Cellular Biology

How Photosynthesis Works



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Organisms can be either autotrophs or heterotrophs. Autotrophs, or literally "self-feeders," are capable of creating their own food through the processes of photosynthesis or chemosynthesis. Heterotrophs, "other-feeders," rely on autotrophs as a food source.

Photosynthesis is the process where photoautotrophs capture light energy and harness it in food molecules. Plants, some bacteria, and some protists are capable of photosynthesis. Literally translated, it means light building.

In photosynthetic organisms, cells contain structures called chloroplasts. Chloroplasts are bean-shaped intracellular organelles ("little organs") that have pigments and membranes inside. The pigment molecules capture light energy. The electrons in the pigment molecules get energized by this light energy, and their energy is chemically transformed and bonded to Carbon, Hydrogen, and Oxygen molecules in the form of starches and sugars.

Photosynthesis begins with the capture of light energy. Just as black clothing absorbs solar energy, pigments in photosynthetic cells absorb energy. Pigment molecules include the well-known Chlorophyll, of which there are actually two types - a and b. There are also accessory pigments, which absorb energy in other parts of the electromagnetic spectrum. These accessory pigments, including xanthocyanins, carotenoids, and phycobilins, are responsible for the fall colors in leaves. They take over when wavelengths of light change in the fall, and chlorophyll production in plants is no longer the appropriate mechanism for photosynthesis.

Pigment molecules are stored in membranous structures within a chloroplast known as Thylakoids. The pigments in thylakoids are arranged into photosystems. Electrons in the pigment molecules get energized by solar energy, bounced around the photosystems to a collecting molecule, and enter into the chemical transformation part of photosynthesis. This capturing of light energy and energizing of electrons is known as the light dependent reactions of photosynthesis, because this step can only occur in the presence of light.

The dark reactions, which can happen with or without light, take the energized electrons and run them through a series of molecular transformations. In the Electron Transport Chain, the electrons "fall" down a series of electron-carrying molecules releasing energy. Then the Calvin Cycle occurs, which is where Carbons, Hydrogens, and Oxygens are bonded together to make 3-Carbon, or eventually 6-Carbon sugar molecules.

Thus, light energy is captured and transformed into food through autotrophic photosynthesis. Chemosynthesis, the other option for autotrophs, is similar except in the use of light. Chemosynthesis relies instead on chemical energy, rather than solar energy. The end product is still the building of food molecules, supporting the rest of the food chain.

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