Oxidative Phosporylation The first cells on the plants were fermentative using simple organic molecules to generate ATP - not too efficient and left a lot of energy stll in the food! Soon after it appears that a much more efficient system evolved - transport of electrons along the plasma membrane it was so successful that today, billions of years later, the very same process is essential to life on Earth! The same principles are used as the basis of both photosynthesis (generation of complex molecules using light energy) and oxidative phosporylation (catabolism of food in the presence of oxygen) As you have heard endlessly - the main energy currency of cells is ATP, and not surprisingly it is the key player here.
Most energy comes from membrane- based mechanisms The process consists of two recognized steps 1) Electron transport chain - Protons (H+) are actively driven across the membrane as electrons are handed between a number of membrane embedded proteins. 2) ATP synthase - harnesses the electrochemical H+ gradient of the membrane (generated by step 1 above) to drive the generation of ATP from ADP and Pi. Students are given the analogy of a dam and turbines. This is not a bad analogy as the parts of the ATP synthase protein do actually spin when making ATP, as protons flow through the protein, as we shall see in the video.
Chemiosmotic coupling The linkage of electron transport, proton pumping, and ATP synthesis is called the chemiosmotic coupling model. This mechanism evolved in bacteria way back in history Then when these bacteria became a constituent of eukaryotic cells first as mitochondrion and then as chloroplasts, they enhanced those cells with the benefits of this very efficient process
Mitochondria Nearly all eukaryotic cells have mitochondria When glucose is converted to pyruvate (glycolysis) only 10% of the energy stored in the chemical bonds of the glucose is harnessed in the formation of the the 2 ATP molecules In mitochondria the sugar is broken down completely to CO2 and H2O and about a net total of 30 ATP molecules are generated from each molecule of glucose.
Mitochondria structure Mitochondria have two membranes - outer and inner Mitochondria have two internal compartments - the matrix inside the inner membrane, and the intermembrane space, which exists between the two membranes The outer membrane is peppered with large protein pores (made by a protein called porin) that let pass all small molecules and proteins up to 5000 daltons in size. The inner membrane is very selective and only permits movement of molecules as per the plasma membrane, using protein channels. The inner membrane is highly folded into cristae to increase surface area The inner membrane of mitochondria is the site of this oxidative phosphorylation in eukaryotes, and the inner membrane surface in prokaryotes.
Food for Mitochondria Pyruvate (from sugars) and fatty acids (from fats) are transported across to the mitochondria from the cytosol before conversion to acetyl CoA, by pyruvate dehydrogenase This molecule then enters the citric acid cycle in the matrix of the mitochondria. It is here that the majority of the high- energy electrons carriers are charged with electrons - the NADHs and FADHs It is these that are then sent over to the membrane for participation in the electron transport chain. SEE SUMMARY…
Respiratory Chain Oxidative phosphorylation is also known as the electron transport chain or the respiratory chain. This chain contains over 40 proteins of which 15 are directly involved in electron transport. Most of these are integral membrane proteins, some are transmembrane in nature. Most of these exist as three large respiratory enzyme complexes –NAHD dehydrogenase complex –Cytochrome b-c1 complex –Cytochrome oxidase complex Each is complexed with metal ions and other chemical groups
Proton Pumps Each of these three complex proteins is really a proton pump They each need energy to work This energy comes from the high energy electrons given to them by NADH or FADH The electrons contain so much energy that each complex takes what it needs to pump protons to the outside of the membrane and then passes the same electron to the next complex in the chain - hence the name electron transport chain…
Electrochemical gradient Once again the difference on the two sides of the membrane of protons generates two forces; 1) Concentration difference - more protons on the outside then in by a factor of 10. –Because protons are important factors in determining the pH of the media - we see that there will be an associated pH difference - the inside of mitochondria is pH 8 and the intermembrane space is pH 7 - as this is highly permeable to ions because of the ‘leaky’ nature of the outer membrane - the whole cell is pH 7 also. 2) Charge difference - Protons are positively charged. So if there are more on the out side surface of the membrane then that will have a positive charge. Therefore, the inside of the mitochondria will have a negative charge. This makes the mitochondria a large battery…
The first battery Just like a real battery the energy stored in maintaining this electrochemical difference can be used by the mitochondria to drive other systems One of these is the ATP synthase - to make ATP from ADP and Pi…
The combination of these two forces is additive and adds to the potential energy
The structure of the enzyme ATP synthase, which makes ATP by using the energy of protons entering the cell
14_21_Redox_potential.jpg The flow of energy along the electron transport chain - as you can see the analogy with a river and dam system is appropriate as the electron can be considered as the water in the river, and each protein complex as the turbines in each dam. Eventually, the water flows to the ocean and cannot flow any longer
O2 enters the picture here! In the very last step. Cytochrome oxidase combines it with protons to generate water
What is the purpose of all this? The whole process has two tasks –One one side it is to use the energy of food to supply the energy build up a difference in the concentration of protons on the two sides of the inner mitochondrial membrane –The other is to utilize this difference to allow the flow of protons back into the matix of the mitochondria and make ATP in the process
Where does all the O2 I breath go? Answer: the vast majority ends up being used in the final step of the electron transport chain, inside the mitochondrial inner membrane! That O2 combines with electrons and protons to make water! The carbon in the form of CO2 which we breath out comes from the the carbon in the food that we consume. The O2 comes from cellular water.