Once cells have been made, they need to be powered. Plants and some other cells can capture the energy of light and convert it into stored energy in ATP. However, most prokaryotic cells and all eukaryotic cells can perform a metabolic process called cellular respiration. Cellular respiration is the process by which the mitochondria of a cell break down glucose to produce energy in the form of ATP. The following is the general equation for cellular respiration:
O2 + C6H12O6 → CO2 + H2O +ATP
Reactions during cellular respiration occur in the following sequence:
Glycolysis: One molecule of glucose breaks down into two smaller sugar molecules called pyruvate. This is an anaerobic process, which means it does not need oxygen to be present. Glycolysis takes place in the cell’s cytoplasm. End product yield from this reaction per one glucose molecule is:
two molecules of ATP
two molecules of pyruvate
two molecules of NADH
Oxidation of pyruvate: Pyruvate is converted into acetyl coA in the mitochondrial matrix. This transition reaction must happen for pyruvate to enter the next phase of cellular respiration. Pyruvate is oxidized, which means it loses two electrons and a hydrogen molecule. This results in the formation of NADH and loss of CO2.
Citric acid cycle: Also called the Krebs cycle, during this cycle an acetyl group detaches from the coenzyme A in the acetyl coA molecule. This process is aerobic, which means it must occur in the presence of oxygen. The net yield per one glucose molecule is:
two molecules of ATP
six molecules of NADH
two molecules of FADH2
four molecules of CO2
Electron transport chain: This process happens in the inner mitochondrial membrane. It consists of a series of enzymatic reactions. Both NADH and FADH2 molecules are passed through a series of enzymes so that electrons and protons can be released from them. During this process, energy is released and used to fuel chemiosmosis. During chemiosmosis, protons are transported across the inner mitochondrial membrane to the outer mitochondrial compartment. This flow of protons drives the process of ATP synthesis. This step of cellular respiration creates an approximate net yield of 34 ATP per glucose molecule. Six molecules of water are also formed at the end of the electron transport chain.
Keep In Mind
Cellular respiration requires oxygen, but there are forms of fermentation that extract energy from food without using oxygen. Fermentation can be either alcoholic (makes ethanol as an end product, like yeast in the brewing of beer) or lactic acid type. Lactic acid is produced in a person’s muscles during strenuous activity when the body cannot move enough oxygen to the cells.
Did You Know?
The citric acid cycle is not identical for all organisms. Plants have some differences in terms of the enzymes used and energy carriers produced.
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