What is a proton gradient and how is it formed?
Basically, a proton gradient occurs when there are more protons (H+ or hydrogen ions) on one side of a membrane than the other. In terms of chemiosmosis, these protons are obtained from the electron acceptors NADH and FADH2 using the electron transport chain (ETC), which strips protons from those particles. This results in the ionized forms of NAD+ and FAD+, which are recycled back into the system. Originally, NADH and FADH2 were the products of the Krebs cycle. A total of three NADH molecules and one FADH2 molecule are produced through one run of the Krebs cycle. Since there are initially two pyruvate (pyruvic acid) molecules derived from the breakdown of glucose during glycolysis, the Krebs cycle must run twice per glucose molecule. Therefore, there are 6 NADH and 2 FADH2 molecules produced (along with 2 ATP molecules). As the electrons travel through the four different protein complexes (NADH's electrons are stripped in protein complex I while FADH2's electrons are removed in protein complex II), the protons are pumped out of the mitochondrial matrix using the proton pumps, which are also embedded into the inner membrane. As more and more NADH and FADH2 are introduced to the system, the proton concentration of the intermembrane space or stroma increases.
In plant cells, this process is somewhat different from other eukaryotic cells in a few key ways. The first is that chemiosmosis would occur in the chloroplast, and the proton gradient would form along the thylakoid membrane. The two "sides" of diffusion are the chloroplast stroma (the liquid/gel-like substance similar to the cytoplasm or cytosol) and the thylakoid lumen (the area inside of the thylakoid discs). Additionally, rather than having protein complexes to pump proteins outside, the chloroplasts make use of their photosystems (Photosystems II and I, respectively) and cytochromes to carry out electron transport.
In plant cells, this process is somewhat different from other eukaryotic cells in a few key ways. The first is that chemiosmosis would occur in the chloroplast, and the proton gradient would form along the thylakoid membrane. The two "sides" of diffusion are the chloroplast stroma (the liquid/gel-like substance similar to the cytoplasm or cytosol) and the thylakoid lumen (the area inside of the thylakoid discs). Additionally, rather than having protein complexes to pump proteins outside, the chloroplasts make use of their photosystems (Photosystems II and I, respectively) and cytochromes to carry out electron transport.
The last protein complex embedded in the membrane is the enzyme ATP synthase. This is the fifth and final complex used in the production of ATP and will come in handy in the last stage of chemiosmosis.
In summary, know that in this stage of chemiosmosis, a proton gradient is formed by the pumping of hydrogen ions by proton pumps in the form of protein complexes or cytochromes across a membrane into the surrounding area. This imbalance of protons (which have a positive charge) on one side of the membrane plays a crucial role in what occurs during diffusion, which you will explore in the next section.
In summary, know that in this stage of chemiosmosis, a proton gradient is formed by the pumping of hydrogen ions by proton pumps in the form of protein complexes or cytochromes across a membrane into the surrounding area. This imbalance of protons (which have a positive charge) on one side of the membrane plays a crucial role in what occurs during diffusion, which you will explore in the next section.