1 00:00:08,820 --> 00:00:22,300 Photosynthesis is vital to life on Earth. 2 00:00:23,579 --> 00:00:26,879 Photosynthesis occurs in all green plants and some algae. 3 00:00:27,239 --> 00:00:30,739 One of the first stages of photosynthesis involves Photosystem II. 4 00:00:31,780 --> 00:00:35,899 The oxygen we breathe is a product of the Photosystem II reaction. 5 00:00:36,439 --> 00:00:41,579 This animation will describe the processes that take place within this important complex. 6 00:00:42,520 --> 00:00:45,579 Photosystem II involves several key components, including 7 00:00:45,579 --> 00:00:51,259 photons, light-harvesting chlorophyll-binding proteins, 8 00:00:52,000 --> 00:00:55,780 a pair of chlorophyll molecules known as the P680 reaction center, 9 00:00:56,359 --> 00:00:59,439 pheophyton molecules, and plastiquinones, 10 00:00:59,859 --> 00:01:03,759 along with water and oxygen. 11 00:01:07,840 --> 00:01:12,659 The reaction center of Photosystem II consists of multiple proteins and pigment molecules. 12 00:01:12,659 --> 00:01:20,299 At the heart of the reaction center is a special pair of chlorophyll molecules, P680, 13 00:01:21,400 --> 00:01:24,239 which donate an electron to the electron transport system. 14 00:01:27,650 --> 00:01:31,269 In photosystem II, the electron is then passed to a pheophyton molecule. 15 00:01:32,230 --> 00:01:37,469 The electron is then passed to plastaquinone QA and then to plastaquinone QB. 16 00:01:38,310 --> 00:01:42,590 These plastaquinone molecules are embedded in the D2 and D1 proteins. 17 00:01:42,590 --> 00:01:51,629 Once QB has accepted two electrons, it then acts as a mobile carrier to the next component of the photosynthetic electron transport system. 18 00:01:53,510 --> 00:01:59,150 Electrons from water are then transferred to the P680 molecules that have lost their electrons in the process. 19 00:02:01,640 --> 00:02:04,840 Additional proteins are involved in splitting the electrons from water. 20 00:02:05,799 --> 00:02:10,159 And still other proteins are necessary to build the complete photosystem reaction center. 21 00:02:11,120 --> 00:02:15,120 Surrounding the reaction center are light-harvesting chlorophyll-binding proteins. 22 00:02:16,360 --> 00:02:20,520 These proteins provide a way to harness the unique energy contained in light. 23 00:02:21,400 --> 00:02:26,939 When one of the many photons of light flooding a leaf hits a chlorophyll molecule surrounding the reaction center, 24 00:02:27,219 --> 00:02:28,719 it creates resonance energy. 25 00:02:29,500 --> 00:02:31,560 You can think of this as vibrational energy. 26 00:02:32,199 --> 00:02:33,780 Here you can see the chlorophyll vibrating. 27 00:02:34,780 --> 00:02:39,159 That resonance, or vibrational energy, is then passed to a neighboring chlorophyll molecule. 28 00:02:39,159 --> 00:02:44,520 It is then passed through several chlorophyll molecules until it reaches the P680 reaction 29 00:02:44,520 --> 00:02:45,520 center. 30 00:02:45,520 --> 00:02:55,020 It is that energy that results in the loss of an electron from the P680 molecule. 31 00:02:55,020 --> 00:02:58,219 Now we can see how these processes work as a whole. 32 00:02:58,219 --> 00:03:02,759 First, a photon of light activates a chlorophyll molecule. 33 00:03:02,759 --> 00:03:08,360 The resonance, or vibrational, energy is transferred to the P680 molecules. 34 00:03:08,360 --> 00:03:11,219 An electron is lost from P680. 35 00:03:11,219 --> 00:03:15,539 It is then donated to QA, then to QB. 36 00:03:15,539 --> 00:03:20,360 The P680 molecules are then reduced by the addition of an electron generated by the splitting 37 00:03:20,360 --> 00:03:25,180 of water molecules at the oxygen-evolving complex. 38 00:03:25,180 --> 00:03:31,099 Since QB needs two electrons to become mobile, a second photon of light is required. 39 00:03:31,099 --> 00:03:35,280 The resonance energy is again transferred to the reaction center. 40 00:03:35,280 --> 00:03:42,159 An electron is lost from P680 and transferred via QA to the QB, which already contains one 41 00:03:42,159 --> 00:03:43,659 electron. 42 00:03:43,659 --> 00:03:50,490 The fully reduced QB is then transferred to the cytochrome B6F complex. 43 00:03:50,490 --> 00:03:55,469 The P680 molecules are again reduced by the oxygen-evolving complex. 44 00:03:55,469 --> 00:04:00,150 Here you can see water being split at the oxygen-evolving complex. 45 00:04:00,150 --> 00:04:06,069 Two water molecules must be split to provide electrons to reduce P680. 46 00:04:06,069 --> 00:04:10,250 The oxygen we breathe is a product of this water splitting process.