The hydrogen ions play critical roles in the remainder of the light-dependent reactions. The oxygen molecules produced as byproducts exit the leaf through the stomata and find their way to the surrounding environment. This splitting releases two electrons and results in the formation of oxygen (O 2) and 2 hydrogen ions (H +) in the thylakoid space. The replacement of the electron enables chlorophyll to respond to another photon. To replace the electron in the chlorophyll, a molecule of water is split. Up to this point, only energy has been transferred between molecules, not electrons. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. Chlorophyll is therefore said to “donate” an electron ( Figure 1).The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. The photon causes an electron in the chlorophyll to become “excited.” The energy given to the electron allows it to break free from an atom of the chlorophyll molecule. ![]() A photon of light energy travels until it reaches a molecule of chlorophyll pigment. It consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids. Each photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center. Both photosystems have the same basic structure: a number of antenna proteins to which chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. There are two photosystems (Photosystem I and II), which exist in the membranes of thylakoids. The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem. This chemical energy will be used by the Calvin cycle to fuel the assembly of sugar molecules. The overall purpose of the light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. ![]() When NADPH gives up its electron, it is converted back to NADP+. The lower energy form, NADP+, picks up a high energy electron and a proton and is converted to NADPH. Photosynthesis uses a different energy carrier, NADPH, but it functions in a comparable way. You should be familiar with the energy carrier molecules used during cellular respiration: NADH and FADH 2. After the energy is released, the “empty” energy carriers return to the light-dependent reactions to obtain more energy. ![]() The carriers that move energy from the light-dependent reactions to the Calvin cycle reactions can be thought of as “full” because they bring energy. The two reactions use carrier molecules to transport the energy from one to the other. In the Calvin cycle, which takes place in the stroma, the chemical energy derived from the light-dependent reactions drives both the capture of carbon in carbon dioxide molecules and the subsequent assembly of sugar molecules. The light-dependent reactions release oxygen as a byproduct as water is broken apart. In the light-dependent reactions, which take place at the thylakoid membrane, chlorophyll absorbs energy from sunlight and then converts it into chemical energy with the use of water. Photosynthesis takes place in two stages: the light-dependent reactions and the Calvin cycle.
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