Help with understanding ATP synthesis

[indrani]

I am having trouble understanding why and when the mitochondria generate ATP. I am trying to explain it without using technical terms such as chemiosmosis etc.

 

Here is what I understood - please correct me if I am wrong. Also, for each point (in bold), I have a few questions.

 

• Mitochondria breaks glucose into ATP.
• Does this happen as soon as we eat?
• What about the rest of the time when the food has been digested? Does the glucose come from breakdown of glycogen, or is that only for the muscles?
• Is ATP ever made from scratch?

• Mitochondria breaks 1 glucose molecule in a series of steps during cell respiration. The by-products (NADH, FADH) form a total of 38 ATP.
• If these NADH and FADHs can form 2/3 molecules of ATP and are supposed to be high energy carriers, why are they not used as energy currency?
• How does an NADH/FADH molecule form ATP?

• Mitochondria sense a ratio of 10:1 ATP:ADP in the cytosol. The imbalance of this ratio (due to ATP being used by other molecules) stimulates the mitochondria to take some of the ADP and convert it into ATP.
• If ATP is continuously recycled from ADP, why is glucose is broken into ATP?
• If ATP is recyclable, why is glucose needed? Why doesn't the mitochondria just recycle the used ADP
• Is the breakdown of glucose what stimulates the production of ATP - so as soon as there is an imbalance in the ratio, glucose will be broken down to generate new ATP molecule? Is that how it works?
• If we get 38 ATP every time glucose undergoes breakdown, does the amount of ATP in a cell increase over time?
• Is there a durability of ATP - like it can only be recycled so many times? Or can one ATP molecule be recycled forever?

• Glucose is a big energy carrier, ATP carries the energy in small pockets. I read somewhere that glucose is like a 10 dollar bill, ATP is like change. You need the change for things like parking meter and washing machines.. but you can't carry around 10 dollars in change all the time. So you store it until you break that 10 dollars for the change.
• Is this true?
• How does glucose store energy? It's breakdown stimulates energy production. But its not like the glucose molecule itself has any energy, right?
• Does every cell store glucose? As glycogen? or is it only muscle and liver cells?
• Can every cell breakdown glycogen to glucose? Or is it only muscle or liver cells?

 

I know I've asked a lot of questions, maybe even redundant ones. But I have been searching for answers to these questions for weeks in journals and books. I'm probably not finding the right articles or understanding what I'm reading.

 

Could you please help me understand? Answers to any question would be highly appreciated. Thanks!!

[CharonY]

Not enough time to answer everything but there are some errors in your assumptions:

 

Mitochondria breaks glucose into ATP.

This is not correct. Depending on how you view it the final degradation is either pyruvate (end of glycolysis) or CO2. ATP is a byproduct of glycolysis or generated via the respiratory chain (assuming you are talking about eukaryotes). The TCA cycle generates reduction equivalents that are used to power proton transport throughout the respiratory chain. These pumps create a potential difference near the mitochondrial membrane that ultimately powers the ATP synthase to generate ATP.

 

This also addresses your next point regarding NADPH, they are the start point of the respiratory chain, and the potential difference between NADPH and the terminal electron acceptor (O2 in aerobic respiration) powers the respiratory chain.

 

I think to understand the relationship of how energy is generated via glucose (and it is obviously not the only molecule) you need to look at the pathways feeding the TCA cycle (including glycolysis but also e.g. beta oxidation) to get an idea how metabolites flow (it is not that glucose gets converted to ATP, for example).

 

But the central hub is really the TCA where the bulk of NADPH is generated for the respiratory chain (and it also serves as a means to convert and replenish metabolite pools).

 

That is on the cellular side of things. But for larger organisms you also need to take into account that the nutrient have to be first transported into the blood stream and targeted to organs such as liver or muscles.

Edited June 10, 2015 by CharonY

[Inhibitor]

You need to read a biochemistry textbook to get proper answers to these questions. Short version: the oxidation of a substrate and subsequent transfer of electrons through the electron transport chain results in a pumping of protons outward across the inner mitochondrial membrane. This electrochemical proton gradients represents a pool of potential energy which is then used to synthesize ATP from ADP + Pi by ATP synthase.

Edited July 3, 2015 by Inhibitor