Cellular respiration is the process by which organisms obtain energy from food. Aerobic respiration uses oxygen and is used by all multicellular and some unicellular life forms. Anaerobic respiration does not involve oxygen. Early life forms used anaerobic processes until organisms evolved to use photosynthesis to produce sugar molecules and oxygen. Aerobic respiration starts with glucose and goes through glycolysis, the Krebs cycle, and electron transport phosphorylation, producing up to 36 ATP molecules per glucose molecule. This process is more energy-efficient than anaerobic respiration, but it also produces small amounts of harmful oxygen forms.
Cellular respiration is the process by which living organisms obtain energy from food. There are two main methods. Aerobic respiration, used by all multicellular and some unicellular life forms, uses oxygen in the atmosphere, or dissolved in water, as part of a complex process that releases and stores energy. Anaerobic respiration is used by a variety of unicellular organisms and does not involve uncombined oxygen.
The emergence of aerobic respiration
The first life forms on Earth arose in a world without free oxygen. They used anaerobic processes to get energy. At some point, still early in Earth’s history, organisms evolved that used photosynthesis to produce sugar molecules using carbon dioxide, obtained from the atmosphere, and water. The sugar served as an energy source and the process produced oxygen as a byproduct. Oxygen was toxic to many anaerobic organisms, but some evolved to use it in a new type of respiration that actually provided much more energy than the anaerobic process.
Early life forms consisted of cells that lacked nuclei or other well-defined structures. These are known as prokaryotes and include organisms such as bacteria and cyanobacteria, also known as blue-green algae. Later cells with nuclei and other structures emerged; these are known as eukaryotes. They include some unicellular organisms and all multicellular organisms, such as plants and animals. All eukaryotes and some prokaryotes use aerobic respiration.
How aerobic respiration works
Cells store energy in a molecule called adenosine triphosphate (ATP). This compound contains three phosphate groups (PO4), but can release energy by losing one of these to form adenosine diphosphate (ADP). Conversely, ADP can acquire a phosphate group to become ATP, thus storing energy.
Another important molecule is nicotinamide adenine dinucleotide. It can exist in two forms: NAD+, which can accept two electrons and one hydrogen ion (H+) to form NADH, which can give electrons to other molecules. The compound is used in respiration to transport electrons from one place to another.
The starting point for respiration is glucose (C6H12O6), one of the simplest carbohydrates. The more complex sugar molecules in foods are first broken down into this compound. Glucose is itself broken down by a process called glycolysis, which occurs in the cytoplasm, or cellular fluid, and is common to both anaerobic and aerobic respiration.
Glycolysis
The process of glycolysis uses two molecules of ATP to convert glucose, which has six carbon atoms, into two three-carbon molecules of a compound called pyruvate in a series of steps. At the end of this process, four molecules of ATP are produced, so that there is an overall gain of two ATP, which represents a gain in stored energy. Glycolysis also results in two molecules of NAD+ each taking two electrons and one hydrogen ion from glucose to form NADH. Overall, therefore, glycolysis results in two molecules of pyruvate, two of ATP and two of NADH.
In eukaryotic cells, the remaining stages of aerobic respiration occur in structures known as mitochondria. It is thought that these tiny organs were once independent organisms embedded in cells in the distant past. Each pyruvate molecule is converted, with the help of NAD+, into a compound called acetyl coA, losing one carbon and two oxygen atoms to form carbon dioxide as a waste product, and forming another NADH molecule.
The Krebs cycle
The next stage is called the Krebs cycle, also known as the tricarboxylic acid cycle (TCA) or citric acid cycle. Acetyl CoA from pyruvate combines with a compound called oxolacetate to produce citrate, or citric acid, which, in a series of steps involving NAD+, produces ATP as well as NADH and another molecule called FADH2, which has a similar function . This results in the citric acid being converted back into oxaloacetate to start the cycle over again. Each cycle completed produces two molecules of ATP, eight of NADH and two of FADH2 from two molecules of pyruvate.
Electron transport phosphorylation
The final stage is known as electron transport phosphorylation or oxidative phosphorylation. At this point in the process, the electrons carried by NADH and FADH2 are used to provide the energy to attach phosphate groups to ADP molecules to produce up to 32 ATP molecules. This takes place on the mitochondrial membrane via a series of five proteins, through which electrons are transported. Oxygen, which readily accepts electrons, is needed to remove them at the end of the process. The oxygen then combines with the hydrogen ions released from the NADH to form water.
Efficiency
Overall, the aerobic respiration process can, in theory, produce up to 36 energy-storing ATP molecules for each glucose molecule, compared to only two for anaerobic respiration, making it a much more energy-efficient process. In practice, however, it is thought that typically about 31 to 32 ATP molecules are produced, as other reactions may occur in the final stages. While this process is a highly efficient way of producing and storing energy, it also produces small amounts of highly reactive forms of oxygen, known as peroxides and superoxides. These are potentially harmful to cells and some scientists believe they may be involved in aging and some diseases.
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