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If you scoop up a single drop of pond water, you are holding a bustling city of millions of tiny living creatures.
These creatures are unicellular organisms, which means their entire body is made of just one independent cell.
How do these tiny cells survive on their own without organs like a heart or lungs? Let's zoom into the simplest one first: bacteria.

Bacteria: The Simplest Cells

Bacteria are prokaryotes, meaning they are cells that do not have a nucleus to hold their DNA.
Instead, their genetic material floats freely in a tangled loop inside the jelly-like cytoplasm.
In 1676, Dutch scientist Antonie van Leeuwenhoek looked through his handmade microscope lenses and discovered these moving creatures, calling them 'animalcules'.
Most bacteria are protected by a rigid outer cell wall, and many use their whip-like flagellum like a mini boat propeller to swim.
What happens when a single-celled organism is much larger and packs a real nucleus inside? That brings us to protists.

Protists: Microscopic Giants

Protists are eukaryotes, meaning they have a distinct nucleus surrounded by a membrane that locks their DNA safely inside.