What you’re seeing, when hydrogen peroxide fizzes up on contact with blood, is a desperate stuggle for life. An enzyme in your blood, and most other living things, rips hydrogen peroxide apart – but not fast enough for bacteria.

Hydrogen peroxide is just H2O2. That’s water with one extra oxygen, which seems simple enough. For cells, though, it’s a destroyer of worlds. If you’re wondering why, have a look at the double oxygen atoms. Oxygen is a greedy atom, hungry for electrons. It will rip the electrons out of any molecule within reach, including molecules inside cells – and they don’t respond well to sudden electron restructuring.

Too much restructuring kills cells, including the ones in your body. This is bad enough for you. A bacterium doesn’t have as many cells as you do. Pouring hydrogen peroxide exposes bacteria and your own cells alike to a relentless onslaught of murderous goo.

There are defenses, though. If you remember the marketing on your bottles of pomegranate juice and kale smoothies, they’re full of antioxidants. Antioxidants take the bullet so your body’s cells, or the bacteria’s cell, don’t have to. And then there’s catalase. Catalase is an enzyme that’s present in most living cells, including yeast cells, your blood cells, or bacterial cells. Catalase rips H202 into regular H20, with a little extra oxygen. The bubbles you see are oxygen harvested from the hydrogen peroxide. It gets a little structure from the protein fragments that the hydrogen peroxide has heartlessly slain.

Note: This entry needed extensive revision, and has been re-written. I was wrong, guys. Sorry!

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[Sources: Decomposing Hydrogen Peroxide, The Oxygen Dilemma.]