The Proton Gradient
Electric potential across membranes is an incredibly interesting concept to consider. Usually when we think electric potential a battery will pop into our mind, not a cell membrane. In this section, I will be talking about how electric potential in a membrane works and then more specifically on how the electric potential within the mitochondrion drives the synthesis of ATP.
To help visualize cell potential in a membrane, we can actually view the cell membrane as a parallel plate capacitor with a dielectric. The reason we can do this is that the basic structure of the cell membrane. A cell membrane is made up of a semi-permeable substance (lipids) that prevent the free-flow of ions across it; because ions can not freely move from one side to the other, it creates charge on one side and a lower charge on the other. The charge gradient is formed and this is how the energy in cells can be stored.
In the case of ATP, while there are other forces that drive the synthesis of ATP we will focus on the proton gradient. In the Electron Transport Chain, protons are pumped to one side of the membrane creating a transmembrane potential. With an abundance of protons on one side of the membrane it will have a strong positive charge and a bunch of protons ready to leave and go back across the membrane - nature will always try to reach equilibrium. Since H+ protons are ions pumped across, they need a little help getting back to the other side of the membrane. Proteins in the membrane utilize the ion's need for help and tendency to want to go across the membrane by making it useful for them. The ion's charge is used to drive their individual processes, like making ATP, and this cycle fulfills both the protein's and the ion's wants. The diagrams below helps illustrate the proton gradient I am referring to.
To help visualize cell potential in a membrane, we can actually view the cell membrane as a parallel plate capacitor with a dielectric. The reason we can do this is that the basic structure of the cell membrane. A cell membrane is made up of a semi-permeable substance (lipids) that prevent the free-flow of ions across it; because ions can not freely move from one side to the other, it creates charge on one side and a lower charge on the other. The charge gradient is formed and this is how the energy in cells can be stored.
In the case of ATP, while there are other forces that drive the synthesis of ATP we will focus on the proton gradient. In the Electron Transport Chain, protons are pumped to one side of the membrane creating a transmembrane potential. With an abundance of protons on one side of the membrane it will have a strong positive charge and a bunch of protons ready to leave and go back across the membrane - nature will always try to reach equilibrium. Since H+ protons are ions pumped across, they need a little help getting back to the other side of the membrane. Proteins in the membrane utilize the ion's need for help and tendency to want to go across the membrane by making it useful for them. The ion's charge is used to drive their individual processes, like making ATP, and this cycle fulfills both the protein's and the ion's wants. The diagrams below helps illustrate the proton gradient I am referring to.
|
|
As the protons come back across the membrane, they are harnessed by ATP synthase (the protein structure within the inner mitochondrial membrane that makes ATP). ATP synthase will twist as the proton moves through. The H+ ions provides the protein with the energy to twist and squish together ADP and a Phosphate group to form ATP and this production continues as long as there is a proton gradient.
We can use the theorized diagram mentioned above to calculate the electric potential across a membrane. To calculate the number of protons needed to build up a potential across the membrane we treat it like a capacitor. In the example below, we are just calculating the number of protons need to be pumped to build up a sufficient charge to produce (some) ATP.
We can use the theorized diagram mentioned above to calculate the electric potential across a membrane. To calculate the number of protons needed to build up a potential across the membrane we treat it like a capacitor. In the example below, we are just calculating the number of protons need to be pumped to build up a sufficient charge to produce (some) ATP.
|
The areal charge density (sigma in the top equation) of a parallel plate capacitor and total charge q can be related and used together to find the number of protons.
|
|
Just using a H+ proton gradient is actually incredibly inefficient and the cell has other means to creating a stronger potential across the cell. Various other ion concentrations like Potassium, Sodium, and Chlorine increase the difference in charge across the membrane and make driving the cell feasible. The number of protons calculated above would only last the body about 1 ms. ATP requires about 4 protons to be produced and the body requires a lot ATP to function on a daily basis.
Daisy Herrman | Physics 212 | Spring 2019 | Web Project | April 12, 2019