Krebs cycle is what's generating the carbon dioxide that you breathe out every time you break down your sugar and then the electron transport system is what generates the load of ATP. the Krebs cycle in general what happens is that pyruvate from glycolysis out in the cytoplasm as it starts to enter into the mitochondria enzymes will take off a couple of high energy electrons and then onto an electron carrier called NAD positive. Then add 2 electrons it becomes negatively charged and then takes a nearby positively charged hydrogen ion, and becomes NADH. So NADH is a full high energy electron carrier full of high energy electrons and it goes off to electron transport system where those high energy electrons can be used to generate a lot of ATP.

In the process of photosynthesis, atmospheric carbon is absorbed by plants.
The carbon is then transferred form plants to the animals feeding on them, and then further moves up the food chain.
Respiration, digestion and metabolism, plants and animals result in some transfer of carbon back to the atmosphere.
Some carbon also moves to the lithosphere when living organisms die or when wood and leaves decay or When animals excrete. Some of these living beings were buried millions of years ago have been converted to fossil fuels.
Burning and mining of fossil fuels cause this carbon to move from the lithosphere to the atmosphere.
Some of this atmospheric carbon gets dissolved in the oceans and then completes the cycle.