Cells come in a variety of shapes and sizes. Prokaryotic cells, such as bacteria, have a diameter of a few micrometers, while eukaryotic cells vary in size due to structural specializations. Sperm cells are only about four micrometers, but neuronal cells may have long cytoplasmic extensions of several meters long. Such variations highlight the vital relationship between volume and surface area. Picture a cell as a cube. When the length of a side increases, its surface area increases as six times its length squared, but the volume increases faster as the length cubed. So when a cell size increases, the surface area-to-volume ratio decreases. A large surface area to volume ratio makes transporting materials quickly in and out of the cell easier. For example, bacterial cells have a high surface area-to-volume ratio, allowing nutrients, gasses, and waste to diffuse through the cell. In larger animals, cells form specialized organs, such as the intestines, which are folded to maximize the surface area for food absorption.