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, 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 as compared to fermentation pathways is the prerequisite for oxygen and the much higher yield of energy per molecule of glucose.
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.
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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 glycolysis is only two molecules of ATP per molecule of glucose. The glucose itself is broken down into pyruvate during glycolysis.
Aerobic respiration occurs in mitochondria in the presence of oxygen and yields the majority of energy for organisms capable of the process. Pyruvate is moved into mitochondria and converted to acetyl CoA, which is then combined with oxaloacetate to produce citric acid in the first stage of the citric acid cycle. The subsequent series converts the citric acid back into oxaloacetate and produces energy-carrying molecules along with way called NADH and FADH2.
Each turn of the Krebs cycle is capable of producing one molecule of 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 utilizes 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, sulfate, sulfur, carbon dioxide and other molecules. These processes are important contributors to the cycling of nutrients within soils as well as allowing these organisms to colonize areas uninhabitable by other organisms.
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 specialized 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 energize 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.