Aerobic respiration, anaerobic respiration and fermentation are methods for living cells to produce energy from food sources. While all living organisms conduct one or more of these processes for energy production, only a select group of organisms are capable of photosynthesis to produce food from sunlight. However, even in these organisms, the food produced by photosynthesis is converted into cellular energy through cellular respiration. A distinguishing feature of aerobic respiration from fermentation pathways is the prerequisite for oxygen and the much higher yield of energy per molecule of glucose. Fermentation and anaerobic respiration share an absence for oxygen, but anaerobic respiration utilises an electron transport chain for energy production much as aerobic respiration does while fermentation simply provides the necessary molecules needed for continued glycolysis without any additional energy production.
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Glycolysis is a universal beginning pathway conducted in the cytoplasm of cells for breaking down glucose into chemical energy. The energy released from each molecule of glucose is used to attach a phosphate to each of four molecules of adenosine diphosphate (ADP) to produce two molecules of adenosine triphosphate (ATP) and an additional molecule of NADH. The energy stored in the phosphate bond is used in other cellular reactions and is often regarded as the energy "currency" of the cell. However, since glycolysis requires the input of energy from two molecules of ATP, the net yield from glycoylysis is only two molecules of ATP per molecule of glucose. The glucose itself is broken down during glycolysis into pyruvate. Other fuel sources such as fats are metabolised through other processes, for example the fatty acid spiral in the case of fatty acids, to produce fuel molecules that may enter respiration pathways at various points during respiration.
Aerobic respiration occurs in the presence of oxygen and yields the majority of energy for organisms capable of aerobic respiration. In this process, the pyruvate produced during glycolysis is converted into acetyl coenzyme A (acetyl CoA) prior to entering the citric acid cycle, also known as the Krebs cycle. The acetyl CoA is combined with oxaloacetate to produce citric acid in the first stage of the citric acid cycle. The subsequent series converts the citric acid into oxaloacetate and produces energy-carrying molecules called NADH and FADH2. These energy molecules are shunted to the electron transport chain, or oxidative phosphorylation, where they yield the majority of ATP produced during aerobic cellular respiration. Carbon dioxide is produced as a waste product during the Krebs cycle and the oxaloacetate produced by one turn of the Kreb's cycle is combined with another acetyl CoA to begin the process again. In eukaryote organisms, such as plants and animals, both the Krebs cycle and electron transport chain occurs in a specialised structure called the mitochondria while bacteria capable of aerobic respiration conduct these processes along the plasma membrane because they lack the specialised cellular organelles found in eukaryotes. Each turn of the Krebs cycle is capable of producing one molecule of guanine triphosphate (GTP), which is easily converted into ATP, and an additional 17 molecules of ATP through the electron transport chain. Since glycolysis yields two molecules of pyruvate for use in the Krebs cycle, the total yield for aerobic respiration is 36 ATP per molecule of glucose in addition to the two ATP produced during glycolysis. The terminal acceptor for the electrons during the electron transport chain is oxygen.
Not to be confused with anaerobic respiration, fermentation occurs in the absence of oxygen within the cytoplasm of cells and converts pyruvate into a waste product to produce the energy carrying molecules needed to continue glycolysis. Since the only energy produced during fermentation is through glycolysis, the total yield per molecule of glucose is two ATP. While the energy production is substantially less than aerobic respiration, fermentation allows the conversion of fuel to energy to continue in the absence of oxygen. Examples of fermentation include lactic acid fermentation in humans and other animals and ethanol fermentation by yeast. The waste products are either recycled when the organism re-enters an aerobic state or removed from the organism.
Found in select prokaryotes, anaerobic respiration utilises an electron transport chain much as aerobic respiration but instead of using oxygen as a terminal electron acceptor, other elements are used. These alternate acceptors include nitrate, sulphate, sulphur, carbon dioxide and other molecules. These processes are important contributors to the cycling of nutrients within soils as well as allowing these organisms to colonise areas uninhabitable by other organisms. These organisms may be obligate anaerobes, able to conduct these processes only in the absence of oxygen, or facultative anaerobes, able to produce energy in the presence or absence of oxygen. Anaerobic respiration produces less energy than aerobic respiration because these alternate electron acceptors are not as efficient as oxygen.
Unlike the various cellular respiration pathways, photosynthesis is used by plants, algae and some bacteria to produce the food needed for metabolism. In plants, photosynthesis occurs in specialised structures called chloroplasts while photosynthetic bacteria typically carry out photosynthesis along membranous extensions of the plasma membrane. Photosynthesis can be divided into two stages: the light-dependent reactions and the light-independent reactions. During the light-dependent reactions, light energy is used to energise electrons removed from water and produce a proton gradient that in turn produces high energy molecules that fuel the light-independent reactions. As the electrons are stripped from water molecules, the water molecules are broken down into oxygen and protons. The protons contribute to the proton gradient but the oxygen is released. During the light-independent reactions, the energy produced during the light reactions is used to produce sugar molecules from carbon dioxide through a process called the Calvin Cycle. The Calvin Cycle produces one molecule of sugar for every six molecules of carbon dioxide. Combined with the water molecules used in the light-dependent reactions, the general formula for photosynthesis is 6 H2O + 6 CO2 + light -> C6H12O6 + 6 O2.
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