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Cellular Respiration (in Eukaryotes)
Glycolysis

Glycolysis means "sugar-splitting". One glucose molecule and two ATP molecules are required to start this process.

The initial phosphorylation of glucose is required to increase the reactivity (decrease its stability) in order for the molecule to be cleaved into two Pyruvate molecules by the enzyme Aldolase.

This reaction produces two net ATP (four ATP are produced, but two ATP are used to start this reaction) and two Pyruvate. In order to create the four ATP, four phosphate groups are transferred to four ADP molecules, turning each of them into ATP.

To summarize, we start off with glucose, and 1 ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate. During energy metabolism, glucose 6-phosphate becomes fructose 6-phosphate. An additional ATP is used to phosphorylate fructose 6-phosphate into fructose 1,6-bisphosphate by the help of phosphofructokinase. Fructose 1,6-biphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate.

Oxidative decarboxylation

Next, the pyruvate produced in Glycolysis is oxidized (The pyruvate lost electrons. Remember "oilrig"). This makes acetyl Coenzyme A (or acetyl-CoA, as often abbreviated) and CO2 by an enzyme PDC.

 Citric Acid Cycle or Krebs Cycle

Now, the acetyl-CoA is oxidized by several enzymes, producing CO2, H2O, 2 ATP, 6 NADH, and 2 FADH2.

Oxidative phosphorylation

Also known as the Electron Transport Chain, this process occurs in the inner mitochondria membrane.

There are 4 proteins, named protein 1, 2, 3, and 4. Proteins 1, 3 and 4 pump protons into the inter-membranal space, while Protein 2 helps Proteins 3 and 4.

High energy electrons donated by the NADH and the FADH2 are passed through redox centers in the Proteins. Each center has a higher electron affinity than the one above it. This causes the electrons to move downwards.

The movement of the electrons from one redox center to the next produce a little bit of energy, which is used to pump protons.

Now that a gradient of protons is formed, ATP Synthase could create ATP since the protons need to pass through it in order to maintain equilibrium. The protons can't succeed to make an equilibrium in a healthy cell, because the Proteins just keep pumping!