Protoplasm is the living substance inside cells that makes life possible. Think of a cell as a tiny room: protoplasm is everything inside that room that's actually alive and working, a jellylike material that contains all the machinery needed for the cell to function, grow, and reproduce.

Scientists in the 1800s discovered that all living things, from bacteria to elephants to redwood trees, are made of cells filled with this remarkable substance. When you look at a drop of pond water under a microscope, you might see an amoeba moving and changing shape. That movement comes from its protoplasm flowing and shifting inside the cell membrane.

Protoplasm includes the nucleus (the cell's control center that holds DNA) and the cytoplasm (the jellylike fluid surrounding the nucleus where the cell does most of its work). It contains proteins, water, salts, and countless tiny structures called organelles that act like miniature organs, each with specific jobs: some produce energy, some build proteins, and some clean up waste.

Modern biologists rarely use the word protoplasm anymore, preferring more specific terms like cytoplasm and nucleoplasm. But understanding protoplasm helps explain something profound: whether you're examining a single-celled organism or your own body's trillions of cells, the same basic living substance makes it all work.