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Cellular respiration is a metabolic process where the body uses organic molecules to produce energy. One of the greatest energy sources that our body uses is glucose. In cellular respiration glucose is broken down, and the energy from those bonds is used to create adenosine triphosphate (ATP). There are two types of respiration: aerobic and anaerobic. In aerobic respiration there are 3 main steps: Glycolysis, Krebs Cycle (Citric Acid Cycle), and Oxidative Phosphorylation. In anaerobic respiration you can have alcohol fermentation or lactic acid fermentation. Overall, cellular respiration can be used in rigor mortis, which helps investigators determine the time of someone’s death.
Aerobic respiration is used in the present of oxygen, and the first step in it is glycolysis. Glycolysis is when glucose enters the cytoplasm through facilitated diffusion, and the six carbon glucose is split in half by enzymes using two ATP. Also, hydrogen is removed from the 3 carbon molecules and transferred to NAD+ making NADH. At the end of glycolysis you have 4 ATP, 2 NADH, and 2 pyruvate (3 carbon molecule) made. Glycolysis is the first step in both aerobic and anaerobic respiration because it does not require oxygen. In the next step, the presence of oxygen determines the type of respiration.
If there is oxygen presence, aerobic respiration continues, and the next step is the Krebs Cycle, but before that the pyruvate is broken down into acetyl (2 carbon molecule) and carbon dioxide by enzymes inside the mitochondrial matrix. Then a coenzyme called CoA attaches itself to the acetyl making acetyl CoA. At the end of this process you have 2 NADH, 2 carbon dioxides, and 2 acetyl CoA. Now the cell is ready for the next main step: the Krebs Cycle. The Krebs Cycle occurs in the mitochondrial matrix where it first takes acetyl CoA and combines it with oxaloacetate (4 carbon molecule) to form citric acid. Then it goes through eight reactions that ultimately end up with oxaloacetate. At the end six NADH, two FADH2, two ATP, and four carbon dioxides are generated.
The next and last step of aerobic respiration is oxidative phosphorylation. This all happens in the mitochondrial matrix and inter membrane space. The NADH and FADH2 that were acquired carry protons (H+) and electrons, and they cross the electron transport chain creating an electrochemical gradient. Then the protons go back across the membrane of the mitochondria through an ATP synthase, which provides enough energy to synthesize ATP from ADP and phosphate. At the end of this process water is formed and around 32-34 ATP are generated.
Now if there was no oxygen present after glycolysis, anaerobic respiration would occur. Lactic Acid fermentation is the process that occurs in humans and mammals when they undergo strenuous activity. The two pyruvate and two NADH are made into two lactic acid and two NAD+, which NAD+ will be used again in glycolysis. Alcoholic fermentation is the other anaerobic process that occurs only in yeasts in many bacteria. The two pyruvate and two NADH are made into two carbon dioxides, two ethanol molecules, and 2 NAD+, which will be used again in glycolysis. Ultimately, in anaerobic respiration no energy is gained, and NAD+ is reused in glycolysis.
Lastly, cellular respiration can help us in rigor mortis and estimating when someone died. Rigor mortis is when the joints of a dead body start to stiffen and lock in place. This occurs in people and animals, and depending on the temperature and other factors, rigor mortis can last up to 72 hours. The joints stiffen because the membranes of muscle cells become more permeable, and start to allow in calcium ions. This causes two fibers, actin and myosin, to contract as long as ATP is present. In order for the muscles to be relaxed the cells need ATP, but all ATP is used up from the muscle contraction and other cellular processes that the muscles stay contracted until the muscles decompose. Rigor mortis provides investigators of a crime an approximate time of death, but they must take into account the variables that affect rigor mortis. First temperature, if it is warmer rigor mortis happens faster while when it is colder rigor mortis tends to happen slower. Also, physical activity before death cause rigor mortis to happen much quickly. Also, fat act like an insulation so rigor mortis develops more slowly. Using these variables an investigator is able to estimate the time of death.
In conclusion, cellular respiration is a necessary metabolic process in all living organisms, and without it we would never exist. Our body has created a system of using organic molecules in order to create energy in the from of ATP in the presence of oxygen and without it. Also, we are able to apply our own bodies metabolic processes in crime investigations in order to approximate a person’s time of death. Cellular respiration is so important to life that it is one of the properties of life.