What are the light reactions of photosynthesis and how is light energy obtained?


light reactions of photosynthesis

The energy of sunlight can be harnessed by the reactions known as ‘light reactions’ for the movement of electrons from H2O water to create chemical energy in the form of ATP and NADPH. Remember in biological systems the chemical energy from electrons are stored in the form of ATP, NADPH, and NADH because the bonds can easily be broken and energy can be released. The bonds between hydrogen and oxygen in water very stable requiring the electrons from the sunlight energy to be broken. The light reactions separate from the dark reactions in which the ATP and NADPH chemical energy generated is then used to fix carbon dioxide, CO2,in the atmosphere to make sugars. I have provided my own diagram which outlines a rather complex rendition of the inner chloroplast membrane and the light reaction molecules.
light reactions of photosynthesis
In the top portion of the example is the aqueous stroma and is separated by the thylakoid membrane from the inner thylakoid lumen on bottom. Refer to my chloroplast design article if the names of the sections are confusing.  The thylakoid membrane is a lipid bilayer where chlorophyll, other light harnessing pigments, and the proteins that shuttle the electrons away from H2O. Note this rendition only depicts the “Z-scheme” of photosynthesis typically found in green plants and cyanobacteria. Photosynthesis can also be accomplished in other living organisms such as purple bacteria and in green-sulfur bacteria in similar but simpler way. For now we will focus on the Z-scheme then back track and explain these less developed systems.

photosynthesis and how is light energy obtained

The light energy is harnessed by light harvesting molecules called pigments which can be chlorophyll or others such as beta-Carotene. The sun constantly gives photons of light that are of many different wavelengths. When photons are absorbed by light harvesting complexes the result is excited electron, or electrons of higher energy. Excited electrons are in an unstable state and will return to their normal state releasing the energy in either the form heat, emit light with less energy (fluorescence), or in our case of pigments the excited electrons transfer the energy in a process known as excitation energy transfer. The excited electrons in one pigment are in such a close proximity, ~10 nm, with the neighboring pigment molecule that the energy can simply be passed along no loss. The efficiency of energy transfer allows photosynthetic organisms to quickly move energy and after the excited electron passes the energy to an adjacent light harvesting complex (LHC) it will return to its normal state and ready for another round. Therefore LHC is typically thought of as antenna ultimately pulling the electrons into the reaction center. The reaction center is special because it absorbs higher wavelengths of lower energy light. By accepting lower energy light the system can utilize most of the suns light energy. The first reaction center in the Z scheme of light reactions is called P680 which describes that it absorbs lower energy light with a wavelength of 680. This is the reaction center because the energy from LHCs that reaches here also excites an electron generated by the breakdown of H2O into O2. The system transfers this electron to a pheophytin electron acceptor molecule through the simple reduction of that molecule. It is important to note this is the first part of the system where an electron has been transferred, previously only energy has been moved. The electron passed from the reaction center results in it becoming an electronegatively positive acceptor ready to shuttle another electron from water.
The rest of the Z scheme follows the movement of the electron ultimately to ultimately generate NADPH while also facilitating the pumping of protons from the stroma into the lumen of the thylakoid. The pheophytin is molecule that is soluble within the hydrophobic environment of the thylakoid membrane so it moves and transfers its electrons energy to the first of two quinone compounds, plastoquinone A (QA). The plastoquinones are responsible for the juggling of the electron to move protons from the stroma when plastoquinone B, (QB) passes the electrons to a cytochrome. The cytochrome b6f complex can be compared to complex III of the electron transport chain of mitochondria. It takes electrons from quinones shuttling them to plastocyanin molecule in a process known as the Q cycle all while pumping protons into the lumen of the thylakoid. The plastocyanin is another membrane soluble molecule that moves the electron to a second reaction center: P700 of photosystem I.
The electron after passing to the reaction center, P700, it can be excited from an additional photon of light energy through another set of light-harvesting complexes. This excited electron is passed first to an acceptor molecule, A0 (this acceptor is still being understood and may be a special form of chlorophyll molecule), then transferred to a second A1. The electron moves through a special iron sulfur (Fe-S) protein before finally ending up in a ferradoxin molecule. The ferradoxin contains a flavin adenine dinucleotide (FAD+) that is reduced to FADH2 with two electrons. Each FADH2 created provides the reducing power to make one NADPH in the stroma.
Now let’s talk a little bit about the energy output of these “light” reactions. The proton pumping in Photosystem II from the Q cycle and the cytochrome b6f complex moves protons from a low concentration within the stroma to a high concentration in the thylakoid lumen. Additionally the water that forms O2 will also leave protons within the lumen. The buildup of protons in the lumen generates a concentration gradient in which protons can flow back into the stroma through the ATP synthase machinery also integrated in the thylakoid membrane and generate ATP in the stroma. For every single O2 molecule generated two H2O molecules are used. Four electrons from the water enter Photosystem II in from these waters and eventually pump a total of twelve protons back into the stroma through the ATP synthase and generate roughly three to four ATP. Four electrons from the water eventually end up making two NADPH molecules. This ratio of three ATP to two NADPH will play a role in the fixation of CO2 in the Calvin cycle.


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