A light dependent reaction (a.k.a. Electron Transport Chain) actually occurs in two stages. In the first stage, electrical energy which consists of a flow of electrons is converted from the solar energy. In the second stage, chemical energy that is stored in chemical bonds is converted from the electrical energy. The electrons within a chlorophyll molecule become (very) excited when struck by solar energy. As the excitement level rises, the electrons start moving from the high-energy area into lower-energy areas. (DuTemple, 2000) This is why this systematic movement of electrons is called (you guessed it) an electron transport chain. The excitation energy trapped by a reaction centre provides the energy needed for electron transfer. During electron transfer, the light split apart the water molecules (called photolysis) and, removes individual hydrogen ions, and by the electron transport chain, to NADP+.
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Fig. 4 (click for a better view) |
In photosynthesis, light energy pushes electrons up an energy hill in the reaction centres (P680, and P700). Figure 7 (http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001) gives us a rough idea of how does a reaction centre work. Robinson (2001) noted that "subsequent energy flow in the electron transport chain is energetically downhill and can be used to do work." (p. 137) Figure 4 (http;//gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001) is called the Z scheme, it shows the pathway of electrons from water to NADP+ producing oxygen and the reducing power (which becomes NADPH). It also shows that the photosynthetic electron transport chain is made up of the electron carriers in a way that reveals the relative electronic energy. Examples of these electron carriers include plastoquinone (PQ) (which is a hydrogen carrier since it can accept two electrons and two protons), cytochrome bf complex (Cyt bf), plastocaynin (PC), NADP+ and many others. (Sound a bit crazy here? Don't worry, we only have to know the abbreviations of the important ones for grade 13 biology) A majority of the carriers are located in the thylakoid membrane, but a few such as NADP+ are located in the stroma. The loss of electrons (oxidation) creates unstable chlorophyll molecules. To regain stability, the chlorophyll molecules take electrons from the water molecules (reduction: gain of electrons). Other than hydrogen ions are been released, same as oxygen gas, though into the atmosphere (and to be used in respiration, by you and me and everyone else). The loose hydrogen ions then form into different compounds, with ATP (woo, energy) being one of them.
As shown in Fig. 5 (http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html, 2001), a three membrane bound protein complexes: Photosystem II, the cytochrome bf complex, and Photosystem I is required for electron transfer from water to NADP+. Moving an electron requires an input of light energy, gathered by the antenna system of reaction centres. Photosystem II sends electrons to photosystem I, note that for historical reasons the reaction centres are not numbered according to their order in the ETC (electron transport chain). Photosystem II catalyses the oxidation (lose of electrons) of water, as well as the reduction (gain of electrons) of PQ. Water oxidation is a critical reaction in photosynthesis since the electrons removed from H2O are ultimately used to reduce CO2 to carbohydrate later in the Calvin Benson cycle. The products created in this process, O2, and H+ ions, will be used later in the formation of ATP (energy!). The photosystem I reaction centre is very similar to photosystem II, it is served by a chlorophyll-containing antenna system. The differences though is that photosystem I catalyzes some different reactions. As each hydrogen ion is passed along the thylakoid membrane, NADP+, an coenzyme, is its final acceptor. Finally the reduction of NADP+ produces NADPH, which will then be sent to the Calvin Benson cycle.
Now, just take one step back to where photolysis occurred at the beginning of this pathway. The two hydrogen ions separated apart from the oxygen molecule go directly to the ATP thynthase (or CP1 particle, the chloroplast F1 particle). As every pair (yes hydrogen ions enjoy being with one another) of hydrogen ions pass through CF1 particle as in completion of one cycle, one ATP (ahha, energy) is produced by chemiosmotic phosphorylation (an electrochemical gradient is produced). Please keep in mind that there are a lot more H+ ions inside the thylakoid than outside, in the stroma. (Home sweet home) In reality, the concentration of H+ is approximately 10,000 times greater inside than outside. Recall from chemistry that the more the H+, the lower the pH level.
In conclusion, the job of this light independent reaction is to provide energy in the form ATP and NADPH for the Calvin Benson cycle. The way each get the carbon dioxide to the cycle varies, yet all plants rely on the Calvin Benson cycle to make carbohydrates. Here, I will explain the most commonly used Calvin Benson cycle, the C3 pathway, for plants live in normal conditions. There are also C4 pathway for plants that are adapted to hot and dry environments, and crassulacean-acid metabolism (CAM) for low-lying plants adapted to warm and arid parts of the world. (Mader, 1990; Robinson, 2001)
Based on some of the information we have already talked about, the next step of photosynthesis is to take the chemically stored energy and use it to form organic compounds. During light independent reaction, one-carbon (inorganic: compounds do not consist both carbon and hydrogen atoms; in this case, with carbon but no hydrogen atoms) molecules are bonded into three- and five- carbon (organic: compounds consist of both carbon and hydrogen atoms) molecules. Let us find out what happens there.
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