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Table of Contents
Table of Contents
Structure of the Mitochondrion
Structure of the Mitochondrion
Overall Mitochondrion Structure
Overall Mitochondrion Structure
Recall that the mitochondrion (according to the endosymbiotic theory) originated as a separate organism. It has its own unique DNA, its own ribosomes, and a double membrane. All of these point toward the mitochondrion originating as a prokaryotic organism that got engulfed by a eukaryotic organism. Instead of digesting the mitochondrion, these organisms lived harmoniously in a mutually beneficial symbiotic relationship.
The double membrane is particularly important for cellular respiration because it will be necessary to track either side of the membrane throughout the process. Familiarize yourself with this basic diagram of the organelle so that you can track where these different processes occur.
Inner Membrane's Importance
Inner Membrane's Importance
The inner membrane of the mitochondrion is highly folded. The folded shape increases surface area, which increases area available for ATP production in aerobic respiration.
You do not need to memorize these structures that you see, but just notice how many membrane proteins there are embedded in that folded inner membrane.
Overview of Cellular Respiration
Overview of Cellular Respiration
Cellular respiration is how a cell transforms a fuel source (glucose) and unleashes that energy in order to capture it by charging some ATP molecules. It is often represented via a simplified equation:
Glucose (C6H12O6) + 6O2 --> 6 CO2 + 6 H2O + energy
In reality, however, cellular respiration is a string of reactions that occur throughout different areas of the mitochondrion. This diagram represents a simplified version of these subsequent steps in order: glycolysis, the link reaction, the Krebs Cycle, the electron transport chain, and chemiosmosis. You do need to be familiar with the basics of these steps, such as where they occur and what their inputs and outputs are, but you do not need to memorize as much as your textbook would have you believe.
A. Glycolysis
A. Glycolysis
Please note that glycolysis occurs in the cytosol - we have not yet entered the mitochondrion.
Glycolysis is the first step of cellular respiration and it sounds a lot more complicated than it is. Let's break down the word: 'glyco' refers to sugars and 'lysis' refers to cutting/chopping. So, we are splitting a sugar molecule here. In fact, since this is the first step of cellular respiration, we have only a glucose molecule so far (oxygen doesn't come in until later).
In reality, glycolysis involves many sub-steps that you do not need to worry about. Do not memorize the steps that make up glycolysis. Just know the following:
Glycolysis involves splitting a single glucose molecule into 2 pyruvate molecules, 4 ATP molecules, and 2 NADH molecules NADH is an electron carrier molecule, and it acts a lot like a charged battery - similarly to ATP). We will see this NADH brought over at the end of cellular respiration (see the arrow leading to the electron transport chain). However, breaking up glucose is difficult, so it actually requires an investment of 2 ATPs. So we spent 2 ATP molecules, but gained 4 for a net gain of 2 ATP molecules. That is some energy, but not much yet.
To summarize glycolysis simply (ignoring molecules such as ADP and NAD+):
Glucose + 2 ATP --> 2 Pyruvate + 4 ATP + 2 NADH
B. Link Reactions (aka Pyruvate Oxidation)
B. Link Reactions (aka Pyruvate Oxidation)
For us, the link reactions are very simple. Basically, we need to bring those pyruvate molecules into the mitochondrion. In doing so, we produce a molecule known as Acetyl CoA, some CO2, and some more NADH. Remember that we have two pyruvate molecules from that single glucose molecule, so this occurs twice.
Simplified Summary of Link Reactions:
2 pyruvate --> 2 Acetyl CoA + 2 CO2 + 2 NADH
Do not memorize any of these chemical structures. Just be familiar with the names and locations of these molecules, and remember that those NADH molecules will be important later in the process of cellular respiration. Please note that now we have entered the mitochondrion! The acetyl CoA we produced will be processed within the mitochondrial matrix, the innermost part of the mitochondria.
C. Citric Acid Cycle (Krebs Cycle)
C. Citric Acid Cycle (Krebs Cycle)
In the textbook, the citric acid cycle (aka the Krebs cycle) is very intimidating. Once again, we are not worried about all the intermediate molecules and reactions that occur. We are looking at the big picture of the process. So do not memorize any of the terrifying diagrams that you may see in the book.
The Krebs cycle is incredibly complex and intimidating, so don't freak out about it. Focus on the inputs and the outputs. We are bringing in the Acetyl CoA molecules from the link reactions. When the cell processes an Acetyl CoA molecule, it produces 3 NADH molecules (remember, these carry electrons and will be important shortly), one FADH2 molecule (another electron carrier much like NADH), three CO2, and a single ATP molecule.
However, there are two Acetyl CoA molecules from the link reactions, so this occurs twice. So, the overall reaction for the Krebs Cycle from a single original glucose molecule is:
2 Acetyl CoA --> 6 NADH + 2 FADH2 + 2 ATP + 4 CO2
Do NOT memorize this cycle shown in the diagram - that is just a visual to show where carbons are coming from and going, as well as showing where those products come from. You do not need to be able to draw this - simply understand the above equation!
D. Electron Transport Chain
D. Electron Transport Chain
The Importance of the Double Membrane
The Importance of the Double Membrane
The electron transport chain (ETC) is where we finally get to see something more interesting and relevant to the biochemistry we've explored up through this point. Finally, those electron carrier molecules, NADH and FADH2 get used and the energy held within those molecules gets used for something besides simply ATP (for now).
However, just like all steps so far, there is a lot that seems intimidating at first. In order to ensure that you get what is most important, it is crucial that we revisit the structure of the mitochondria. Recall that the mitochondria has a double membrane. This is important because much of this process involves the innermost matrix and the intermembrane space - the area between the two membranes. Recall that the Krebs cycle took place within the innermost matrix. But much of this process will involve transport across that inner membrane.
The Flow of Electrons - Playing Hot Potato
The Flow of Electrons - Playing Hot Potato
Throughout the electron transport chain, electrons from the electron carriers (NADH and FADH2) will be passed around like it's a game of hot potato. You do not need to know any of the molecules that pass the electrons around aside from the original electron carriers and where they end up. In the diagram, pay close attention to the orange arrow labeled electron flow. This arrow represents the path along which this game of hot potato is played. You can see that there are quite a few players in this hot potato game such as integral membrane proteins (represented with Roman numerals) and mobile electron carriers. You do not need to memorize these structures.
If you follow the electron chain to the very end, you can see that the electrons are eventually brought into contact with oxygen - FINALLY we get to see where the O2 comes into cellular respiration! This is why we need to breathe in oxygen, after all. These electrons need to go somewhere; we cannot let them float around our cells. That would be known as a free radical, and can be extremely dangerous, sometimes causing cancers. So those electrons get shoved onto oxygen to produce water. This is why a significant amount of water is produced as a result of cellular respiration - look back at the original equation for all of cellular respiration and you will remember that water is a product.
Forming a Hydrogen Ion (H+) Gradient
Forming a Hydrogen Ion (H+) Gradient
Every time an electron gets passed, some energy is released. You do not need to know the biochemical reasons for this, just know that every single time an electron exchanges hands, some energy is released. Think about the game of hot potato - if someone grabs the potato, some of the heat energy is transferred into that player's hands. In this case, some of that energy will be transferred to the membrane proteins. That energy is useful, so these proteins are going to use it to pump hydrogen ions (H+) against their concentration gradient (from the matrix into the intermembrane space). Recall that this requires energy, so represents active transport. In this particular case, the energy was not provided by ATP, but by the electrons being passed around.
Remember that energy is never destroyed. That energy spent in active transport goes somewhere - specifically it goes into the hydrogen gradient itself. Now we have a LOT of hydrogen ions (H+) in the intermembrane space that want to leave! That is potential energy. We used the energy of all of those electrons to charge up this battery of hydrogen ions wanting to leave the intermembrane space. That is going to be crucial for our final step of cellular respiration - chemiosmosis.
Overall Inputs and Outputs
Overall Inputs and Outputs
The electron transport chain finally shows us the utility of those molecules we've been making up until this point. We see that the NADH and FADH2 provide the electrons that provide energy to form a hydrogen ion gradient. This gradient is of utmost importance because it will allow us to form a BUNCH of ATP in the final step of cellular respiration: chemiosmosis. So, to summarize the electron transport chain, we can use an unconventional equation such as:
O2 + NADH + FADH2 --> Hydrogen ion gradient + H2O
Please not that this is an incredibly simplified version of what we have seen occur - I have chosen not to include the numbers of each of these molecules because we don't need to know that for AP Biology.
E. Chemiosmosis
E. Chemiosmosis
We have finally made it to the final step of cellular respiration: chemiosmosis. This has one of the scariest-sounding names, but really boils down to something quite simple. The cell wants to make ATP. In order to do so, it needs energy (we need energy to charge the battery), so it will use the energy from the hydrogen ion gradient we just formed in the electron transport chain in order to power an enzyme known as ATP Synthase (literally named this because it synthesizes, or makes, ATP).
That's it. These diagrams for the ETC actually have shown chemiosmosis as well - it is occurring on the far right of this image. All of those H+ ions will move through ATP Synthase (shown in blue here), which acts exactly like a rotary motor. The kinetic energy of the H+ ions moving back into the matrix (along their concentration gradient via facilitated diffusion) is used to power the rotary motor of ATP synthase, catalyzing the reaction that charges ADP back up to ATP.
Overall Efficiency of Cellular Respiration
Overall Efficiency of Cellular Respiration
Recall that most ATP was made during the final step of cellular respiration, chemiosmosis. This is where the vast majority of the ATP during cellular respiration is harvested. Everything we have done up until this point has been leading up to this. Think about all of those electron carriers we kept making - NADH and FADH2 - finally we get to see what all of it was for! For a single glucose molecule, this final step results in 26 or 28 (depending on conditions) ATP molecules, far outweighing the measly 4 ATP the other steps have produced. But every single step along the way relied on those beforehand. So for a single glucose molecule, about 30 or 32 ATP molecules are produced (depending on conditions). That is how a cell gets its useful energy. That is why we must eat food.
Why Do Cells Do This Across So Many Steps???
Why Do Cells Do This Across So Many Steps???
What we are effectively doing throughout this entire process is burning glucose. In fact, this is why we refer to losing weight as burning fat. You will recall from chemistry that combustion, or burning, reactions have a general formula:
Fuel + O2 --> CO2+ H2O + energy (heat)
Wait a second, that looks awfully familiar. That's because it is shockingly similar to our overall equation for cellular respiration:
Glucose (C6H12O6) + 6O2 --> 6 CO2 + 6 H2O + energy (ATP)
Glucose is our fuel for cellular respiration - we are literally just combusting, or burning, it to release energy.
Burning things is dangerous - think about how dangerous explosions are. And molecules hold a lot of potential energy - think about how atomic bombs are made! You wouldn't want explosions occurring within your cells, obviously.
So instead of exploding a single molecule of glucose, the cell has broken up that one large explosion into many smaller explosions. This is what makes the burning useful to us. If you took your car (which has an internal combustion engine, by the way...) and immediately burned all of the gasoline in a full tank, you'd have a huge explosion, but that isn't very useful to you.
Instead, your car engine is designed to allow for little 'explosions' that power the movement of the car. The cell works the same way in cellular respiration.
Fermentation - When There Is No Oxygen
Fermentation - When There Is No Oxygen
We've been operating under the assumption that cellular respiration always occurs this way. In fact, this only occurs this way in eukaryotes, and even within eukaryotes there is some diversity. The general conditions for cellular respiration involve oxygen, as we've seen above. However, in anaerobic conditions, or conditions without oxygen, cells will undergo a process known as fermentation, a form of anaerobic respiration. That really is the extent of the topic that you must know.
However, you should know that this occurs within your own cells. When you exercise intensely, you burn through your oxygen more quickly than you can bring it in. As a result, your muscle cells resort to fermentation, which happens to produce lactic acid. This acid crystallizes and creates that soreness that you either love or hate the next day after intense exercise.
Many different kinds of products can be made via fermentation depending on the conditions and organisms involved. Things from alcohol to cheese are only possible due to our understanding of fermentation.
Also note that fermentation resembles cellular respiration at first. These cells still undergo glycolysis. After that point, however, the paths diverge.
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