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6.9: Oxygenic Photosynthesis

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    Compared to the salt loving archaea Halobium with its purple bacteriorhodopin-rich membranes, photosynthetic cyanobacteria (which are true bacteria), green algae, and higher plants (both eukaryotes) use more complex molecular systems through which to capture and utilize light. In all of these organisms, their photosynthetic systems appear to be homologous, that is derived from a common ancestor, a topic we will return to later in this chapter. For simplicity’s sake we will describe the photosynthetic system of cyanobacterium; the system in eukaryotic algae and plants, while more complex, follows the same basic logic. At this point, we consider only one aspect of this photosynthetic system, known as the oxygenic or non-cyclic system (look to more advanced classes for more details.)

    The major pigment in this system, chlorophyll, is based on a complex molecule, a porphyrin (see above) and it is primarily these pigments that give plants their green color. As in the case of retinal, they absorb visible light due to the presence of a conjugated bonding structure (drawn as a series of alternating single and double) carbon-carbon bonds. Chlorophyll is synthesized by a conserved biosynthetic pathway that is also used to synthesize heme, which is found in the hemoglobin of animals and in the cytochromes, within the electron transport chain present in both plants and animals (which we will come to shortly), vitamin B12, and other biologically important prosthetic (that is non-polypeptide) groups associated with proteins and required for their normal function177.

    Chlorophyll molecules are organized into two distinct protein complexes that are embedded in membranes. These are known as the light harvesting and reaction center complexes. Light harvesting complexes (lhc) act as antennas to increase the amount of light the organism can capture. When a photon is absorbed, an electron is excited to a higher molecular orbital. An excited electron can be passed between components of the lhc and eventually to the reaction center (“rc”) complex. Light harvesting complexes are important because photosynthetic organisms often compete with one another for light; increasing the efficiency of the system through which an organism captures light can provide the organism with a selective advantage.

    In the oxygenic, that is molecular oxygen (O2) generating (non-cyclic) photosynthesis reaction system, high energy (excited) electrons are passed from the reaction center to a set of membrane proteins known as the electron transport chain (“etc”). As an excited electron moves through the etc its energy is used to move H+s from inside to outside of the cell. This is the same geometry of movement that we saw previously in the case of the purple membrane system. The end result is the formation of a H+ based electrochemical gradient. As with purple bacteria, the energy stored in this H+ gradient is used to drive the synthesis of ATP within the cell’s cytoplasm.

    Now you might wonder, what happens to the originally excited electrons, and the energy that they carry. In what is known as the cyclic form of photosynthesis, low energy electrons from the electron transport chain are returned to the reaction center, where they return the pigments to their original (before they absorbed a photon) state. In contrast, in the non-cyclic process that we have been considering, electrons from the electron transport chain are delivered to an electron acceptor. Generally this involves the absorption of a second photon, a mechanistic detail that need not trouble us here. This is a general type of chemical reaction known as an reduction-oxidation (redox) reaction. Where an electron is within a molecule's electron orbital system determines the amount of energy present in the molecule. It therefore makes sense that adding an electron to a molecule will (generally) increase the molecule’s overall energy and make it less stable. When an electron is added to a molecule, that molecule is said to have been "reduced", and yes, it does seem weird that adding an electron "reduces" a molecule. If an electron is removed, the molecule's energy is changed (decreased) and the molecule is said to have been "oxidized"178. Since electrons, like energy, are neither created nor destroyed in biological systems (remember, no nuclear reactions are occurring), the reduction of one molecule is always coupled to the oxidation of another. For this reason, reactions of this type are referred to as “redox” reactions. During such a reaction, the electron acceptor is said to be “reduced”. Reduced molecules are generally unstable, so the reverse, thermodynamically favorable reaction, in which electrons are removed from the reduced molecule can be used to drive various types of thermodynamically unfavorable metabolic reactions.

    Given the conservation of matter and energy in biological systems, if electrons are leaving the photosynthetic system (in the non-cyclic process) they must be replaced. So where could they be coming from? Here we see what appears to be a major evolutionary breakthrough. During the photosynthetic process, the reaction center couples light absorption with the oxidation (removal of electrons) from water molecules:

    \[\text{light} + 2H_2O \rightleftharpoons 4H^+ + 4e^– + O_2.\]

    The four electrons, derived from two molecules of water, pass to the reaction center, while the 4H+s contribute to the proton gradient across the membrane179. O2 is a waste product of this reaction. Over millions of years, the photosynthetic release of O2 changed the Earth’s atmosphere from containing essentially 0% molecular oxygen to the current ~21% level at sea level. Because O2 is highly reactive, this transformation is thought to have been a major driver of subsequent evolutionary change. However, there remain organisms that cannot use O2 and cannot survive in its presence. They are known as obligate anaerobes, to distinguish them from organisms that normally grow in the absence of O2 but which can survive in its presence, which are known as facultative anaerobes. In the past the level of atmospheric O2 has changed dramatically; its level is based on how much O2 is released into the atmosphere by oxygenic photosynthesis and how much is removed by various reactions, such as the decomposition of plant materials. When large amounts of plant materials are buried before they can decay, such as occurred with the formation of coal beds during the Carboniferous period (from ~360 to 299 million years ago), the level of atmospheric O2 increased dramatically, up to an estimated ~35%. It is speculated that such high levels of atmospheric molecular oxygen made it possible for organisms without lungs (like insects) to grow to gigantic sizes180.

    Contributors and Attributions

    • Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.

    This page titled 6.9: Oxygenic Photosynthesis is shared under a not declared license and was authored, remixed, and/or curated by Michael W. Klymkowsky and Melanie M. Cooper.

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