The electron transport chain is a series of membrane-embedded molecules that transfers electrons from one molecule in the chain to the next. The energy released by this process is used to pump hydrogen ions across a membrane and store energy.
An electron transport chain (ETC) is analogous to a pump forcing water up a hill against gravity -- the process takes energy, but once the water is at the top of the hill it has potential energy and can drive a turbine as it flows back down. By pumping protons across a membrane, the ETC creates a concentration gradient that other membrane-embedded proteins can use to power the manufacture of adenosine triphosphate or ATP.
ETCs are vital for photosynthesis and for cellular respiration. Photosynthesis takes place in structures called chloroplasts, while cellular respiration occurs in structures called mitochondria. ETCs involved in photosynthesis are found inside the chloroplast in the membranes of small sacs called thylakoids. ETCs involved in cellular respiration, by contrast, are found in the inner membrane of the mitochondria.
Although ETCs perform similar functions in both cellular respiration and photosynthesis, these two processes actually have opposite effects. Photosynthesis makes sugar molecules as a starting point for the synthesis of other organic compounds and is thus an anabolic process -- a process that builds up larger molecules from smaller components. Cellular respiration, by contrast, is a catabolic process, meaning that it breaks down sugars to yield smaller molecules of CO2 and water.
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