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Overview of Photosynthesis Electrons taken from water molecules are boosted "uphill" by the energy from sunlight. The chloroplast uses these "excited" electrons, along with carbon dioxide and hydrogen ions, to produce sugar molecules. The reaction steps add up to the overall chemical equation for photosynthesis shown in Figure 1 Figure 1 - the chemical equation for photosynthesis summarizes many reaction steps. Photosynthesis occurs in two main stages, each with many steps: the light reactions and the Calvin cycle (light independent reactions) (Figure 2). Figure 2 This "road map" shows the two main stages of photosynthesis: the light reactions, which occur in the thylakoids, and the Calvin cycle, which occurs in the stroma. The light reactions convert the energy in sunlight (light energy) to chemical energy. These reactions depend on molecules built into the membranes of the thylakoids. First, the green pigment chlorophyll found in the membranes captures light energy. Then the chloroplasts use the captured energy to remove electrons from water. This splits the water into oxygen and hydrogen ions. The oxygen formed is a "waste product" of photosynthesis eg it is not used in the process. It escapes to the atmosphere through the stomata of leaves. So what becomes of the water's electrons and hydrogen ions? Chloroplasts use them to make an energy-rich molecule called NADPH. (NADPH is an electron carrier that delivers the energy rich electrons to the Calvin cycle so sugar can be made). The chloroplasts also use the captured light energy to generate ATP. The overall result of the light reactions is the conversion of light energy to chemical energy stored in two compounds: NADPH and ATP. The light reactions convert light energy to the chemical energy of ATP and NADPH. But recall that photosynthesis also produces sugar. So far no sugar has been produced. That is the job of the Calvin cycle, which uses the ATP and NADPH produced by the light reactions. The Calvin Cycle has chemical inputs and outputs. The inputs are carbon dioxide from the air and the ATP and NADPH produced by the light reactions. The Calvin cycle uses carbon from the carbon dioxide, energy from the ATP, and highenergy electrons and hydrogen ions from the NADPH. The cycle's output is an energy-rich sugar molecule. That sugar is not yet glucose, but a smaller sugar named G3P. The plant cell uses G3P as the raw material to make glucose and other organic molecules it needs. The Calvin cycle is sometimes referred to as the "light-independent reactions" because, unlike the light reactions, it does not directly require light to begin. However, this doesn't mean that the Calvin cycle can continue running in a plant kept in the dark. The Calvin cycle requires two inputs supplied by the light reactions, ATP and NADPH. Concept Check 1. Explain why a leaf appears green. 2. Besides oxygen, what two molecules are produced by the light reactions? 3. Where in the chloroplast do the light reactions take place? Summary of Photosynthesis Lets take a look at the overall process again. Recall that the overall equation for photosynthesis is: 6 CO2 + 6 H2O C6H12O6 + 6 O2 The light reactions, which take place in the thylakoid membranes, convert light energy to the chemical energy of ATP and NADPH. The light reactions use the reactant water from the equation and release the product oxygen. The Calvin cycle, which takes place in the stroma, uses ATP and NADPH to convert carbon dioxide to sugar (Figure 7). Figure 7 The light reactions and the Calvin cycle together convert light energy to the stored chemical energy of sugar. The plant can use the sugar to build other organic molecules. By converting light energy to chemical energy, photosynthesis is the first step in the flow of energy through an ecosystem. Some of that chemical energy then passes from producers to consumers. Even when people eat meat, you can trace its stored energy back to photosynthesis. For example, the beef in a hamburger came from cattle that ate plants. Photosynthesis is the ultimate source of all the food you eat and all the oxygen you breathe. Concept Check 1. What are the inputs and outputs of the Calvin cycle? 2. Which stage of photosynthesis uses each reactant from the overall photosynthesis equation? Which stage generates each product from the overall photosynthesis equation? 3. What molecule is the direct product of photosynthesis? How is that molecule then used by plant cells? Concept Check 1. Explain why a leaf appears green. 2. Besides oxygen, what two molecules are produced by the light reactions? 3. Where in the chloroplast do the light reactions take place? Concept Check 1. What are the inputs and outputs of the Calvin cycle? 2. Which stage of photosynthesis uses each reactant from the overall photosynthesis equation? Which stage generates each product from the overall photosynthesis equation? 3. What molecule is the direct product of photosynthesis? How is that molecule then used by plant cells?