U3 AOS2 Topic 5: Aerobic Cellular Respiration

All living organisms need constant supply of energy to carry out life processes. Most of this energy comes from breakdown of glucose by the process of cellular respiration. This energy is released in the form of ATP. There are two different pathways involved in breakdown of glucose. First is aerobic cellular respiration which requires oxygen and can make 32 ATPs per glucose molecule and second one is anaerobic fermentation which does not need oxygen and can make only 2 ATPs per glucose molecule. 

In aerobic cellular respiration glucose reacts with oxygen and breaks down into CO2 and water to release energy. This reaction is not simple but involves complex biochemical pathways catalyzed by different enzymes and has following stages:

1.   Glycolysis

2.   Krebs cycle

3.   Electron transport chain

Glycolysis occurs in cytosol where glucose is broken down into two molecules of pyruvic acid and releases 2 ATPs and 2 NADH. NADH being an electron and proton carrier can be used in the third stage of cellular respiration i.e. electron transport chain to make even more ATPs.

Krebs cycle occurs inside the mitochondrial matrix. Two pyruvate molecules produced in glycolysis can be used in krebs cycle to make 4 CO2, 2 FADH2, 6 NADH and 2 ATP. NADH and FADH2 being high energy electron and proton carriers can be used in electron transport chain to make more ATPs. Before entering Krebs cycle, pyruvate molecule combines with coenzyme A to make acetyle-CoA.  This reaction is called as link reaction and produces NADH and CO2. As there are two pyruvic acid molecules involved therefore 2CO2 are produced in link reaction. At the end of krebs cycle, total 6CO2 molecules are exhaled.

Electron transport chain is the third stage which occurs in inner membrane of mitochondria also known as cristae. In this stage high energy electron proton carriers NADH and FADH2 are used to generate proton gradient across the cristae to make ATP. There are a number of proton complexes present on inner mitochondrial membrane which help in electron transport chain. At first two protein complexes NADH and FADH2 unload their electrons and protons. The excited electrons can transport through different protein complexes powering the protons to move from mitochondrial matrix into the intermembrane space and therefore create proton gradient across the membrane.

The high concentration of protons in intermembrane space causes the protons to move down the concentration gradient via a protein called as ATP synthase which powers the formation of ATPs. For each glucose molecule, 28-26 ATP molecules are generated. The free protons and electrons in mitochondrial matrix are picked up by O2 to generate water.