Cellular Respiration and Photosynthesis
Cellular respiration and photosynthesis are totally different processes, but they are connected together through an important relationship. As we know, photosynthesis make glucose which will be used in cellular respiration in order to make ATP. This glucose will turn back into carbon dioxide which will be used in photosynthesis to produce glucose again. However, these two different metabolic processes are really important for carbon cycle and continuity of life.
Cellular respiration is the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen. Glucose is often the main source of energy for cellular respiration, ( C6H12O6 + 6O2 ? 6CO2 + 6H2O ). However, cellular respiration does not take place in just one step, it is involves four metabolic pathways, which are: glycolysis, the breakdown of pyruvate, the Krebs cycle (citric acid cycle), and oxidative phosphorylation. Most of the energy comes from the last stage. Glycolysis means the breakdown of glucose, and this is the first stage of cellular respiration. This process involves breakdown the glucose into two molecules of pyruvic acid. Glycolysis can occur in the presence of oxygen and occur in the absence of oxygen. Glycolysis happen in 10 steps, these 10 steps can be grouped into three phases. In the first phase, energy investment phase, two ATP are attached to glucose in order to converted to fructos-1,6-bisphosphate. In the second phase, cleavage phase, fructos-1,6-bisphosphate breaks into two molecules of glyceraldehyde-3-phosphae. In the last phase, energy liberation phase, 4 ATP, 2 NADH, and 2 pyruvate are produced. However, the net yield is 2 ATP only, because 2 ATP are used in the energy investment phase. After glycolysis finish in the cytosol, The two molecules move to mitochondrial matrix where the second metabolic phase takes place. Breakdown of pyruvate is the second stage of cellular respiration. Pyruvate molecules are oxidized by pyruvate dehydrogenase, this enzyme will remove one molecule of CO2 from each pyruvate in order to convert it to acetyl group. An organic molecule called coenzyme A (CoA) is attached to acetyl group to produce acetyl CoA . NADH molecule is produced during this process for each pyruvate, which means 2 NADH for the whole process. Krebs cycle (citric acid cycle) is the third stage of cellular respiration which occur in the mitochondrial matrix also. The acetyl group is removed from acetyl CoA and attached to oxaloacetate to form citrate acid. This cycle is a series of several steps, during this series of reactions, two molecules of CO2 are released. While this cycle occurs, 3 NADH, 1 FADH2, and 1 ATP are made for each citrate acid. Which means 6 NADH, 2 FADH2, and 2 ATP are made for the whole stage. After this series of steps ends, oxaloacetate is regenerated so Krebs cycle can occur again when CoA is available. Oxidative phosphorylation is the last stage of cellular respiration, and this is where caries from the first three stages go. This process requires oxygen and occur in the inner mitochondrial membrane. Electron transport chain (ETC), provides most of the energy that come from cellular respiration. During this stage, NADH and FADH2 donate their electrons to different points in the ETC. High-energy electrons are passed from one carriers to the next. Every time 2 electrons pass down the chain, their energy used to move (H+) across the inner mitochondrial membrane. As this happens, the intermembrane space become more positively than the matrix. When the intermembrane space become more positively than the matrix the ATP synthesis starts. The charge difference makes the H+ move through channels in these enzymes. Each time a H+ ions move through it, the ATP synthesis turns, and the enzyme attach a phosphate to ADP to produce ATP. This stage makes 34 ATP, because 6 NADH = 30 ATP, and 2 FADH2 = 4 ATP. And by adding 2 ATP from glycolysis and 2 ATP from Krebs cycle we will get 38 ATP which is the maximum amount of ATP for the whole process of cellular respiration.
Photosynthesis is the process in which the energy from light is captured and used to synthesize glucose and other organic molecules