Biochemistry/Electron Transport Chain

The electron transport chain is a system of molecules through which electrons are transferred to generate ATP. It has an important role in both photosynthesis and cellular respiration.

ETS in photosynthesis

In photosynthesis, when sunlight is absorbed in photosystem 2, electrons are energized. They are transferred to the reaction center. From the reaction center, the electrons enter the electron transport chain and pass the etransport chain molecules. Then the 2 deenergized electrons are reenergized in Photosystem 1 (is second because PSII was discovered first)and they go to the NAPD+ reductase which transfers ther electrons to a coenzyme, converting it to NADPH+ The two electrons used must be replaced, so water is broken down producing 2 protons (H+) which concentrate in the thylakoid membrane, 2 electrons replaced in PSII and oxygen released as O2. Protons go down proton pumps and a concentration gradient forms as protons move from the stroma into the thylakoid space. Protons move down gradient through ATP synthase which forms ATP. Energy is stored.

ETS in respiration

In respiration, an electron transport chain is used to break the fall of electrons to oxygen into several energy-releasing steps. It slows down the releasing process of energy. There are a number of molecules that consists an electron transport chain. Most of these molecules are proteins, built into the inner membrane of mitochondria of eukaryotic cells and the plasma membrane of aerobically respiring prokaryotes. On the "top" end of the chain, where the energy is highest, electrons removed from oxidation of glucose are shuttled by NADH. On the "bottom" end, where the energy is lowest, Oxygen molecules pick up the electrons, and together with protons existing in the cellular environment, oxygen molecules form water. The total energy released from the oxidation of glucose by oxygen is huge, about 2880kJ/mol. (The overall oxidation of glucose is shown by reaction C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O). This energy can't be picked up by cells in a short amount of time. Therefore, electron transport chain applies a cascade of reactions to slowly release the energy, maximizing the usage of glucose. In the cascade, each "downhill" carrier is more electronegative than their "uphill" neighbors, so that the electrons can be passed "downhill" by oxidation. Oxygen is the last electron acceptor.

the detailed graph of the mechanism of electron transfer chain

image citation:http://en.wikipedia.org/wiki/File:Mitochondrial_electron_transport_chain%E2%80%94Etc4.svg