Welcome to My AP Biology Thoughts podcast, my name is Helena Holley and I am your host for episode #78 called Unit 3 Cellular Energetics: Light Dependent Reactions. Today we will be discussing the mechanisms of the first stage of photosynthesis known as the light dependent reactions or non-cyclic photophosphorylation. 

The light dependent reactions is a process in which light energy is converted to chemical energy. In plants, this reaction takes place in thylakoid membranes of chloroplasts. The process involves the use of two photosystems called photosystem I and II. These are embedded in the membrane and they contain many pigments which optimize them for harvesting light. The process involved the use of an electron transport chain to create a proton gradient which is used to make ATP and NADH. 

Now that you have an overview of the light dependent reactions, I'll take you through the process step by step. The reactions begin when photosystem II absorbs light causing an electron to be boosted to a higher level. This electron is passed to an acceptor molecule and it is replaced by an electron from water, which causes h20 to split into ½ O2 and 2 H+ molecules. The O2 released is the oxygen we breathe. The high energy electron that is energized by the light is passed down an electron transport chain. The electron releases energy as it travels through the chain which drives the pumping of H+ from the stroma into the thylakoid lumen, building a gradient. Since this proton gradient is created, the H+ ions will naturally want to flow down it. In order to flow down their gradient back into the stroma, they have to go through the ATP synthase enzyme. This creates ATP from ADP and Pi through chemiosmosis. One of the last locations of the electrons in their electron chain is photosystem I where the electron is boosted to an even higher energy level and transferred to an acceptor molecule. This high energy electron travels down a short final leg of the electron transport chain and at the end it is passed to NADP+ resulting in the creation of NADPH. 

The light dependent reactions are a significant part of the process of photosynthesis. The ATP and NADPH created in the light dependent reactions is used to power the production of carbohydrates from carbon dioxide. Carbohydrates are a main food source for organisms and are essential for survival. Without light dependent reactions, photosynthesis cannot occur and energy for the photosynthetic organisms would not be created. This would result in the organism dieing. The light dependent reactions are essential to the organisms survival. Enzymes are also a key component of light dependent reactions such as the ATP synthase enzyme. As discussed in the course, enzymes require certain conditions in order to function and their functioning is important to the light dependent reactions. 

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