What is the citric acid cycle? See all questions in Aerobic Respiration. Impact of this question views around the world. You can reuse this answer Creative Commons License. Light in the form of photons supplies the energy needed to excite two e - s in PSII photosystem II , which are then passed along the transport chain. Oxidative phosphorylation occurs in the membrane of mitochondrial christae during cellular respiration.
In both processes e - s are passed down a chain of electron transfer agents in a series of redox reactions. The ultimate replacement source of electrons is water, but water must lose four electrons and PS II can only accept one at a time. After four electrons have been donated by the OEC to PS II, the OEC extracts four electrons from two water molecules, liberating oxygen and dumping four protons into the thylakoid space, thus contributing to the proton gradient.
The excited electron from PS II must be passed to another carrier very quickly, lest it decay back to its original state.
Pheophytin passes the electron on to protein-bound plastoquinones. The first is known as PQA. PQH2 passes these to the Cytochrome b6f complex Cb6f which uses passage of electrons through it to pump protons into the thylakoid space.
Cb6f drops the electron off at plastocyanin, which holds it until the next excitation process begins with absorption of another photon of light at nm by PS I. PS I gains a positive charge as a result of the loss of an excited electron and pulls the electron in plastocyanin away from it. Meanwhile, the excited electron from PS I passes through an iron-sulfur protein, which gives the electron to ferredoxin another iron sulfur protein.
The electrons have made their way from water to NADPH via carriers in the thylakoid membrane and their movement has released sufficient energy to make ATP. Energy for the entire process came from four photons of light. The two photosystems performing all of this magic are protein complexes that are similar in structure and means of operation. They absorb photons with high efficiency so that whenever a pigment in the photosynthetic reaction center absorbs a photon, an electron from the pigment is excited and transferred to another molecule almost instantaneously.
This reaction is called photo-induced charge separation and it is a unique means of transforming light energy into chemical forms.
Besides the path described above for movement of electrons through PS I, plants have an alternative route that electrons can take. Instead of electrons going through ferredoxin to form NADPH, they instead take a backwards path through the the proton-pumping b6f complex.
The ability of plants to switch between non-cyclic and cyclic photosystems allows them to make the proper ratio of ATP and NADPH they need for assimilation of carbon in the dark phase of photosynthesis. We begin with descriptions of the components of the electron transfer chain in mitochondria, the sequence in which these carriers act, and their organization into large functional complexes in the mitochondrial inner membrane.
We then look at the chemiosmotic mechanism by which electron transfer is used to drive ATP synthesis, and the means by which this process is regulated in coordination with other energy-yielding pathways.
The evolutionary origins of mitochondria, touched upon in Chapter 2, are further considered. With this understanding of mitochondrial oxidative phosphorylation, we turn to photophosphorylation. Light-absorbing pigments in the membranes of chloroplasts and photosynthetic bacteria transfer the energy of absorbed light to reaction centers where electron flow is initiated.
Electron flow occurs through a series of carriers and, as in mitochondria, this flow drives ATP synthesis by the chemiosmotic mechanism. The discovery in by Eugene Kennedy and Albert Lehninger that mitochondria are the site of oxidative phosphorylation in eukaryotes marked the beginning of the modern phase of studies of biological energy transductions. Mitochondria are organelles of eukaryotic cells, believed to have arisen during evolution when aerobic bacteria capable of oxidative phosphorylation took up symbiotic residence within a primitive, anaerobic, eukaryotic host cell see Fig.
Mitochondria, like gramnegative bacteria, have two membranes Fig. The outer mitochondrial membrane is readily permeable to small molecules and ions; transmembrane channels composed of the protein porin allow most molecules of molecular weight less than 5, to pass easily.
The inner membrane bears the components of the respiratory chain and the enzyme complex responsible for ATP synthesis.
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