Cracking the AP Biology Exam


Cellular Energetics


At the same time that electrons are being passed down the electron transport chain, another mechanism is at work. Remember those hydrogen ions (also called protons) that split off from the original hydrogen atom? Some of the energy released from the electron transport chain is used to pump hydrogen ions across the inner mitochondrial membrane to the intermembrane space. The pumping of hydrogen ions into the intermembrane space creates a pH gradient, or proton gradient. The potential energy established in this gradient is responsible for the production of ATP.

These hydrogen ions can diffuse across the inner membrane only by passing through channels called ATP synthase. Meanwhile, ADP and Pi are on the other side of these protein channels. The flow of protons through these channels produces ATP by combining ADP and Pi on the matrix side of the channel. This process is called oxidative phosphorylation.

Three other things you’re expected to know for the AP Biology Exam:

  • Every NADH yields 3 ATP (except NADH from glycolysis produces 2 ATP).
  • Every FADH2 yields 2 ATP.
  • The total number of ATP produced in Stage 4 is 32 ATP.

Don’t concern yourself with the exact number of NADH, FADH2, CO2, and ATP produced at each stage. For the AP Biology Exam, all you need to remember are the big steps and the overall outcome: