This whole thing started with an email from Margaret Lee, a professor at Ohlone College in Fremont and Newark, CA, asking for an explanation regarding the chemiosmotic theory. Not being comfortable with things electrical, I turned to my friend and esteemed colleague, Frank Harold for a more reasoned response. Frank is a professor emeritus from Colorado State University and an Affiliate Professor at the University of Washington in Seattle. He is one of the main contributors to the understanding and elucidation of Mitchell's chemiosmotic theory.
Dear Dr. Schaechter (Elio),
I listen to TWIM and read Small Things Considered regularly, ever since I started teaching Microbiology at the local community college about a year ago. I'm not a microbiologist by training, though now I wish I had been, as I find it be such a fascinating area of biology.
I was reading your recent blog about Leftover Science and your work with bacterial membranes. I'm not able to comment specifically on the research you described there-- it's a little/or a lot over my head. However, I have had a more basic question about membranes for a while, and can't seem to find a satisfactory answer when I search the internet. I hope you don't mind if I ask. How do gram positive bacteria carry out chemiosmosis with a seeming lack of an enclosed membrane space? In some papers I see references to a very small periplasmic space in gram positives. Does that mean that the thick peptidoglycan layer is able to serve as a not-so-porous outer boundary?
Thanks so much for your help.
Elio forwarded your inquiry to me, since he still believes that I understand this stuff. You raise a good question, I remember that a version of it gave me conniptions when I was struggling to master Mitchell nearly 50 years ago.
The answer is that the cell wall has nothing to do with energy coupling by a proton current. Membrane vesicles are very good at linking the respiratory chain to various transport systems, but have no cell wall whatsoever. The way it works is that the respiratory chain pumps protons out of the cytoplasm, an enclosed space bounded by an impermeable membrane. The proton carries a positive charge, so its expulsion leaves its former partner behind holding a negative charge. Globally, this means that there is now an electrical potential across the plasma membrane, interior negative. That membrane potential draws protons (indeed, any positive charges) back into the cytoplasm. That is the proton-motive force that couples the extrusion of protons to useful work. Now, the particular proton that was expelled has long since been lost in the ocean outside the cell, but that ocean makes an infinite reservoir of protons (mostly virtual, available from the dissociation of water). Any proton out there will feel the pull, and if it finds itself near a pathway that allows it to flow in, it will do so. If that happens to be a ATP synthase, the downhill flux of protons can be harnessed to the synthesis of ATP. But a K+ ion will do too, helping to accumulate potassium.
NO, it isn't easy, but once you master it all of bioenergetics comes together to make sense. You will find longer discussions in some of my works (e.g. Harold, F M, The Way of the Cell, or The Vital Force), in David White' The Physiology and Biochemistry of Prokaryotes, or in Elio Schaechter et al.'s Microbe. If I can be of help, please feel free to contact me.
With best wishes,
I much appreciate the time you took to answer my question. Your explanation does make sense. I didn't think the cell wall had anything to do with the process, since textbooks say they are fairly porous. (I am not sure if the teichoic acids play a role in affecting ion movement--this was not part of my initial question.) So, it appears that you don't really need a membrane-enclosed compartment such as the mitochondrial intermembrane space or gram neg periplasmic space to get a proton motive force; the cytoplasmic space relative to the outside of the cell will work as well.
Thank you very much!
Hi Again, No, that's not right, you absolutely do need a closed compartment. I don't know how to draw a diagram on the computer, but if you send me a mailing address I shall send you one. Or else, look at a printed discussion. The mitochondrion works because it is a closed compartment: protons are pumped out of it, generating an electrical potential across the membrane, with the interior negative. If you poke a hole in the membrane, the potential collapses (there are substances that do this). It is just that the outside does not matter.
Dear Dr. Harold,
I did think a closed compartment is needed for the ATP synthase to work, as that's how the textbooks teach it. Most biology textbooks mention the bacterial cell membrane, but the discussion and the figures are usually with the mitochondrion. This figure in the link below shows ATP synthases in mitochondrion, chloroplast, and gram neg bacteria. My original question was, if you need a closed compartment for this kind of chemiosmosis synthesis of ATP, what/where is the closed compartment in the gram positive bacteria membrane? (Is it that in this case, "outside does not matter.")
Much appreciate your help.
in this case what's good for mitochondria is also good for a Gram-positive bacterium. The closed compartment is the whole cell, bounded by its impermeable plasma membrane. Protons are extruded from the cell, and the proton-motive force acts across the plasma membrane. The cell wall and periplasmic space, if any, have nothing to do with transport across the plasma membrane, nor with chemiosmotic energy coupling.
Thank you: your answer leaves no further questions on this matter. I much appreciate your help over these emails!