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  Respiration

Respiration:

In respiration oxidation of organic compounds takes place and the oxidizable substrate is the primary electron donor. Glucose (CH O) is oxidized into pyruvic acid (CH3COCOOH) that is ultimately oxidized to CO2.The energy contained in glucose is completely discharged as the C is totally oxidized to CO2. In such respiration the terminal electron acceptor is oxygen (i.e., oxidant) Anaerobic respiration also takes place in some organisms where other inorganic compounds (i.e., other than oxygen) such as NO3- , SO4- or organic compounds such as fumarate serve as final electron acceptor. In the fermentation an organic compound is the final electron acceptor and the oxidizable substrate is the electron donor. For example in glucose (C6H12O6) fermentation by yeasts ethanol (C2H5OH) and CO2 are produced. Ethanol is more reduced and still has energy whereas the CO2 does not have any energy. A part of the glucose carbon is oxidized to CO2 and a portion decreased to ethanol serving as   electron donor and acceptor. In photosynthetic bacteria bacteriochlorophyll serves as e-donor and acceptor. In the photosynthesis of plants; eucaryotic algae and procaryotic cyanobacteria (i.e., Blue Green algae) water is the main electron donor and NADP+ is the terminal acceptor. The flow of electrons in different processes is termed as electron transport chain. The series of oxidation-reduction reactions are mediated by a range of electron carriers or electron carrying enzymes.  The free energy liberated during the flow of e- in the electron transport chain is conserved in the form of ATP. Since this is an oxidative procedure this type of formation of ATP is termed as Phosphorylation. The respiratory chain is an electron transport chain where a pair of e- or hydrogen atoms having electron from the substrate oxidized is coupled to reduction of oxygen to water.

Here the flow of electron discharges energy that is transformed in the form of ATP.  Enzymes containing prosthetic groups or coenzymes include the respiratory chain. All of them are an O/R system with an oxidized and reduced form. These are Nicotinamide Adenine dinucleotide (i.e., NAD), Nicotinamide Adenine Dinuclotide Phophate (i.e., NADP), Flavin mononucleotide (FMN), Flavin Adenine Dinucleotide (FAD), Coenzyme Q (also termed as Ubiquinone) and Cytocromes.

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Three ATP molecules are made per molecule of NADH2 oxidized. Energy production is also completed in photosynthesis. Photosynthesis is a procedure of reduction of CO2 to carbohydrate employing the light as energy source employing water as reductant.  Photoautotrophs such as plants, algae and blue green algae employ light as their energy source and CO2 as sole source of carbon. Anoxygenic photosynthetic bacteria do not employ water for photosynthesis though use inorganic compounds such as H2, H2S or H2S2O3, organic compounds such as lactate or succinate. Photo-phosphorylation takes place on both the types of photosynthesis and during the electron flow energy is liberated with synthesis of ATP. Cyclic photophosphorylation takes place in photosynthetic bacteria as the Bacteriochlorophyl excited by light adoption and discharges an electron that passes high ferredoxineubiquinone cytochrome b and cytochrome f and turns to bacteriochlorophyll.  ATP is synthesized whenever e- flows from cyt b to cyt f.

In plant photosynthesis, the absorption of light in pigment system-II liberates an e- that flows via plastoquinone, cytochrome b and f and lastly pigment system. Whenever pigment system I absorbs light an e- is liberated that flows via ferredoxin, flavoprotein and to NADP+.  The e- does not return to its position. Therefore, termed as non cyclic flow. Two ATP molecules are synthesized whenever e- flows from cyt b to cyt f in system II and other whenever e- flows to ferrodoxin from the excited pigment. This kind of phosphorylation is non cyclic photo-phosphorylation.

In both oxidative phosphorylation and photosynthetic phosphorylation ATP synthesis is coupled with e- transfer. The energy liberated during oxidation of an organic molecule into another can also be employed for ATP synthesis. This is termed as substrate level phosphorylation and takes place in glycolysis.

 

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