Only a small minority of prokaryotes cause disease; most are harmless or essential (microbiome, nutrient cycling, oxygen production).
- Stromatolites (~3.5-billion-year-old layered microbial structures) are among the oldest evidence of life; cyanobacteria drove the Great Oxidation Event.
- Endosymbiotic theory: mitochondria and chloroplasts descend from engulfed prokaryotes — supported by their own DNA and 70S ribosomes.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List four structural differences between prokaryotic and eukaryotic cells.
Show answer
Prokaryotes lack a nucleus (DNA in the nucleoid) and membrane-bound organelles; they are generally smaller; their ribosomes are 70S (vs 80S in eukaryotic cytoplasm); most bacteria have peptidoglycan cell walls, which eukaryotic cells do not.
What evidence led to the three-domain system, and what are the three domains?
Show answer
Comparisons of small-subunit ribosomal RNA sequences (Woese and colleagues) showed prokaryotes split into two deeply separate lineages. The three domains: Bacteria, Archaea, Eukarya.
Give one example each of a thermophile, a halophile, and a Methanogen Archaeon producing methane in oxygen-free environments Full entry →, with the environment each thrives in.
Show answer
Thermophile: hot-spring or vent archaea; halophile: archaea of salt lakes or salt ponds; methanogen: archaea of swamps, ruminant guts, or the human colon. (Any accurate example works.)
How do archaea differ from bacteria in membrane lipids and cell walls?
Show answer
Archaeal membrane lipids are ether-linked (bacteria use ester-linked); archaeal cell walls generally lack peptidoglycan, using other polymers or protein layers instead.
What is the endosymbiotic theory, and what two lines of evidence support it?
Show answer
Mitochondria and chloroplasts originated from ancient prokaryotes (an alpha-proteobacterium and a cyanobacterium) engulfed by early eukaryotic cells. Evidence: both organelles have their own circular DNA and 70S ribosomes, and divide independently of the host cell.
Why are biofilms harder to eliminate than free-floating bacteria?
Show answer
Biofilms are dense communities embedded in a secreted polymer matrix that shields cells from antibiotics, disinfectants, and immune cells; biofilm cells also show altered metabolism that increases tolerance.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Prokaryote
- Single-celled organism lacking a nucleus and membrane-bound organelles
- Nucleoid
- Region where the prokaryotic circular chromosome sits, without a membrane
- Domain
- Highest taxonomic rank: Bacteria, Archaea, Eukarya
- Archaea
- Domain of prokaryotes molecularly distinct from Bacteria; many extremophiles
- Extremophile
- Organism thriving in extreme conditions (heat, salt, acidity, pressure)
- Methanogen
- Archaeon producing methane in oxygen-free environments
- Cyanobacteria
- Photosynthetic bacteria that release oxygen
- Stromatolite
- Layered rock-like structure built by photosynthetic prokaryote communities
- Endosymbiotic theory
- Idea that mitochondria and chloroplasts originated from engulfed prokaryotes
- Biofilm
- Community of microbes attached to a surface, embedded in secreted polymers
Sources & references
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