Cellular respiration is the process by which cells (both eukaryotic and prokaryotic cells) harvest energy aerobically. In order to harvest this energy, organisms must ingest some type of food. Even salad (shown in the picture), the low-calorie meal that every child under 12 hates, provides us with energy that fuels our daily activities. Usually a sugar known as glucose is used to depict how cellular respiration works. The process of cellular respiration can be shown in three main stages:
The first stage is Glycolysis. Glycolysis occurs in the cytoplasm of cells. When one molecule of glucose enters the cellular respiration cycle, two molecules of pyruvate come out. The six carbon atoms in glucose end up in the two molecules of pyruvate (three carbon atoms in each pyruvate). Two molecules of NAD+ are reduced (electrons are added onto the molecules) to NADH. Then, substrate-level phosphorylation occurs. Phosphorylation is the process of transferring a phosphate group to ADP in order to create ATP. All the energy produced in Glycolysis is stored in two ATP molecules (four ATP molecules were generated but two were consumed) and two NADH molecules, however most of the energy from the glucose is in the pyruvate molecules. Different from the other two stages of cellular respiration, Glycolysis is universal (it occurs in every and any type of cell), does not require oxygen, and does not take place in a membrane-bound organelle. Some organisms, such as yeast and some bacteria, do not need to make any more energy than what is produced in Glycolysis. For most other organisms, including humans, they need to continue through the other two steps in cellular respiration.
The second step in cellular respiration takes place in the mitochondria and is called the Citric Acid Cycle/Krebs Cycle. Before the two pyruvate molecules can enter the Krebs Cycle, though, they must first be groomed. First, one carbon from each pyruvate is released into CO2. Then, an electron gets transferred from the remaining two-carbon molecule into NAD+ to make NADH (the two-carbon molecule is oxidized and the NAD+ is reduced). Finally, a compound called coenzyme A joins the two-carbon molecule to form acetyl coenzyme A (acetyl CoA). Two acetyl CoA compounds are made for every one glucose molecule. And now the Krebs Cycle can finally begin! The steps of the Krebs Cycle are as follows:
1) The CoA part of acetyl CoA is taken off. The remaining two-carbon acetyl compound is joined with oxaloacetate. This combination produces citrate, a six carbon molecule.
2) Redox reactions release two carbon atoms into two CO2 molecules. One molecule of ATP is produced by phosphorylation and two molecules of NAD+ are reduced to NADH. By the end of these reactions, there is now a four carbon molecule called succinate.
3) Succinate turns to malate which then turns to oxaloacetate to start the cycle over again. During this time, FAD is reduced to FADH2 and NAD+ is reduced to NADH.
By the end of the Citric Acid/Krebs Cycle, one molecule of acetyl CoA will yield one molecule of ATP, 3 NADH, and 1 FADH. Because there are two molecules of aceytl CoA entering the Krebs Cycle, the total yield is 2 ATP, 6 NADH, and 2 FADH.
The final stage in cellular respiration is oxidative phosphorylation. The electron transport chain used in oxidative phosphorylation is located on the mitochondria membrane. NADH and FADH carry electrons to this chain. As electrons go down the chain, hydrogen ions are pumped across the membrane to create a concentration gradient of H+ across the membrane. This gradient is used in chemiosmosis to produce ATP molecules- As H+ goes through the ATP synthase, ATP molecules are made. About 34 ATP molecules are produced through oxidative phosphorylation. Altogether, a maximum of 38 ATP are created in cellular respiration.