Here's why:

* Humans use specialized organs for essential functions: We have lungs for breathing, a stomach and intestines for breaking down food, a heart for pumping blood, and so on. These organs are complex structures made of tissues and cells that work together.

* Bacteria are single-celled organisms: They are far smaller and simpler than humans. They don't have specialized organs or tissues.

* Bacteria perform essential functions differently: Instead of dedicated organs, bacteria rely on their cell membrane and internal processes to:

* Breathe: They take in oxygen or other molecules for energy production through their cell membrane.

* Break down food: They have enzymes within their cells that break down nutrients.

Therefore, bacteria use different mechanisms to perform functions like breathing and digestion, while humans rely on specialized body parts.