Unit two. The Living Cell
6.4. How Photosystems Convert Light to Chemical Energy
Plants use the two photosystems discussed in series, first one and then the other, to produce both ATP and NADPH. This two-stage process is called noncyclic photophosphorylation, because the path of the electrons is not a circle—the electrons ejected from the photosystems do not return to them, but rather end up in NADPH. The photosystems are replenished instead with electrons obtained by splitting water. As described earlier, photosystem II acts first. High-energy electrons generated by photosystem II are used to synthesize ATP and then passed to photosystem I to drive the production of NADPH.
Within photosystem II (represented by the first purple structure you see on the left in figure 6.8), the reaction center consists of more than 10 transmembrane protein subunits. The antenna complex, which is the portion of the photosystem that contains all the pigment molecules, consists of some 250 molecules of chlorophyll a and accessory pigments bound to several protein chains. The antenna complex captures energy from a photon and funnels it to a reaction center chlorophyll. You can also see the antenna complex in the photosystem illustrated in figure 6.7. The reaction center gives up an excited electron to a primary electron acceptor in the electron transport system. The path of the excited electron is indicated with the red arrow. After the reaction center gives up an electron to the electron transport system, there is an empty electron orbital that needs to be filled. This electron is replaced with an electron from a water molecule. In photosystem II the oxygen atoms of two water molecules bind to a cluster of manganese atoms embedded within an enzyme and bound to the reaction center (notice the light gray water-splitting enzyme at the bottom left of photosystem II). This enzyme splits water, removing electrons one at a time to fill the holes left in the reaction center by the departure of light-energized electrons. As soon as four electrons have been removed from the two water molecules, O2 is released.
Figure 6.8. The photosynthetic electron transport system.
Electron Transport System
The primary electron acceptor for the light-energized electrons leaving photosystem II passes the excited electron to a series of electron-carrier molecules called the electron transport system. These proteins are embedded within the thylakoid membrane; one of them is a “proton pump” protein, a type of active transport channel. The energy of the electron is used by this protein to pump a proton from the stroma into the thylakoid space (indicated by the blue arrow through the electron transport system). A nearby protein in the membrane then carries the now energy-depleted electron on to photosystem I.
Making ATP: Chemiosmosis
Before progressing onto photosystem I, let’s see what happens with the protons that were pumped into the thylakoid by the electron transport system. Each thylakoid is a closed compartment into which protons are pumped. The thylakoid membrane is impermeable to protons, so protons build up inside the thylakoid space, creating a very large concentration gradient. As you may recall from chapter 4, molecules in solution diffuse from areas of higher concentration to areas of lower concentration. Here, protons diffuse back out of the thylakoid space, down their concentration gradient, passing through special protein channels called ATP synthases. ATP synthase is an enzyme that can use the concentration gradient of protons to drive the synthesis of ATP from ADP. ATP synthase channels protrude like knobs on the external surface of the thylakoid membrane (figure 6.9). As protons pass out of the thylakoid through the ATP synthase channels, ADP is phosphorylated to ATP and released into the stroma (the fluid matrix inside the chloroplast). Because the chemical formation of ATP is driven by a diffusion process similar to osmosis, this type of ATP formation is called chemiosmosis.
Figure 6.9 Chemiosmosis in a chloroplast.
The energy of the electron absorbed by photosystem II powers the pumping of protons into the thylakoid space. These protons then pass back out through ATP synthase channels, their movement powering the production of ATP.
Now, with ATP formed, let’s return our attention to the right half of figure 6.8, with photosystem I accepting an electron from the electron transport system. The reaction center of photosystem I is a membrane complex consisting of at least 13 protein subunits. Energy is fed to it by an antenna complex consisting of 130 chlorophyll a and accessory pigment molecules. The electron arriving from the first electron transport system has by no means lost all of its light-excited energy; almost half remains. Thus, the absorption of another photon of light energy by photosystem I boosts the electron leaving its reaction center to a very high energy level.
Like photosystem II, photosystem I passes electrons to an electron transport system. When two of these electrons reach the end of this electron transport system, they are then donated to a molecule of NADP+ to form NADPH (one electron is transferred with a proton as a hydrogen atom). This reaction, which takes place on the stromal side of the thylakoid (as shown in figure 6.8), involves an NADP+, two electrons, and a proton. Because the reaction occurs on the stromal side of the membrane and involves the uptake of a proton in forming NADPH, it contributes further to the proton concentration gradient established during photosynthetic electron transport.
Products of the Light-Dependent Reactions
The light-dependent reactions can be seen more as a stepping stone, rather than an end point of photosynthesis. All of the products of the light-dependent reactions are either waste products, such as oxygen, or are ultimately used elsewhere in the cell. The ATP and NADPH produced in the light-dependent reactions end up being passed on to the Calvin cycle in the stroma of the chloroplast. The stroma contains the enzymes that catalyze the light-independent reactions, in which ATP is used to power chemical reactions that build carbohydrates. NADPH is used as the source of “reducing power,” providing the hydrogens and electrons used in building carbohydrates. The next section discusses the Calvin cycle of photosynthesis.
Key Learning Outcome 6.4. The light-dependent reactions of photosynthesis produce the ATP and NADPH needed to build organic molecules, and release O2 as a by-product of stripping hydrogen atoms and their associated electrons from water molecules.