Glycolysis and Cellular Respiration
Topics in Depth
The Theme of Oxidative Phosphorylation in Glycolysis and Cellular Respiration
This is our bonus game, but also the more important one, because we will convert the tokens from the citric acid cycle (NADH and FADH2) to tickets (ATP). This is where the bulk of ATP comes from in cellular respiration—not glycolysis or the citric acid cycle, but oxidative phosphorylation.
If we break it down, it is not too hard to figure out what this long phrase means: oxidative must have something to do with oxidation, which involves a transfer of electrons. Phosphorylation is a term we saw earlier, and it means adding phosphate. So put them together, and we are talking about redox reactions and phosphate additions, which is not all that different than what we dealt with in glycolysis and the citric acid cycle.
The main point of oxidative phosphorylation is the transfer of electrons from NADH and FADH2 to power ATP production. Similarly, the main purpose of playing arcade games is to win tickets for prizes (okay, and also maybe to have fun and earn high scores in the games). NADH is more often the electron donor because more NADH than FADH2 result from the citric acid cycle.
Oxidative phosphorylation happens in two steps:
1. The electron transport chain: no ATP here, just lots of electrons being transported.
2. Chemiosmosis: a gradient of protons (H+) drive the enzyme ATP synthase to make ATP.
We like to call the electron transport chain the ETC. It is sort of like the EAC (East Australian Current) in Finding Nemo, except that instead of transporting fish and sea turtles hundreds of miles the ETC transports hundreds of electrons.
In the ETC, NADH (or less frequently FADH2) starts the electrons down the chain. NADH and FADH2 donate electrons to protein complexes, which are basically blobs of protein. These blobs are numbered I, II, III and IV, and they pass electrons down the chain from protein to protein in the blobs I to IV. One of the electron carriers, ubiquinone, is not a protein, but a lipid. Yet another example of protein jobs getting outsourced to lipids. Oxygen is the terminal electron acceptor, which means it takes the electrons from the last protein complex. Oxygen takes electrons from NADH or FADH2, then also picks up two hydrogen ions to make water.
When FADH2 is the electron donor, it binds to complex II, later in the chain than where NADH starts. This means fewer protons are released (no protons go through complex I) which makes FADH2 a less energetic electron carrier and results in fewer ATP molecules being made.
Since electrons and protons travel together, all this electron shuffling results in some loose protons (H+) floating around. The protons end up on the other side of the inner mitochondrial membrane that the NADH and FADH2 (and their reduced forms) are.
At the end of the ETC, the protons flow back across the membrane, which powers the enzyme ATP synthase to make ATP. This flowing of protons across the inner mitochondrial membrane is chemiosmosis. The word chemiosmosis means ions moving across a membrane. When water flows across a membrane, it is called osmosis, and chemiosmosis is a very similar process. In both cases, particles diffuse from an area of high concentration to an area of lower concentration. The gradient of high to low proton concentration has potential energy, and it can be used by ATP synthase.
ATP synthase makes ATP from ADP and inorganic phosphate (Pi). The enzyme ATP synthase is like a motor driven by protons because a steady stream of H+ moving back across the inner membrane of the mitochondria provides the power to make ATP. In the same way that flowing water can move a water wheel, protons move the ATP synthase wheel.
Go ATP synthase, go!
In this round of pinball, we won 34 tickets. This means the process of oxidative phosphorylation produces 34 molecules of ATP per glucose molecule.
Chemiosmosis happens not just in mitochondria, but also in bacteria and chloroplasts.
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