Aerobic Cellular Respiration
In the last part of this lesson, we learned how all living things do glycolysis to start harvesting energy from the breakdown of glucose. Here’s a recap:
We also learned that the preferred next step is to continue to completely break down glucose into carbon dioxide, harvesting all of the energy out of cell respiration:
Let’s now learn how this process continues when cell respiration is possible.
The Intermediate Step
In the intermediate step, also called the transition step, pyruvate is turned into acetyl CoA, releasing 1 carbon dioxide and making 1 NADH (per pyruvate, for a total of 2 CO₂ and 2 NADH per glucose). Fats get turned into acetyl CoA by another mechanism and enter the next step, the Krebs cycle, just like acetyl CoA from glucose.
The Krebs Cycle
In the Krebs Cycle, also called the citric acid cycle, we break acetyl CoA down into 2 carbon dioxides (per acetyl CoA, for a total of 4 CO₂ per glucose). Again, there is a whole process for how this happens, which you don’t have to know for this class, but here it is to prove that it’s not just magic:
This video gives a great overview of the intermediate step and the Krebs cycle. It shows you some of the details of the reaction, which you do not need to know, but may be helpful for your conceptual understanding:
The main idea is that we are continuing to break down glucose all the way into carbon dioxide, releasing energy that we are storing. In the citric acid cycle, this energy is stored in the form of NADH and another high-energy molecule, FADH₂, which both get saved for later and are ultimately used to make ATP. A little bit of a molecule called GTP (guanosine triphosphate), which is basically ATP, is also made.
This step does not directly require oxygen, but it doesn’t happen unless oxygen is present.
This step does not directly require oxygen, but it doesn’t happen unless oxygen is present.
The Krebs Cycle happens in the mitochondria. Mitochondria are very special and important. They are so special and important that I don’t want to distract you by talking about them now, so you’ll have to wait until the end of this lesson.
The Electron Transport Chain
The electron transport chain is another very cool process that you don’t have to know all of the details of for this class. Here’s a video, if you’re curious:
The main idea here is that we are taking all of the energy that was stored in NADH and FADH₂ and turning it into ATP. This happens in the mitochondria (again, more on this later) and involves an exceptionally neat protein called ATP synthase, which acts like a turbine to make ATP. (As electrons move down the electron transport chain, hydrogen ions are pumped into the intermembrane space, which is the space between the two membranes of the mitochondria. As these hydrogen ions flow back through, down their concentration gradient, they spin the turbine). This video shows ATP synthase in action:
The electron transport chain directly uses oxygen. The oxygen gets broken apart and ultimately turned into water. Roughly 30 ATP are made per glucose.
Mitochondria Are The Powerhouse Of The Cell
The Krebs Cycle and the electron transport chain happen inside of a very special, important compartment of the cell called the mitochondria. Mitochondria have pretty much just one job: to make energy for the cell. For this reason, they are often called “the powerhouse of the cell.” Mostly in memes about the broken educational system, but I digress:
Memed though they may be, mitochondria are actually pretty important. They are an essential part of the energy flow for all life on earth, they are the reason you’re able to make enough energy to read this, their dysregulation is implicated in at least 5 of the top 10 leading causes of death in the U.S. (type II diabetes, heart disease, cancer, stroke, and Alzheimer’s disease), and they’re a great example of an important biological principle, which is that:
This is a major theme in biology: Things have special structures that give them special functions. Organelles have special structures that give them special functions. Mitochondria have special structures that give them special functions. The function of mitochondria is to do aerobic respiration efficiently, and its structure helps it do that. The structure of mitochondria looks a little something like this:
You’ll see an outer membrane, an inner membrane with folds, an intermembrane space, and a matrix. These all have a purpose:
You don’t have to memorize the functions of all the structures of the mitochondria. But, you should understand that the unique structure of the mitochondria gives it its unique function, and it’s helpful if you can explain some specific examples of this.
- The outer membrane regulates what comes into and goes out of the mitochondria. This is what traps all the right stuff in the mitochondria and keeps all the wrong stuff out. Much like the cell membrane creates a unique environment inside of the cell, localizing together all of the stuff that’s important for life, the outer membrane of the mitochondrion creates a unique environment inside of the mitochondrion, localizing together all of the stuff that’s important for aerobic respiration. This is really the point of having organelles to begin with: Organelles make life easier because of localization, or putting all the right stuff for reactions in one smaller spot.
- The inner membrane and intermembrane space allow the electron transport chain to occur. (The proteins of the electron transport chain are located on the inner membrane, and ATP generation relies on a proton gradient created across that membrane by pumping H+ ions into the intermembrane space).
- The inner membrane has folds to so that you can squish more membrane, and therefore more reactions of the electron transport chain, into a smaller space. This is efficient. You see this idea in a lot of biology, and we usually use the shorthand “increased surface area” to mean that an increased amount of space on the surface, which can interact with the surrounding area, allows more of any particular thing on that surface to happen. You’ll see this idea again.
- The matrix is an open space (full of mitochondria goo) where the Krebs Cycle occurs.
You don’t have to memorize the functions of all the structures of the mitochondria. But, you should understand that the unique structure of the mitochondria gives it its unique function, and it’s helpful if you can explain some specific examples of this.
Summary
This video gives a great overview of the most important points that we’ve discussed about aerobic respiration and the importance of mitochondria:
Here it is with a little more detail than you really need, but as a super fun song:
The main things you should make sure you understand about each step of cellular respiration are:
You should also understand these important concepts:
Finally, for meme’s sake, here is some other important information that you should know:
- The starting and ending products of each step (you don’t need to know the exact numbers for this class)
- Where each step occurs
- What type of energy we get out. It’s also helpful to have a rough idea of how much energy comes out (i.e., a little energy from glycolysis, a little from the Krebs cycle, and a lot from the electron transport chain).
You should also understand these important concepts:
- Glucose (sugar) is being broken down gradually so that we can turn the energy released by the combustion reaction into ATP.
- Compared to glycolysis only, which does not require oxygen, the full process of aerobic respiration (all the way through the electron transport chain), which requires oxygen, makes A LOT more ATP.
- The steps of aerobic respiration that by far produce the most energy happen in the mitochondria, which is an organelle that localizes the important molecules involved in aerobic respiration into one spot, to make it more efficient. The unique structure of the mitochondria—especially its folded inner membrane—is what gives it its unique function.
- Pretty much everything in biology has a special structure which gives it a special function.
Finally, for meme’s sake, here is some other important information that you should know:
- In the United States, your taxes are due on April 15, every year.
- Everyone who has an income, even if it’s just a small amount of income from self-employment, needs to file taxes. Even non-US citizens who earn money in the US need to file taxes, except in certain specific circumstances.
- There are many free online resources that will guide you through the process of filing your taxes. They will generally ask for your personal information, your wage statements (W2 form) from your employer, any income that you earned from self-employment, and any specific tax situations that might apply to you, like whether you donated to charity that year, whether you have kids, whether you bought a house that year, and so on. Most people who only have one or two jobs and don’t fall into a lot of those special circumstances can do their taxes pretty quickly using one of these tools. If your income was withheld properly through your paycheck, you shouldn’t owe money, and you might get a little back.
- People with more unique tax situations, such as those who are running a new business and may not know the ins and outs of business taxes, or who fall into a lot of the special categories, may find it useful to consult a tax accountant.
- Here is a helpful video on how to file your taxes.
Learning Activity
Content contributors: Kathleen Yu, Eli Levine, Emma Moulton