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Essentially, cellular aerobic respiration and photosynthesis are the opposite of each other. Photosynthesis begins with energy and makes organic molecules, while cellular aerobic respiration begins with organic molecules and breaks it down into energy. Photosynthesis is a series of reduction reactions, respiration is a series of oxidation reactions. Photosynthesis is endergonic, respiration is exergonic. Photosynthesis is anabolic, respiration is catabolic.
Respiration begins with glucose entering the glycolysis cycle (in the cytoplasm of the cell), where it is broken down through a series of reactions into pyruvate. Pyruvate undergoes oxidation (now in the matrix of the mitochondria), and is bonded to CoA, forming Acetyl CoA (CoA = Coenzyme A). Acetyl CoA is used in the Krebs cycle and is oxidized, releasing electrons to NADH and FADH2. These reduced molecules are then used in the electron transport chain (ETC), where they pass their electrons to protein complexes in the mitochondria's membrane. These proteins are channels for hydrogen ions (protons). When these channels open protons flow through to the other side of the membrane. A gradient is then created and this causes the protons to flow through another enzyme complex called ATP synthase. This enzyme is somewhat like a turbine, and using the proton gradient creates ATP. The protons then bind to oxygen molecules and pick up an electron, making water, one of the by products of cellular aerobic respiration. CO2 is also a by-product, through decarboxylation (removal of carbon, along with an oxygen molecule - in this case (CO2)) during pyruvate oxidation, and the Krebs Cycle.
Photosynthesis on the other hand converts photons to ATP through the light dependent reactions within the chloroplasts, via use of photosystems. Water gives up an electron to a photosystem which becomes excited when hit by photons. This excited electron is passed along a chain on the thyllakoid disk, similar to the membrane of the mitochondria, and used to produce the same product. The excited electron goes to a proton pump, which creates a gradient, and the gradient through diffusion causes protons to goes through another enzyme, similar to ATP synthase. The electrons are then picked up by NADPH, similar to NADH and FADH2 in animal cells. These molecules are then used in the light independent reactions - Calvin Cycle. In which CO2 is converted into glucose through introduction of ATP and NADPH.
- This is somewhat of a rough overview, hopefully it's enough for you. Sorry I kinda skimped out on photosynthesis at the end.
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