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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.

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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
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!

ATP Synthesis

Pumping all these protons outside across the inner membrane of the mitochondria creates a high concentration of protons between the inner and outer mitochondrial membranes and, therefore, a concentration gradient of protons. In the last step of the electron transport chain, an enzyme called ATP synthase is used. This is a channel protein in the inner mitochondrial membrane.

The protons flow through this pump at max speed, back into the mitochondrial matrix; this causes part of the enzyme to spin in circles like a whirling dervish. This spinning motion provides the final dance and song number of cellular respiration. ATP synthase catalyzes the phosphorylation of ADP, creating the last 28 molecules of ATP. How’s that for a final act?

Cellular Respiration Summary

Formula for cellular respiration
Formula for Cellular Respiration

Let’s bring all this back to our formula for cellular respiration in order to summarize the reactants and products from the process as a whole. One glucose molecule was broken down in glycolysis to net two ATP molecules and electron carriers.

Pyruvate oxidation and the citric acid cycle produced two more ATP molecules, more electron carriers, and six molecules of carbon dioxide. Finally, in the electron transport chain, the electron carriers were used to donate electrons and protons that turned oxygen molecules into water and created the remainder of the 32 ATP molecules – all from one glucose molecule.

Lesson Summary

In this lesson, we learned how we took the products of glycolysis and the citric acid cycle and used them in the electron transport chain, the third stage of cellular respiration.Electron carriers are reduced during glycolysis and the citric acid cycle to NADH + H+ and FADH2.

These carriers then donate electrons and protons to the electron carrier proteins of the electron transport chain. The final electron acceptor is oxygen. Together with oxygen, electrons and protons form molecules of water.In addition, protons are actively pumped to the other side of the inner mitochondrial membrane, creating a proton gradient. These protons flow through ATP synthase, a channel protein that uses this power to phosphorylate ADP to make ATP.

The electron transport chain, therefore, produces 28 ATP molecules as well as water.

Learning Outcome

After viewing this lesson, you’ll be able to:

  • Explain how the electron transport chain uses the products of glycolysis and the citric acid cycle to finish the process of cellular respiration and turn our food into energy
  • Name the reactants and products of each stage of cellular respiration

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