Chapter 7 Anabolism and the sources of nutrients for organisms
Chapter summary:
In the previous chapter, we have seen the way life has found to release the energy stored in high energy electrons on organic molecules. The secret is that in an aerobic environment with the atoms of oxygen in the dioxygen molecule being so eager to capture electrons, the electrons can freely and will naturally transfer from the organic molecules to oxygen. This electron transfer releases energy. We have seen that in the case of methane combustion, the electron transfer is direct, leading to the release of ephemeral heat and light, resulting in a large and sudden increase of entropy. But this type of energy release is just not sustainable for organisms which must find ways to minimize the energy release in the form of heat, and have a way to release the energy exactly where it is needed.
It does need an intermediate and ubiquitous storage of energy. The secret is the diphosphoanhydride bond of the ATP molecule, which can temporarily store lots of energy without the involvment of direct electron transfer. In the case of the oxidative phosphorylation, the formation of the diphosphoanhydride bonds is thought to be mechanically induced between ADP and a phosphoryl group during the rotation of the ATPase, itself powered by a proton flow maintained between compartments. In the end, the electron transfer only indirectly leads to the formation of ATP, by powering the proton pumps. It is thus fascinating that the release of the energy chemically stored onto high energy electrons on organic molecules involves many small steps, which essentially prevent the direct transfer from the electron donor to the acceptor, which would lead to combustion…
All of this works in the long run if somehow there is a source of organic molecules, otherwise with a finite amount of organic molecules, the respiration process would eventually deplete all organic molecules on earth. So the correlary to the energy release in respiration, is the capture and storage of energy. And this is where it is very useful to be on a planet close enough to its star such that it receives enough but not too much energy. One of life’s secret has been to be able to capture this allochthonous source of energy, i.e., sunlight, and store it, and now you know in what form: store it in the form of high energy electrons onto organic molecules. This chapter provides enough of the details of photosynthesis processes that lead to the formation of carbohydrates.