Why are cells small?
Key takeaways
- Two physical constraints — surface area and diffusion — explain why cells are so small.
- A human body is built from 30 trillion cells excluding microbes that each arise from a lone, fertilized egg.
- A simplistic answer is that evolution has made each cell the size best suited to its function.
Two physical constraints — surface area and diffusion — explain why cells are so small.
By Niko Mc Carty, Fellow at Astera Institute. A human body is built from 30 trillion cells excluding microbes that each arise from a lone, fertilized egg. These cells come in a multiplicity of shapes and sizes, with internal volumes spanning five orders of magnitude. The smallest human cell, a sperm, has a volume of just 30 µm³, whereas an oocyte has a volume of 4,000,000 µm³, making it the largest cell in the human body.1
What accounts for this huge range? A simplistic answer is that evolution has made each cell the size best suited to its function. Maybe sperm are small because the body needs to make many of them, and tiny cells cost less energy to make. (Sperm consist of little more than DNA and a few mitochondria, which are necessary for providing energy to spin their whip-like tails.) By contrast, an oocyte needs massive reserves of mitochondria and nutrients to support early embryonic growth. In short, every cell is as large or small as it needs to be within reason.