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So on a planet with a chlorine based atmosphere and only a small amount of oxygen, here is my proposed mechanism for chlorine based photosynthesis.

Carbon tetrachloride, since it is a liquid at 70 degrees Fahrenheit or 23 degrees Celsius, needs to be absorbed through the soil mostly though some does come from the air. HCl vapors and HOCl vapors are also absorbed into the black leaves. The leaves are black because since Chlorine absorbs UV and violet and blue light, all that is left for the plants is the range from red light to green light. As a result, they evolved a black pigment to absorb as much as they can of this light.

This also means that the plants get hot because they absorb a ton of infrared and then release some of it. So animals have evolved infrared vision to find these plants in the dark.

But anyway, back to what I was saying about photosynthesis.

HCl and HOCl are both hydrogen sources, HOCl is an oxygen source, and CCl4 is a carbon source. 6 of each are required to make 1 glucose. I haven't figured out exactly how this occurs but the black pigment that makes the plants get hot and release some infrared definitely helps this process by absorbing a lot of energy which can then be used to split the chlorine off of these molecules.

So you get this equation for chlorine based photosynthesis:

${6HCl + 6HOCl + 6 CCl_4 + light -------> 1 C_6H_{12}O_6 + 18Cl_2}$

Cellular respiration would essentially be the reverse of this except that the energy would be in a different form. So is this a plausible form of photosynthesis for plants adapted to living on a planet with a chlorine based atmosphere or is there a better alternative that still produces the glucose and chlorine but requires less energy for it to happen?

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I feel like oxygen is kind of crashing the chlorine party here.

On our world, oxygen is the readily available oxidizer and carbon is the energy currency. CO2 is the maximally oxidized carbon and CH4 the least oxidized. Some creatures oxidize CH4 to CO2. Eukaryotic life uses partly oxidized carbons like sugars and fats and alcohol - presumably because it is easier to handle metabolically. Glucose makes sense in an oxygen world. Alkanes or aromatics would also be good fuel - and are good fuel for internal combustion engines.

I think that in a chlorine world, chlorine should take the place of oxygen altogether. The chlorine analogs would be chlorocarbons not terminally chlorinated like carbon tetrachloride - chloroform, dichloromethane. You could make a long chain of these with chlorine hanging off, like a chlorine fat. You could not close a ring with chlorine like you can with oxygen.

I like the idea of a chlorinated aromatic filling the glucose role.

tetrachlorobenzene

A molecule like tetrachlorobenzene would be stable and would have loads of energy to be released on chlorination back to carbon tet.

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  • $\begingroup$ +1 My first thought was ‘why is oxygen required at all here’, but I lack the chemistry-ism to actually answer. $\endgroup$ – Joe Bloggs Oct 13 '18 at 7:52
  • $\begingroup$ "like a chlorine fat" wow. This makes me wonder about the food chain on that planet. $\endgroup$ – Tom Oct 13 '18 at 8:55

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