I would like to understand what the term H+ Gradient means. I googled this question and found terms such as chemiosmosis and ion gradient being tossed around. I am very new to biology and I do not know any advanced terms or anything of the like. What I do understand is that the mitochondria produces ATP and has something called a proton pump.

Thank you in advance.

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    $\begingroup$ A gradient is defined as the difference in concentration of a solutant between two places. For example a sodium gradient along a cellular membrane is the concentration difference between the inside of the cell and the outside space. A H+ gradient (also called proton gradient) is therefore the concentration difference between the inner mitochondrial membrane and the outer mitochondrial membrane. Does this help you? I can expand this later today in a proper answer. $\endgroup$ – Chris Oct 29 '14 at 16:40
  • $\begingroup$ This is really helpful, thank you! Why is it called proton, though? Shouldn't it be ion? And why is this necessary to maintain? I know that lysosomes break down cells that cannot maintain their H+ Gradient, what does ion concentration have to do with the organisms need to break it down? Sorry to ask so many questions. Thank you very much for the help! $\endgroup$ – Sciiiiience Oct 29 '14 at 16:55
  • $\begingroup$ @Sciiiiience An H+ ion is a proton $\endgroup$ – tel Oct 29 '14 at 18:42
  • $\begingroup$ Oh I see! Sorry, I didn't think that through. I forgot hydrogen is just proton-electron. Thanks $\endgroup$ – Sciiiiience Oct 29 '14 at 18:44

Lets start with a general definition: A gradient is the difference in the concentration of a substance (this can be a solutant in liquids, a gas or whatever) between two places. The image below shows the general definition. If nothing else happens what keeps the gradient stable, over time diffusion will cause an equilibrium between both places. The flow of substance always goes from the high concentration to the low (indicated by the arrows).

enter image description here

In biological systems the two parts of the gradient are usually divided by a membrane and are more stable. Gradients can be generated by active transport which requires the input of energy to work against the gradient. Examples would be the concentration gradient along a membrane like protons (positively charged hydrogen-ions), as this can be seen in the mitochondria. Here three complexes of the respiratory chain (I, III and IV) pump protons from the matrix in the intermembrane space, which requires energy. The fourth complex (V) pumps the protons back and uses the released energy from this process to make ATP. See the figure below which shows this process schematically (from here):

enter image description here


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