Oxidative Phosphorylation
The final step in cellular respiration is oxidative phosphorylation. Here the energy stored in electron carriers is finally released through the electron transport chain and harnessed to make more ATP through chemiosmosis.
A series of proteins are embedded in the inner membrane of the mitochondrion. The first protein (flavoprotein) accepts electrons from NADH (gets reduced) and then passes them onto the next protein, releasing some of the electron's energy in the process. The electrons in FADH2 enter the chain at a lower level and therefore releases less energy. Oxygen is at the end of the chain and it accepts electrons from the last protein. Reduced oxygen then binds with hydrogen ions produced earlier and form water.
Electrons getting passed down the electron transport chain is an exergonic process. The energy released is used to power hydrogen pumps, which move hydrogen ions against their concentration gradient into the intermembrane space.
Then chemiosmosis occurs. The ATP synthase embedded in the inner membrane uses the energy stored in the H+ concentration gradient to attach inorganic phosphates to ADP, thus forming ATP. This process yields 32 - 34 ATP molecules. If we add the 4 ATP produced from glycolysis and the citric acid cycle, we get 36 - 38 ATP in total from a single glucose molecule. This is a very efficient process compared to other biological systems. About 40% of the energy originally present in glucose is converted to ATP.
The three reasons why there is a variation in how much ATP produced is:
1. Not all the energy from chemiosmosis is used to convert ADP to ATP. Some of the energy may be redirected to perform other cellular work.
2. 1 NADH molecule moves 10 H+ across the inner membrane, but it takes between 3 to 4 H+ flowing back into the matrix through the ATP synthase to generate 1 ATP. So each NADH can generate 2.5 - 3.3 ATP. We usually round it off to 3. FADH2 on the other hand moves 6 H+ across the membrane, which means 1.5 - 2 ATP can be generated.
3. The type of shuttle used to transport electrons from the NADH generated in the cytosol (from glycolysis) into the mitochondria varies between FAD and NAD+. If it's FAD, less ATP will be produced, and if it's NAD+, slightly more will be made.
A series of proteins are embedded in the inner membrane of the mitochondrion. The first protein (flavoprotein) accepts electrons from NADH (gets reduced) and then passes them onto the next protein, releasing some of the electron's energy in the process. The electrons in FADH2 enter the chain at a lower level and therefore releases less energy. Oxygen is at the end of the chain and it accepts electrons from the last protein. Reduced oxygen then binds with hydrogen ions produced earlier and form water.
Electrons getting passed down the electron transport chain is an exergonic process. The energy released is used to power hydrogen pumps, which move hydrogen ions against their concentration gradient into the intermembrane space.
Then chemiosmosis occurs. The ATP synthase embedded in the inner membrane uses the energy stored in the H+ concentration gradient to attach inorganic phosphates to ADP, thus forming ATP. This process yields 32 - 34 ATP molecules. If we add the 4 ATP produced from glycolysis and the citric acid cycle, we get 36 - 38 ATP in total from a single glucose molecule. This is a very efficient process compared to other biological systems. About 40% of the energy originally present in glucose is converted to ATP.
The three reasons why there is a variation in how much ATP produced is:
1. Not all the energy from chemiosmosis is used to convert ADP to ATP. Some of the energy may be redirected to perform other cellular work.
2. 1 NADH molecule moves 10 H+ across the inner membrane, but it takes between 3 to 4 H+ flowing back into the matrix through the ATP synthase to generate 1 ATP. So each NADH can generate 2.5 - 3.3 ATP. We usually round it off to 3. FADH2 on the other hand moves 6 H+ across the membrane, which means 1.5 - 2 ATP can be generated.
3. The type of shuttle used to transport electrons from the NADH generated in the cytosol (from glycolysis) into the mitochondria varies between FAD and NAD+. If it's FAD, less ATP will be produced, and if it's NAD+, slightly more will be made.