Slide 1 There are two Electron Transport Chains in many photosynthetic organisms Just as in the electron transport chain in the mitochondria, the electron transport chains in photosynthesis are a series of redox reactions. The free energy that is released during these redox reactions is used for two reasons: One electron transport chain moves protons (H+) across a membrane to generate ATP - this happens in the same way the generation of ATP in the ETC in respiration happens - using the enzyme ATP synthase and oxidative phosphorylation. The other electron transport chain reduces the high energy electron carrier NADP+ to NADPH. NADP+ is similar to the NAD+ used by cellular respiration - it is a high energy electron carrier molecule.
Slide 2 This image is a represenation of the movement of electrons through the two electron transport chains and two photosystems associated with photosynthesis. Photosystem I is associated with the electron transport chain that reduces NADP+ to NADPH and Photosystem II is associated with the electron transport chain that generates ATP by building a voltage across a membrane. This movement of electrons is known as the Z-scheme.
Slide 3 When a photon of light hits Photosystem II - electrons are pushed to a high energy state. They bounce around the photosystem until they are grabbed by an electronegative electron acceptor molecule. Then the electron moves through the electron transport chain and hydrogen ions are pumped across the membrane. Again, this is just like the electron transport chain in mitochondria - a voltage is produced and ATP synthase uses this voltage to generate ATP.
Slide 4 Similarly - when a photon of light hits Photosytem I the electrons bounce around until they are grabbed by an electronegative electron acceptor molecule. These electrons then get passed down the electron transport chain. But they are not used to create a voltage or generate ATP - instead they are used to reduce the high energy electron carrier NADP+ to NADPH. Remember - building a sugar is going to require both energy and electrons. The energy comes from the ATP produced by Photosystem II and the electrons come from the NADPH produced by Photosystem I.
Slide 5 Photosystem II reduces NADP+ to NADPH. This means the photosynthesizing cell needs a constant supply of electrons. Without electrons - the systems would stop functioning. So, where do these electrons come from?
Slide 6 The answer is water. Photosystem II is associated with an enzyme whose job is to split a molecule fo water. It is the splitting of water that supplies the electrons needed by photosynthesis. The splitting of water also releases oxygen. This is where the oxygen by-product of photosynthesis is produced.
Slide 7 This image allows us to compare respiration in the mitochondria with photosynthesis in the chloroplast. What is important to notice is that both organelles contain electron transport chains that move electrons from a high energy state to a low energy state. In respiration, the movement of electrons builds a voltage that the enzyme ATP synthase uses to generate ATP. The same thing happens in photosynthesis but there is an additional electron transport chain that does not create a voltage but instead reduces NADP+ to NADPH.
Slide 8 So, why do plants need water for photosynthesis?
Slide 9 The answer is that the water supplies electrons that will be used to reduce carbon dioxide to sugar in the Calvin cycle.
Slide 10 The sugar that is the ultimate goal of photosynthesis is not produced during the light-dependent reactions. The light-dependent reactions produce the ATP and NADPH that will be used in the Calvin cycle to build the sugar. We will learn about this in the next tutorial.