Why do we inhale oxygen and exhale carbon dioxide? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Why do we inhale oxygen and exhale carbon dioxide? Short and long answer, you ready?
The short answer is that you inhale oxygen because you need oxygen for some biological processes. A fairly important one is the production of ATP, the energy all of our cells use. In the process, electrons are used and oxygen has a high affinity for electrons. The waste products of this process are Carbon Dioxide and Water, in different steps along the way.
The long answer needs some pictures. This one is a seriously long answer and will explain the production of ATP. CO2 is involved in the citric acid cycle and water is involved in the electron transport chain.
You know how we eat to live? Well that’s where it starts. The major source of energy we get from food is sugar, more specifically glucose. Now things get a bit funky so bear with me. Glucose needs to be broken down in steps. This has to be done slowly because glucose contains plenty of energy and we don’t want to blow stuff up.
Step 1: Glycolysis
The glucose molecules are broken down into two pyruvate molecules. It takes ten steps to go from glucose to pyruvates. This all happens in the cytosol, which is all the fluid inside a cell between the organelles.
The big 6-carbon glucose molecule first needs to be split into two smaller 3-carbon molecules (phosphoglyceraldehyde, PGAL), this split uses ATP. It might sound counterproductive since we are trying to make ATP, but the investment will pay off. One ATP is used by each kinase reaction, and step one and step three require it, so a total of two ATPs are used to split glucose into the smaller PGAL molecules.
These PGAL molecules are then transformed into Pyruvate, and during that process two ADPs are turned into ATP by a kinase reaction (in steps seven and ten). Because we have two PGALs we create four ATPs (so we gain two because we used two before).
The whole process therefore uses glucose and two ATP and then produces two pyruvates, two NADH, and four ATP. The net gain is two ATP, the investment paid off since we doubled it.
Step 2: Pyruvate Oxidation / Decarboxylation / Pyruvate Dehydrogenase
In the last step we were left with pyruvate after breaking apart glucose, in fact we have two pyruvate molecules for each glucose model. The next step is Pyruvate Oxidation, which takes place inside mitochondria. Remember the famous saying: “Mitochondria are the powerhouses of the cell”? We’ll get there soon enough. The transformation takes place in a few steps.
- The first step is breaking off a carbon molecule, this carbon takes two oxygens with it(so CO2 is removed).
- In the second step the 2-carbon molecule that is left is oxidized (electrons lost), these electrons are picked up by the NAD+ turning it into NADH.
- The 2-carbon molecule is attached to Coenzyme-A, this turns it into Acetyl CoA. This is just a carrier molecule to bring the 2-carbon group to the next step.
From one glucose molecule two pyruvate molecules are made, these are turned into two acetyl-CoA molecules. Two carbons are released as carbon dioxide, these are two carbons from the original six in glucose. Lastly, two NADH are produced from NAD+.
Step 3: The Citric Acid Cycle / Krebs cycle / Tricarboxylic Acid (TCA)
This cycle (whatever name you choose) is an essential step in the process. It takes the Acetyl-CoA produced in the last step and squeezes out every tiny bit of potential energy it can. Just like the last step this process takes place in the matrix of mitochondria. It’s called a cycle for a reason, the reason being that it is a closed loop. The last part reforms the molecule used in the first step.
- In the very first stage acetyl-CoA is combined with oxaloacetate (a 4-carbon molecule) into the 6-carbon molecule citrate (hence the name).
- In a two- stage process a water molecule is removed and added again to citrate to turn it into isocitrate.
- Then in a series of reactions it breaks of two carbon molecules, these are released as Carbon Dioxide. This happens in a similar manner as in Pyruvate Oxidation with the help of NAD+. This part of the cycle has a regulatory function, the enzymes doing this can speed up or slow down depending on energy needs. In the third step we are left with a 5-carbon molecule called a-ketoglutarate.
- In stage four we have a repeat of stage three, where a 4-carbon molecule is created, which is again hooked up to Coenzyme A to form Succinyl-CoA.
- We are now left with the 4-carbon molecule of Succinyl-CoA. The CoA part is replaced by a phosphate group, and the phosphate group then immediately transfers to ADP to make ATP. Some cells also use Guanine instead of Adenosine, turning GDP into GTP. These two are basically the same, energy carriers. What is left of the Succinyl is now Succinate.
- We are working with the Succinate now and in stage six it gets oxidized into fumarate, it loses 2 H+. The hydrogen atoms are transferred onto FAD, turning it into FADH2. FAD is used instead of NAD+ because Succinate doesn’t like to give away electrons. FAD has a higher electron affinity and is able to get them from Succinate, NAD+ is not strong enough. FADH2 production is done by an enzyme embedded into the inner membrane of the mitochondria, so the electrons go straight into the electron transport chain.
- In stage seven water is added to the fumarate, turning it into malate.
- Stage eight Oxidizes the Malate using NAD+ again, this results in Oxaloacetate the molecule we added in the first step.
- In each cycle two carbons enter with Acetyl-CoA, two molecules of Carbon Dioxide are released in the process (in steps three and four).
- Three NADH molecules are formed (in steps three, four, and eight), and one molecule of FADH2 (in step six).
- One molecule of ATP/GTP is produced (in step five).
Per Glucose (two Acetyl-CoA are produced)
- 4 CO2
- 6 NADH
- 2 FADH2
- 2 ATP/GTP
Step 4: Oxidative Phosphorylation
From the last step we have quite a lot of NADH and FADH2 molecules, the actual ATP produced by the Citric Acid Cycle isn’t a lot, but the important molecules are in fact this abundance of NADH and FADH2. This is what we are going to use in the last step, Oxidative Phosphorylation. This is actually a two stage process consisting of the Electron Transport Chain and Chemiosmosis.
Electron Transport Chain
The Electron Transport Chain is composed of several proteins and organic molecules that are embedded in the membrane of the mitochondria. These proteins are bundled together into complexes, four of them in this case.
We start with the NADH and FADH2 molecules that were created in the previous step. These are the ones we got via glycolysis, pyruvate oxidation, and then the citric acid cycle.
- In complex 1 NADH transfers its electrons, turning back into NAD+ and H+ which is moved to the intermembrane Space. The electrons are transferred to Ubiquinone (Q). FADH2 holds onto its electrons a bit tighter (they are at a lower energy level), so Complex 1 can’t do anything with it but pass it on.
- In Complex 2 the same thing happens to FADH2 using the same enzyme that made it during the citric cycle. The electrons are taken and passed onto Ubiquinone (Q) via iron-sulfur proteins.
- The electrons are now in Ubiquinone (Q), which in the process has become QH2 and travels through the membrane to deliver the electrons to Complex 3. Complex 3 uses the energy to pump more H+ into the intermembrane space.
- The electrons are passed on to another carrier: Cytochrome C (Cyt C), transporting them to complex 4. Complex 4 makes good use of the gradient and pumps a few more H+ across the membrane. The electrons eventually end up attached to O2 which splits up into separate oxygen atoms. The separate oxygen atoms then need Hydrogen to share a proton, and as we know oxygen plus hydrogen equals water (good old H2O).
So what happens is that NADH and FADH2 are turned back into NAD+ and FAD, we need this because they are required in glycolysis and the citric acid cycle. If they wouldn’t be turned back there wouldn’t be any available for the former cycles and the whole thing breaks down.
Secondly a gradient is created, H+ is pumped to the intermembrane space changing the concentrations and creating stored energy to be used later. It’s like winding up a toy, the winding stores energy to be released later.
The “waste” product is water; Oxygen is used because it has a high affinity for the electrons. This is why we breath, we need the oxygen to take away the electrons at the end. If there is no oxygen to pick up the electrons the chain ends, production stops, and energy production grinds to a halt.
In the first stage protein complex 1, 2, and 3 actively pump H+ to the intermembrane space. With this difference in concentration of H+ a gradient is created, also called the proton-motive force (hydrogen/H+ are called protons). Because of the gradient H+ wants to move back into the matrix, like a ball wants to move downhill. But the membrane won’t allow the H+ to go, there is only one path it can take. A protein called ATP synthase forms a channel across the membrane. Similar to how a hydroelectric dam uses the force of water, the ATP Synthase protein uses the flow of H+. The process of using a proton gradient to do something is called chemiosmosis (hence the name).
When H+ flows through the protein the top part (poking out into the intermembrane space) turns, the base (inside the matrix) stays stationary. Turning the inner part inside the base grabs ADP and adds a Phosphate to it. In a sense the ADP is energized as ATP (you go from di-phosphate to tri-phosphate). For each 4 H+ ions that flows through the channel, a single ADP molecule is turned into ATP.
This is why the mitochondria are called “powerhouses of the cell”, this is almost a quite literal description of what is going on. Just like how a hydroelectric dam generates power for a town, ATP synthase creates the energy used by everything.
ATP Synthase is a true ATP monster, producing more than 80% of the ATP yield collected from breaking down glucose. This way each molecule of glucose yields an additional 26-28 ATP by using the gradient created by NADH and FADH2. The grand total of ATP produced for each glucose molecule is then about 30-32 ATP.
- Two ATP are made in Glycolysis and two more are made during the Citric Acid Cycle. The rest comes from the NADH and FADH2 converted in the ATP synthase. When NADH moves through the transport chain about 10 H+ ions are pumped through the membrane, so for each NADH 2.5 ATP can be made (10/4=2.5).
- FADH2 enters the chain a bit later (during complex 2), so they missed the first pump. FADH2 leads to 6 H+ being pumped though the membrane. So for each FADH2 about 1.5 ATP can be made (6/4=1.5).
This is why all that NADH and FADH2 pays off, this is where the majority of the eventual ATP comes from. It drives the proton (H+) pump that establishes the gradient for ATP synthase.
The yield from Glycolysis isn’t exact, it can be either three or five. This is because Glycolysis occurs in the cytosol and NADH can’t pass through the membrane into the mitochondria. Because it can’t deliver the electrons to complex 1 it needs an intermediary, a shuttle system.
- Some cells hand it over to FADH2 inside the inner mitochondrial membrane, this results in 3 ATP (2 NADH -> 2FADH2 -> 12 H+ -> 3 ATP).
- Other cells use NADH inside the inner mitochondrial membrane, resulting in 5 ATP (2 NADH -> 2NADH -> 20 H+ -> 5 ATP).
30-32 ATP is the upper bound of the estimate, in reality it is probably lower. Sometimes the intermediates are siphoned off to be used by other biological systems, ATP production is but one process of many.
This is the entire process where glucose is turned into energy that a cell can use. Oxygen is vital since it is the receiver for electrons used in the process. Without oxygen the process halts and you get no energy. The waste product is Carbon Dioxide and Water, where oxygen bonds to either a carbon or two hydrogen (can’t have them flying around on their own can we?)
So you breath to live, because you need the oxygen to turn glucose into energy. Without oxygen the production stops. Carbon Dioxide is the waste product of this process.
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