How is energy produced in mitochondria(ATP and ADP)?

Can someone please explain to me in simple terms how energy is is produced in Mitochondria and How ATP and ADP forms.

I am confused and don't understand when i research this because its too complex.

I need help, i need to understand this, so can anyone explain in simple terms.

How much detail would you like? Starting from the electron transport chain or all the way from the start of aerobic(?) respiration?

It would be better to start from the aerobic respiration.

Okay - I will write up a response tomorrow

Whoops - sorry - I had this on my mind earlier, but completely forgot.

I am going to try to be as concise and simple as possible so that you can understand the bigger picture (which I think is very important).

Glycolysis

So we start off with glucose. The glucose is then phosphorylated to become glucose-6-phosphate. Why? This is so that it can be prevented from diffusing (and leaking) out of the cell (by adding a phosphate it can no longer diffuse through the bilayer easily). Then, to make the molecule unstable, it is converted into fructose-6-phosphate and then finally into fructose-1,6-bisphosphate. Now, wherever you're adding a phosphate, you need to use an ATP molecule. The ATP molecule is hydrolysed into ADP and an inorganic phosphate - the phosphate joins to the glucose or fructose and the ADP is recycled most probably. Since we add two phosphates during these stages, we end up investing two ATP molecules (our investment pays off later!)

Then we undergo some further reactions that produce 2 triose phosphates and then, after a further series of very quick reactions, we end up with pyruvate. The key thing to remember here is that in these series of very quick reactions we end up producing 4 ATP molecules and 2 NADH molecules.

Making the link (the link reaction):

In effect, this is actually a very complicated process, involving a lot of compounds. But the essence of it is that the pyruvate from glycolysis (which happens in the cytoplasm) is shuttled into the mitochondrial matrix, where it is dehydrogenated and decarboxylated to give acetate. This acetate quickly combines with coenzyme A (coA) to form acetyl coenzyme A, which shuttles it to the Krebs Cycle.

In terms of products, let's think: we have decarboxylated the pyruvate (in other words, taken carbon atoms from it). Now these carbon atoms combine with oxygen and are released as carbon dioxide. We have also dehydrogenated the pyruvate, which releases hydrogen atoms. The coenzyme NAD accepts these hydrogen atoms to form some more reduced NAD (or NADH, same thing).

The Krebs / Citric Acid Cycle:

Again, it is a very complicated process, but we will go over the basics. The acetyl coA offloads the acetate which combines with a 4 carbon compound called oxaloacetate to give citrate (which is a 6 carbon compound). Throughout this cycle, you decarboxylate and dehydrogenate the compounds, and this cycle keeps turning and turning and turning... Overall, this ends up producing carbon dioxide, some more NADH, some FADH (which is slightly different to NADH) and some ATP. How much I won't say, because we're going for an overview.

So far:

So far, we have some carbon dioxide, reduced NAD, reduced FAD and ATP.

The carbon dioxide will just be breathed out, that's fine. Now we need to deal with our NADH, FADH and the ATP (which can just go and be used in the cells directly). Their use comes in the next stage: the electron transport chain.

The Electron Transport Chain:

This is quite interesting I think, but quite confusing at first. In your inner mitochondrial membrane, you have your normal phospholipid bilayer but you also have these embedded proteins called electron carriers. Precisely you have 4 of them.

What happens is the NADH comes along and one NADH will split up into NAD, 2 hydrogen ions (which are sometimes called protons) and 2 electrons. Now, the electrons are absorbed by the first electron carrier and they are passed onto the next electron carrier. As they do this, the electrons (which are in an excited atomic state) start to de-excite and lose energy. It is this loss of energy that is then used to also pump in the hydrogen ions from the mitochondrial matrix to the inter membrane space.

So we have all of these electrons being transported along these chain of electron carriers (hence it's called an electron transport chain) and all this energy being used to pump hydrogen ions in. Now the electrons get to the fourth electron carrier and, essentially, hit a dead end. They can't go anywhere and no real surplus of energy either. So, what they do is simply combine with any remaining hydrogen ions in the matrix that haven't been pumped in (or that are just free) and some electrons to produce water. Oxygen is hence often called the final electron acceptor. Why? Well, because it is the last molecule to accept these electrons that have been given off by the NADH molecule.

As for the protons, well, they are now in the inter membrane space. In addition to the electron carriers, you have this awesome molecule called ATPsynthase, which protons love to flow through. They cannot flow through the bilayer because of their charge. But they still want to flow down their electrochemical concentration gradients and enter the matrix again. So, the ATPsynthase enzyme provides a path for them to travel down. But this is the really clever bit: as they move through the ATPsynthase, there is a 'motor' part on the enzyme that turns and catalyses the reaction between ADP and an inorganic phosphate that produces ATP. The reason for this is because of the potential energy these protons have. As they rush through the ATPsynthase enzyme, the are releasing energy and this energy causes the motor to turn and allows the reaction to be catalysed.

And what you end up with is ATP! Of course, you produce it before, but this is where the most comes from. That is why your body wants to get as much reduced NAD in the previous reactions, so that it can bring all these hydrogen ions to the electron transport chain where they can flow through ATPsynthase and produce tonnes of ATP.

Feel free to ask any questions. Hope it helped!