Cellular Respiration is a chemical process with the following equation: C6H12O6 + O2 → H2O + CO2. All organisms, including those capable of photosynthesis, go through the process of cellular respiration. The overall reaction breaks down a carbohydrate, most frequently modeled by glucose, and converts the energy stored in that molecule into the most basic cellular energy, ATP. Respiration is almost the complete opposite of photosynthesis. So if you understood photosynthesis, understanding respiration should be relatively easy.

Cellular Respiration is broken down into three major steps which are dependent on one another: glycolysis, the Krebs cycle, and the electron transport chain. While glycolysis takes place in the cytoplasm of the cell, the Krebs cycle and the electron transport chain take place inside of the mitochondria.

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Glycolysis

Glycolysis is the most evolutionarily conserved process in cellular respiration. The process takes place in all living organisms in almost the exact same way. Fundamentally, glycolysis involves breaking down glucose, which possesses 6 carbons, into two 3-carbon molecules of pyruvate.

In the process, a small amount of energy is released due to the breaking of bonds. This is captured as 2 molecules of ATP. Similarly, the breaking of bonds releases a few electrons that are picked up by electron carriers, NADH. These electrons will be dropped off to the electron transport chain later.

Before pyruvate can continue on into the mitochondria to enter the Krebs cycle, pyruvate oxidation takes place. Oxidation is the loss of electrons. In this process, pyruvate becomes a 2-carbon molecule called acetyl CoA. A molecule of carbon dioxide is released from each pyruvate molecule that is oxidized.

Krebs Cycle

The Krebs Cycle takes place in the mitochondria. In this cycle, similarly to the Calvin Cycle, a number of enzymes process a number of reactions that… you DON’T need to know about! (unless you go to medical school one day… good luck!)

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The moral of the story is that a number of highly specific enzymes break down acetyl CoA in reactions that create a number of electrons and a little bit of energy. The process results in the creation of a lot of electron carriers (around 8) such as NADH and FADH2. These electron carriers will allow a lot of ATP production in the electron transport chain. 2 ATP are also produced in the Krebs Cycle.

Electron Transport Chain

The electron transport chain is where the majority of ATP is produced. The chain works in the same way as the electron transport chain in photosynthesis. A concentration gradient is formed, and ATP synthase is responsible for creating ATP.

When hydrogen ions are dropped off by electron carriers to the electron transport chain, the hydrogen ion is pumped across the plasma membrane to form a high concentration gradient of hydrogen ions. These will be used by ATP synthase.

The electron travels through the electron transport chain on a number of electronegative proteins. It eventually ends up binding with oxygen, the final electron acceptor. When oxygen accepts the electron, it forms a bond with hydrogen ions and water is created.

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The concentration gradient of hydrogen travels through ATP synthase, in the same way as it does in photosynthesis, the kinetic energy is used to phosphorylate ADP into ATP. This process is called chemiosmosis, as ions are moving down their concentration gradient. This process produces somewhere between 30 and 40 ATP molecules. Don’t worry, you don’t need to know specific numbers! Just know that a TON more ATP is produced through this process than through either glycolysis or the Krebs cycle.

Another important aspect of the electron transport chain is the recycling of electron carriers. This takes place when they drop off their electron and can then be refilled in glycolysis or the Krebs cycle. If these carriers were not emptied, the cycle would not be able to continue.

Fermentation

In organisms without access to oxygen, anaerobic respiration takes place. This happens in a number of bacteria, and in other organisms when oxygen is being used up faster than it can be inhaled (think crazy workout). 

Without oxygen, the Krebs cycle and electron transport chain cannot take place, because there is no final electron acceptor. Instead, electron carriers must be recycled elsewhere. This happens through the process of fermentation. 

Organisms find other molecules to drop off their electrons. Some examples include creating lactic acid, ethanol, and carbon dioxide. This is how beer and wine are fermented by various bacteria and yeast. In humans, our body produces lactic acid when oxygen is in short supply, such as in a tough workout. This can create sore muscles the next day.

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The main takeaway about fermentation is that cells MUST recycle their electron carriers in order to continue to reuse them to produce ATP. They will find another molecule to drop their electrons off on. Secondly, during anaerobic respiration, glycolysis, alone, is producing ATP. This means that ATP production is MUCH lower than in aerobic respiration.