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Life comes in an enormous range of sizes. It can be as small as a bacterium, which is invisible to the unaided eye. Or it can be large enough to fi t half a dozen humans in its jaws alone, such as the school-bus-sized, prehistoric Megalodon shark. Scientists can only observe Megalodon through the fossils it left behind. Due to its huge size, however, it was made up of hundreds of trillions of cells. A single-celled organism relies on the organelles within its one cell to carry out its life functions. A multi-celled organism requires a greater degree of organization to survive. Most of the cells in a multi-celled organism have diff erent structures and abilities that help them carry out specifi c functions. But even these specialized cells are limited in what they can do on their own. As a result, the survival of a multi-celled organism relies on teamwork. Working together, specialized cells carry out life functions, such as digestion, breathing, and circulation. Such organization is key to developing complex life forms such as Megalodon and you. Key Concepts • All cells begin alike and differentiate into specialized cells. • Specialized cells have different structures that allow them to perform unique functions. • Groups of cells working together form tissues. • Groups of tissues working together form organs, which work together in systems. All cells begin alike and differentiate into specialized cells. Imagine that you are building a shelving unit. You cut the wood, join the shelves together, and mount the unit on the wall. Now imagine that you have only one tool, such as a hammer, to work with. Will you have a harder time completing the job? Aft er all a hammer is great for pounding nails but is inefficient for cutting wood. To build a shelving unit successfully, you need tools that are specialized for the different tasks you must complete. As the construction task becomes more complex, such as building a shed, the tools you need become more specialized. In this sense, organisms have a lot in common with construction jobs—as they become more complex, more specialized tools (or cells) are required to perform different functions.