Efficacious Universe

June 26, 2011

The earliest form of photosynthesis on Earth was thought to be the result of a mutated gene that coded for a sunscreen pigment. When the earliest life on Earth spread out from the hydrothermal vents, these organisms eventually spread across the entire ocean and found their way closer to the surface in search of food. However, the surface of the oceans were all but sterilized because the radiation being belched from our rambunctious Sun, which was more dangerous than today’s more placid Sun, would have killed anything that got too much exposure. Some cells adapted to this environment by capitalizing on a novel pigment that could protect the cell’s machinery by absorbing any UV radiation that might strike the cell’s surface. This pigment could transmit visible light away from anything inside the cell that could be damaged and over generations a species found a way to utilize that light to transport electrons through its membrane.

The electron transport chain is a metabolic pathway that creates ATP, the energy source for our cells. Animals use oxidative respiration to replenish a chemical called NADH that fuels the electron transport chain. Other organisms like bacteria use anaerobic respiration to generate NADH, but this isn’t as efficient and it doesn’t produce as much ATP from this older pathway. In either case, the NADH produced from these pathways donates its proton and carries electrons that can be harvested for use in the electron transport chain. These electrons are picked up by an electron carrier molecule that shuttles the electrons across a section of the cell membrane. The price to ferry these electrons through the cell membrane is to push a proton from outside the cytoplasm to inside of the cell membrane. This process of paying protons to move electrons culminates when those electrons get passed across the entire electron transport chain, then those electrons get attached up by an atom like an oxygen or sulfur along with two hydrogen atoms that got pumped inside the cell.

In photosynthesis, a photon that strikes a structure within a cell called a thylakoid gets passed through a pigment like chlorophyll until it reaches the end of the line and dumps that energy onto an electron; like a game of hot-potato that photon gets passed through proteins called antenna systems until it reaches that electron and excites it. Plants and other organisms that undergo photosynthesis have two separate and consecutive photosystems that work in series. Electrons are normally in a so-called ground state, but when they suck up the energy of a photon, it causes them to jump into a higher energy state. That electron flies off of the chlorophyll molecule and gets funneled into the electron transport chain. Photosystem II funnels photons through the thylakoid membrane of a chloroplast to split water apart; this replenishes the electrons lost when that photon carries an excited electron from the pigment molecule chlorophyll through the electron transport chain. This leaves the chlorophyll molecule with a positive charge and in order to reset that molecule back to a more stable form, water molecule gets ripped apart; one of its electrons gets incorporated into the chlorophyll. Photosystem I funnels another photon in to replenish NADH, an electron donor that powers the Calvin cycle that creates sugar for plants. The hydrogen atoms from water float around inside the cell until they get used to pay the ferry that moves electrons across the thylakoid membrane while the oxygen atom gets released into the atmosphere. This is how oxygen got built up in our atmosphere on Earth; the combined photosynthesis of early bacteria and plants belched out so much oxygen into the atmosphere that it poisoned most of the anaerobic life on Earth at the time. More on that later.

Photosynthesis went on before the advent of oxygen in our atmosphere; that and a process called chemosynthesis used a different molecule to accept electrons at the end of the electron transport chain. In these processes, sulfur and hydrogen sulfide are used instead of oxygen and water as the final recipient of electrons channeled through the electron transport chain. A form of anaerobic photosynthesis used by purple and green sulfur bacteria, for instance utilizes the same mechanisms of photosynthesis to create sugars from visible light, but this pathway only requires photosystem I and not II.

This diagram outlines the path an electron takes across the thylakoid membrane to complete the electron transport chain and power ATP synthesis in a cell.

 

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