The Electron Transport Chain: Products and Steps

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  • 0:05 The Last Step of…
  • 2:07 Steps of the Electron…
  • 3:57 ATP Synthesis
  • 4:44 Cellular Respiration Summary
  • 5:22 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

In this lesson, we'll learn about the finale of cellular respiration. The electron transport chain uses products from the first two acts of glycolysis and the citric acid cycle to complete the chemical reaction that turns our food into usable cellular energy.

The Last Step of Cellular Respiration

Now, before you go to any big show, there's some preparation to be done. You can think of the steps of cellular respiration as the opening acts to the main event. We've been doing a dance of cellular respiration for a few lessons now, building up to the finale. Remember that cellular respiration is the process that converts food into chemical energy. In this spectacular show of how our cells perform this process, we opened up with glycolysis, followed by the citric acid cycle. Collectively, these two acts of cellular respiration get us ready for the main event, the electron transport chain. If you were an electron, this would easily be the most fun you've ever had, and we'll show you why.

But first, let's recap. In cellular respiration, our cells use glucose and oxygen to produce water, carbon dioxide, and energy - this is in the form of ATP. Remember that we get glucose from the food we eat, like from that delicious pie from that family picnic we attended when we were learning about cellular respiration.

In glycolysis, the sugar glucose was broken down into two pyruvate, or three-carbon sugars. These pyruvate molecules were further modified through pyruvate oxidation before they entered the citric acid cycle. The products of both glycolysis and the citric acid cycle combined totaled four net ATP molecules and six carbon dioxide molecules. You'll notice this doesn't yet account for all the products of cellular respiration. We still have to make water, as well as another 28 molecules of ATP. You'll also notice we haven't yet used the oxygen we breathe for this process.

However, if you remember, the steps of glycolysis and the citric acid cycle collectively oxidized carbon molecules, transferred electrons to electron carriers, and produced a whopping total of ten NADH + H+ and two FADH2 molecules. Now these reduced electron carriers are poised to donate these electrons to the fireworks of the show - the electron transport chain.

Steps of the Electron Transport Chain

If these electrons were all actors in cellular respiration, this would be their time to shine. The electron transport chain is the third stage of cellular respiration.

Four protein complexes in the inner mitochondrial membrane form the electron transport chain. These complexes exist in a descending order of energy. Here the electron carriers come along to drop off all their electron and proton cargo that they picked up during the glycolysis and citric acid cycle stages. NADH + H+ and FADH2 become oxidized, donating electrons to the first and second protein complex respectively. These complex proteins now become electron carriers themselves and are now reduced. They become oxidized as they pass these electrons down the electron transport chain. You can think of this like someone taking a slinky and dropping it onto the first step of a staircase. The slinky will continue to move down the steps, just like an electron moves down energy levels, until it hits the bottom.

Illustration of the electron transport chain
Electron Transport Chain

Now, as the electrons are moving down the stairs, the protons donated from NADH + H+ and FADH2 also are put to good use. They are pumped to the other side of the inner mitochondrial membrane through the protein complexes I, III, and IV. They are moved by active transport from the mitochondrial matrix to the space between the inner membrane and outer membrane of the mitochondria. They do this by using the energy derived from the electrons that flow down the electron transport chain stairs.

The last 'stair' of the electron transport chain is oxygen. You were wondering when we were going to use that, right? A single oxygen molecule accepts two electrons and two protons from the final protein complex. This produces a molecule of water. Why do all of us need oxygen? To complete cellular respiration!

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