Plants and trees grow and position themselves to places where they can get the most sunlight. Those left in the dark, unless specifically adapted, will struggle to survive, or simply wither away. Light is essential to photosynthesis, the plant's primary means of energy generation. Without the energy produced by photosynthesis, the plants will not be able to continue the thousands of chemical reactions occurring every second necessary to maintain itself.
The process of photosynthesis occurs in an organelle called the chloroplast. Inside these chloroplasts are membranes known as thylakoids. Embedded on the thylakoid membranes are the photosystems, large multiprotein complexes, responsible for capturing the light's energy. The plants break apart water molecules to take its electrons, releasing oxygen in the process. The electrons are sent to a photosystem called Photosystem II. Chlorophyll molecules, present in the photosystems, capture light energy and excite the electrons into what is called high-energy electrons. These high-energy electrons are transported to other proteins along the membrane to aid in the production of ATP, the primary "fuel" of most living things. The electron's energy has been lowered due to the work it did, but there is still more to be done! The electron is sent to Photosystem I to get another dose of energy so it can complete its work. Once it is reenergized, the electron is then sent to make NADPH, another common form of fuel used by living things. This whole process is called the "light reactions," and is continually going on as long as there is sunlight.
The second part of the photosynthesis process is called the "dark reactions," and as the name suggests, this part does not require light. Now, with all this fuel from the light reactions, the plant needs to put it to good use. Carbon dioxide, ATP, and NADPH are sent to what is called the "carbon fixation cycle." The details of how this cycle works are beyond the scope of this site, for it is quite complex and require some knowledge of biochemistry. The most important thing to note is that the carbon fixation cycle transforms the CO2, ATP, and NADPH into proteins, sugars, and fatty acids. These products are used to growth, repair, and storage of energy for later use.
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