Cellular Respiration Cellular respiration is a chemical process that produces adenosine triphosphate, or otherwise known as ATP for energy that is also needed to survive. It leaves waste products, carbon dioxide and water, which is needed for photosynthesis, a process that only plants use. Production of ATP through the process of cellular respiration occurs in the mitochondria of the cytosol inside plant and animal cells.
Cellular respiration occurs in three stages, Glycolysis, which happens in the cytosol, Krebs cycle, which takes place in the matrix of the mitochondria, and electron transport chain, which happens in the cristae of the mitochondria. The first stage of this process is Glycolysis: Glycolysis first breaks down a glucose molecule, which is a very important sugar molecule for living things. Since glucose is a six-carbon molecule, it splits into two pyruvic acids (pyruvate).
In this stage, two ATP molecules are used and four ATP molecules are made, so it makes a sum of two ATP molecules. Pyruvic acid gives high-energy electrons to NAD positive which makes two NADH. In conclusion, glycolysis produced two ATP molecules, two NADH, and two pyruvate molecules. The Krebs cycle, the second stage of respiration, first starts with breaking down pyruvic acid from the glycolysis into Acetyl CoA. It is to make the pyruvate more usable in the Krebs cycle but in this “process”, carbon dioxide diffuses out.
After acetyl CoA forms, the eight steps of the cycle start. First, Acetyl CoA is transferred to the oxaloacetate group by CoenzymeA to form citrate. NAD+ (Nicotinamide adenine dinucleotide), an enzyme carrier, is used and turned into NADH when an electron is loaded on it. The electrons are taken to the electron transport chain. Oxalosuccinate molecules go to Succinyl CoA when two carbon dioxides are taken out and an electron carrier (NAD+) is loaded with electrons. Succinyl CoA follows up.
In this part, GTP (guanosine triphosphate) turns into GDP (guanosine diphosphate) and vice versa. ADP (Adenosine diphosphate) changes into ATP and succinyl CoA then makes the molecule succinate. From succinate to malate, FAD is loaded with an electron and changes into FADH2 and sent to the electron transport chain. Malate is the last of the cycle and it is repeated after that. NAD+ is loaded with an electron once again and sent to the electron transport chain. The results are two ATP molecules. The electron transport chain is aerobic, which means it requires oxygen.
Most of the ATP is made here. It is a series of electron carriers in the cristae. Through a series of reactions, the electrons are passed to oxygen from one carrier protein to another. The carriers of electrons are NADH and FADH2. Electrons flow through the chain and pumps hydrogen ions from the inside out into the mitochondria (outer compartment). Then, through ATP synthase, the hydrogen molecules are back into the inner compartment and ATP is formed. At the end of the chain, oxygen (O2) is waiting, accepting H+ and electrons, which forms H2O.
In the electron transport chain, about 32-34 ATP molecules are produced. The ATP is then stored into the form of a hydrogen ion gradient. After glycolysis, fermentation can occur when the presence of oxygen is not around. This is known as anaerobic respiration since throughout this process, oxygen is not required. Even though, this is an alternative to aerobic respiration, fermentation is only used sparingly. There are two forms of fermentation, lactic acid fermentation and alcoholic fermentation.
Lactic acid fermentation can occur in humans, when muscles are under strenuous activities such as sprinting. This process starts with pyruvate, where NADH drops off the hydrogen transferring it to pyruvate. As for results, it ends with two ATP molecules, lactate, and NAD. While, others like yeast and beer go through alcoholic fermentation while brewing and baking. Alcoholic fermentation goes through a similar process, but the hydrogen molecule from NADH is transferred into pyruvate to form ethanol, carbon dioxide, two ATP molecules, and NAD.
Fermentation can help produce ATP for cells that need energy when oxygen is short in supply, but both forms of fermentation only yield a sum of two ATP molecules with each glucose molecule. Which means fermentation produced much less energy than cellular respiration in total. Since cellular respiration produced a net of thirty-six to thirty-eight molecules of ATP per glucose molecule while fermentation only produced a net of four which include alcoholic and lactate fermentation.