The Electrochemical Proton Gradient



The electrochemical proton gradient is movement of hydrogen ions across a membrane diffusing from an area of high hydrogen concentration to an area of lower hydrogen concentration, producing a concentration gradient and an electrical potential gradient. These gradients together store potential energy in the cell and available for work. And so cellular respiration is dependent on the function of the electrochemical gradient.

During oxidative phosphorylation a high energy electron is passed along an electron transport chain the inner mitochondrial membrane in eukaryotes. This linked set of proteins carry out redox reactions which release energy. The energy released by electrons flowing through this transport chain, pumps hydrogen out of the matrix space across the inner mitochondrial membrane in a process called electron transport. This produces an electrical potential across the membrane and a store of potential energy as a pH gradient. The gradient created by this allowes hydrogen back across the membrane, through an enzyme called ATP synthase. This process is called chemiosmosis and it occurs during cellular respiration or photosynthesis As this happens, the enzymatic activity of ATP synthase synthesizes adenosine triphosphate, ATP, from  adenosine diphosphate, ADP in a phosphorylation reaction which is driven by the proton flow. There are five main protein complexes involved in eukaryotes whereas in prokaryotes there are many different enzymes present that use different electron donors and acceptors located in the cells membrane or intermembrane space.

Although cellular respiration is dependant on the function of the proton gradient the oxidative phosphorylation also generates a significant proportion of the formation of the harmful ROS, reactive oxygen species, in cells such as superoxide and hydrogen peroxide. Resulting in free radicals that damage cells and contribute to disease. Mitochondria therefore also has mechanisms that protect the cells from the reactive oxygen. As an example the increase of ethanol concentration in a cell changes the conditions so that the mitochondria produces more reactive oxygen. Ethanol may also damage the mitochondrial DNA leading to the deterioration of the protective mechanisms. Hence mitochondria have a central role in the emergence of liver disease due to alcoholism.

References:

Essential Cell Biology 3rd Edition, Bruce A. & al., read 1.12.2015

Chemiosmosis, Wikipedia article, https://en.wikipedia.org/wiki/Chemiosmosis, accessed 3.12.2015

Electron transport PDF file, http://home.ku.edu.tr/~okeskin/Biol200/lecture10-biol.pdf, accessed 3.12.2015

Oksidatiivinen Fosforylaatio,Solunetti c 2006,http://www.solunetti.fi/fi/solubiologia/oksidatiivinen_fosforylaatio/1/, accessed 27.11.2015