The H+ gradient is across the inner membrane
- The idea
- Predict
- See it
- Why
- Check
The idea
“The outer mitochondrial membrane holds the H+ gradient and the ATP synthase.”
The outer membrane is the first wall you meet, so it seems like the one that matters. Its pores make it more like a sieve.
The short version
The mistake: The outer mitochondrial membrane holds the H+ gradient and the ATP synthase.
What's actually true: The inner membrane holds the electron transport chain, the H+ gradient and ATP synthase. The outer membrane is leaky.
- The outer membrane is full of pore proteins called porins. Ions and small molecules pass through, so it can't hold an H+ gradient.
- The inner membrane barely lets H+ through. The ETC in it pumps H+ out of the matrix.
- ATP synthase sits in that same inner membrane. H+ flow back through it, and that flow drives ATP synthesis. This is chemiosmosis.
- In endosymbiosis, the inner membrane came from the engulfed bacterium's plasma membrane, where bacteria still make ATP this way.
On the exam: The ETC, the H+ gradient and ATP synthase all belong to the inner membrane. The outer one just leaks.