All living things require chemical energy to grow and reproduce. Heterotrophs are organisms (like humans) that obtain chemical energy from the food they eat. There are a few autotrophic organisms capable of making their own food. Photoautotrophs (like plants) ****capture solar energy and convert it into chemical energy by photosynthesis.
Photosynthesis creates chemical energy which is passed up the food web to all organisms. Photosynthesis also releases oxygen, which is necessary for cellular respiration.
Likewise, photosynthesis removes a significant amount of carbon dioxide from the air. Without this, carbon dioxide gas would accumulate and trap heat in the atmosphere. This is known as the greenhouse effect, and it is a factor in climate change.
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The word photosynthesis gives us clues about its purpose. The prefix (photo-) means 'light', while the suffix (-synthesis) means 'to make'. Hence, photosynthesis uses light to make something. Specifically, it uses photons (light energy) to make sugars (glucose).
This glucose provides plants with fixed carbon, produced through a process called carbon fixation. Carbon fixation occurs when inorganic carbon (carbon dioxide) is incorporated into an organic molecule (like a sugar molecule).
Glucose produced by photosynthesis provides plants with chemical energy, which can be extracted as ATP. This extraction occurs by cellular respiration or fermentation, which we discussed in the previous chapter.
Of course, photosynthesis is not as simple as a plant using photons to make sugar. Broadly speaking, photosynthetic organisms ‘split’ water molecules (photolysis) to release electrons that can be excited by solar energy. The excited electrons are then used to fix carbon dioxide into an organic glucose molecule, as was discussed above.