What does that sentence mean?
Oxygen is what it is all about. Ironically, the stuff that gives us life eventually kills it. The ultimate life force lies in tiny cellular factories of energy, called mitochondria, that burn nearly all the oxygen we breathe in. But breathing has a price. The combustion of oxygen that keeps us alive and active sends out by-products called oxygen free radicals. They have Dr. Jekyll and Mr. Hyde characteristics. On the one hand, they help guarantee our survival. For example, when the body mobilizes to fight off infectious agents, it generates a burst of free radicals to destroy the invaders very efficiently.

"But breathing has a price."

What does that sentence mean?

Question

What does that sentence mean?
Oxygen is what it is all about. Ironically, the stuff that gives us life eventually kills it. The ultimate life force lies in tiny cellular factories of energy, called mitochondria, that burn nearly all the oxygen we breathe in. But breathing has a price. The combustion of oxygen that keeps us alive and active sends out by-products called oxygen free radicals. They have Dr. Jekyll and Mr. Hyde characteristics. On the one hand, they help guarantee our survival. For example, when the body mobilizes to fight off infectious agents, it generates a burst of free radicals to destroy the invaders very efficiently.

"But breathing has a price."

What does that sentence mean?

Answer

When something "has a price" it means that something isn't free. It will cost you something.

In the case above, breathing gives us oxygen which keeps us alive. However, "the cost" of breathing is that the combustion of the oxygen leads to the formation of by-products called oxygen free radicals which can have bad effects (as the paragraph suggests). Therefore "breathing has a price".

Answer

"But breathing has a price."

This means, there are also negative effects from breathing.