Cell Division

Growth at the cellular level isn't subtle. Cells don't gradually expand like balloons inflating. They grow by splitting in half - a process called mitosis that transforms one cell into two identical daughter cells.

Here's what actually happens: The cell duplicates all its chromosomes (the instruction manuals), organizes them into neat pairs, then literally tears itself in two. The entire cell cycle - from one cell to two daughter cells - takes about 24 hours in typical human cells (though this varies by cell type: some divide faster, some slower). When it's done, you have two cells where you had one. Each has a complete copy of the original's DNA, the same organelles, the same basic capabilities.

This is profoundly inefficient. Why not just make cells bigger? Because cells have surface area problems.

A cell needs to take in nutrients through its membrane (surface area) to feed its entire volume. As a cell grows, its volume increases as the cube of its radius (V = 4/3πr³), but its surface area only increases as the square (A = 4πr²). Double a cell's diameter, you quadruple its surface area but octuple its volume. The bigger you get, the harder it is to feed yourself.

So cells divide instead. Two small cells have more total surface area for the same total volume than one big cell.