Aerobic respiration

Aerobic respiration requires oxygen in order to generate energy. It is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion to be fully oxidized by the Krebs cycle. The product of this process is energy in the form of ATP (Adenosine Triphosphate), by substrate-level phosphorylation, NADH and FADH2. The reducing potential of NADH and FADH2 is converted to more ATP via an electron transport chain with oxygen as the "terminal electron acceptor". Most of the ATP produced by cellular respiration is by oxidative phosphorylation, ATP molecules are made due to the chemiosmotic potential driving ATP synthase. Respiration is the process by which cells obtain energy when oxygen is present in the cell.

Theoretically, 36 ATP molecules can be made per glucose during cellular respiration, however, such conditions are generally not realized due to such losses as the cost of moving pyruvate into mitochondria. Aerobic metabolism is rather more efficient than anaerobic metabolism. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and at the cell membrane in prokaryotic cells.

Glycolysis

Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms and does not require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced but two are consumed for the preparatory phase. The initial phosphorylation of glucose is required to destabilize the molecule for cleavage into two triose sugars. During the pay-off phase of glycolysis four phosphate groups are transferred to ADP by substrate-level phosphorylation to make four ATP and two NADH are produced when the triose sugars are oxidized. Glycolysis takes place in the cytoplasm of the cell. The overall reaction can be expressed this way:

Glucose + 2 ATP + 2 NAD+ + 2 Pi + 4 ADP --> 2 pyruvate + 2 ADP + 2 NADH + 4 ATP + 2 H2O + 4 H+

Oxidative decarboxylation

Produces acetyl-CoA from pyruvate inside the mitochondrial matrix. This oxidation reaction also releases carbon dioxide as a product. In the process one molecule of NADH is formed per pyruvate oxidized.

Krebs cycle/Citric Acid cycle

When oxygen is present, acetyl-CoA enters the citric acid cycle inside the mitochondrial matrix, and gets oxidised to CO2 while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidise the equivalent of one glucose molecule two acetyl-CoA must be metabolised by the Krebs cycle. Two waste products, H2O and CO2 are created during this cycle.

Oxidative phosphorylation

In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. It comprises of the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidising the NADH produced from the Krebs cycle. ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP.

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