Photosynthesis is the process of capturing energy from the sun and converting it into a chemical form that can be stored (as glucose) for harvested at a later time.
solar energy + H20 + CO2 –> glucose + O2
Cellular respiration is the action of harvesting that energy and converting it into a form usable to power cellular processes.
glucose + O2 —-> H20 + CO2 + ATP
If solar energy and ATP are equated with simply ‘energy’ , then these two reactions are the same, only in opposite directions.
Cellular respiration begins in the cytoplasm, where the reaction occurs regardless of whether O2 is present or not. This first stage, glycolysis (from sugar + breaking) produces a Net of 2 ATP as well as the electron carrier molecules, NADH. If there is no Oxygen present (Anaerobic conditions), then this is the only energy-producing step that can occur and fermentation occurs in order to regenerate the NAD+ required to continue gylcolysis.
If O2 is preset (and if we are talking about eukaryotic organisms) then the products of glycolysis (two pyruvate molecules) will be transported into the mitochondrion for further processing. In this space, pyruvate is converted into AcetylCoA, which subsequently enters the Krebs / Citric Acid Cycle. No substrate survives beyond this point. There is a small amount of ATP formed by substrate level phosphorylation, but high energy electrons in the form of NADH and FADH are passed to the Electron Transport Chain.
The Electron Transport Chain is the last step of cell respiration and occurs when a high energy electron is passed into the chain, the electron is passed from one member to the next drawing H+ ions from the Matrix into the inter membrane space. As H+ ions accumulate in the intermembrane space both chemical and electrical concentration gradients. When this gradient is released and H+ ions are allowed to pass through specialized enzyme / channel proteins, the energy is harvested to form ATP via the process of oxidative phosphorylation / chemiosmosis.
I will attach a diagram of this series of reactions tomorrow and perhaps add some additional materials. Right now, I’ve already fallen asleep three times just in typing this.