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IB DP Biology — A: Unity and Diversity | SL | A3.1 Diversity of Organisms
📋 What You Will Learn
- Identify the structural features that distinguish prokaryotic cells from eukaryotic cells, including features unique to bacteria and archaea
- Explain the diversity of prokaryotes in terms of metabolism, habitat, and ecological role, including autotrophic and heterotrophic strategies
- Describe the defining characteristics of protists as a diverse, paraphyletic group of eukaryotes
- Analyse how structural features of prokaryotes and protists relate to their functions and ecological roles
- Apply knowledge of prokaryote and protist diversity to unfamiliar biological contexts, including data on cell size, morphology, and metabolic activity
🔍 Guiding Question How does the diversity of life at the cellular level reflect the unity of life's fundamental processes?
🌍 Why This Matters A single gram of soil contains up to one billion prokaryotic cells — more individual organisms than there are humans on Earth. These microscopic entities fix nitrogen, decompose organic matter, and drive the carbon cycle. Without prokaryotes, every ecosystem on the planet would collapse within weeks.
📋 Real-Life Context
In 1977, Carl Woese and George Fox made a discovery that rewrote the tree of life. By comparing ribosomal RNA sequences across organisms, they found that what had been called "bacteria" was actually two profoundly different domains of life — Bacteria and Archaea — united only by their lack of a membrane-bound nucleus. This single observation forced biologists to reconsider what it means to be "simple." Prokaryotes are not primitive failures on the way to becoming eukaryotes. They are extraordinarily successful, metabolically diverse, and ancient — the dominant life form for most of Earth's history.
Protists complicate the story further. They are eukaryotes — cells with nuclei, membrane-bound organelles, and complex internal architecture — but they are not animals, plants, or fungi. They are everything else. Understanding both groups is essential to understanding the full breadth of cellular life.
What Is a Prokaryote? Defining the Cell Without a Nucleus
Here is the problem that defined the first two billion years of life on Earth: how do you run a cell without compartmentalisation? Eukaryotes solved this by evolving a nucleus, mitochondria, and an endomembrane system. Prokaryotes never did — and yet they thrive in every environment on Earth, from hydrothermal vents to the human gut. Their simplicity is not a limitation. It is a different solution to the same problem.
📖 Key Definition — Prokaryote A prokaryote is a unicellular organism that lacks a membrane-bound nucleus and other membrane-bound organelles. Genetic material is located in a region of the cytoplasm called the nucleoid. Prokaryotes include all members of the domains Bacteria and Archaea.
Universal Prokaryotic Features
All prokaryotes share a set of structural features that define the prokaryotic cell plan. These features are present in both Bacteria and Archaea, regardless of their metabolic diversity or habitat.
- No membrane-bound nucleus: DNA is located in the nucleoid region — a defined area of the cytoplasm but not enclosed by a membrane.
- Single circular chromosome: The main genetic material is a single, circular DNA molecule, typically not associated with histone proteins in bacteria (though archaea do use histone-like proteins).
- Ribosomes (70S): Prokaryotic ribosomes are smaller than eukaryotic ribosomes — 70S compared to 80S. This difference is clinically significant: many antibiotics target the 70S ribosome specifically.
- Plasma membrane: A phospholipid bilayer that controls the movement of substances into and out of the cell.
- Cytoplasm: The aqueous interior of the cell where all metabolic reactions occur.
- Cell wall (in most prokaryotes): A rigid layer external to the plasma membrane that provides structural support and prevents osmotic lysis.
Features Present in Some Prokaryotes
Not all prokaryotes are identical. Several additional structures appear in some species and are directly linked to specific functions. Knowing these structures — and what they do — is essential for understanding prokaryotic diversity.
- Plasmids: Small, circular, double-stranded DNA molecules separate from the main chromosome. They