One gradient to rule them all…

Here’s something new about the proton motive force in photosynthesis.

If you think about bioenergetics at all, and I realize that many of you probably don’t do this, you may remember that the universal underlying mechanism for creating biochemical energy involves a chemical gradient across a biological membrane system. The ultimate goal is to make ATP, the fundamental energy currency of living systems. Photosynthesis and Cellular Respiration are often presented as opposing processes, but their function is the same- to make ATP. These processes just use different energy sources to accomplish this feat (photosynthesis-light; cellular respiration-glucose). The chemical gradient used by biological systems is a proton gradient. So, energy sources (light or glucose) are used to pump protons across a biological membrane. The proton imbalance across this barrier results in potential energy that can be used for biochemical purposes because energetics favors the flow of these protons back down the gradient to the side of the membrane with fewer protons. The enzyme ATP synthase takes advantage of this proton potential, allowing protons to flow back over the membrane, but coupling that energy to form ATP.

made for chemiosmosis article from Chemiosmosi...

made for chemiosmosis article from Chemiosmosis1.svg (Photo credit: Wikipedia)

This coupling of a proton gradient to generate biochemical energy is called the chemiosmotic theory. This mechanism has been well-established for decades and the Nobel Prize for its conception and proof has long since been awarded. You wouldn’t think that anything related to something as basic as the proton motive force and chemiosmosis would be fodder for a glam* journal like Science, but you’d be wrong. It turns out scientists are still finding new components of this established theory.

In the October 4th issue of Science, Carraretto and colleagues report on a potassium channel that is essential for balancing the components of the proton motive force and fine-tuning photosynthesis in response to environmental conditions.** The proton motive force has two components: the chemical pH gradient (∆pH) and the electrochemical gradient (∆Ψ) from the ionic imbalance. The ∆pH component plays a major role in facilitating mechanisms of light energy dissipation to shunt energy away from photosynthesis when the system is functioning at maximal capacity. The physical relationship as to how the two components are regulated remains unclear, but something must be controlling them since the photosynthetic machinery is particularly sensitive at optimizing the competing processes of photochemical efficiency and photoprotection. Carraretto and co-authors report on AtTPK3, a potassium channel in the thylakoid membrane that controls the magnitude of the ∆Ψ component. It does this by counterbalancing ions across the thylakoid membrane. As H+ ions are translocated to the thlykaoid lumen, AtTPK3 moves K+ ions into the stroma to maintain overall charge neutrality. Without this protein, plants cannot optimally partition the components of the proton motive force and have problems with optimizing photosynthesis under changing environmental conditions.

TPK3 moves potassium ions out of the lumen and into the stroma as protons are pumped into the lumen

TPK3 moves potassium ions out of the lumen and into the stroma as protons are pumped into the lumen

While the chemiosmosis theory is accepted dogma among today’s scientists, when the theory was first postulated by Peter Mitchell in his 1961 Nature paper***, it was controversial. Mitchell had to be tenacious and careful in his arguments for the chemiosmotic theory and against substrate-level phosphorylation (an opposing theory at the time with its own loud and credible proponents). It is also worth noting that the scientific career path of Peter Mitchell is an interesting read. I recommend checking out the links below. He successfully worked his way through a traditional academic career track, but not long after publishing his initial paper on the chemiosmotic theory, health problems necessitated a change in research venue. He left his research for two years while he renovated Glynn House as a personal residence and laboratory. He, along with his research colleague Jennifer Moyle, founded Glynn Research Ltd, an independent research laboratory funded by personal holdings from Peter and his brother. He continued at Glynn for the remainder of his research career.

With respect to the personal careers of scientists, maybe there isn’t anything new under the sun. The same struggles of work-life balance, advocacy for controversial research conclusions, and alternative funding strategies have been played out before. Maybe we ‘early career’ scientists are being too conservative and selling ourselves short. Maybe the structure of the science community would be vastly different if more of us said “yes” to the mad ideas we have as did Peter Mitchell and Jennifer Moyle. From Passionate Minds: The Inner World of Scientists:

“But when I got ill and had to leave, I went down to live in a cottage next door to Glynn, and started to think, well, what am I going to do now, because I can’t be idle. So the possibility arose that one might found an independent institute. As it was most unlikely that you could do all that on your own, I wrote to Dr. Moyle, and said ‘I have a mad idea. Would you be prepared to give up science for two years and work with me to restore Glynn House, and then try and establish an independent charitable foundation?’ Of course, she should have written back and said ‘No’, but she didn’t. She wrote back and said ‘Yes’.”


*glam journal: Cell, Nature or Science, the upper echelon of the scientific publishing community, if your work can attract the attention of these guys, you’ve got a golden ticket for your career path.

** Sorry, as with all glam journals, it’s behind a paywall.

***Yes, it is fifty years old, but still behind a paywall.



One thought on “One gradient to rule them all…

  1. Ian

    Very nice. You know, I’ve been pondering what keeps us from taking more chances/being riskier as young scientists these days. Ethan Perlestein on Twitter talks about this- why don’t more Ph.D.’s try start-ups? And I think part of the answer is because we don’t have a lot to venture and lose- being poor makes you really cautious I think (even if it’s a good chance to take). And some of us just don’t have research to translate into an independent place (that would lead to a company that would lead to a career). You can look up the company Applied Minds too- they’re as close as you might get to a collection of independent scientists working on what they’re interested in.


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