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Photosynthesis Basics - Study Guide

How Plants Make Food - Key Concepts


Concord Grape Leaf

Photosynthesis Occurs in Leaves

Keith Weller, USDA Agricultural Research Service


  • In plants, photosynthesis is used to convert light energy from sunlight into chemical energy (glucose). Carbon dioxide, water, and light are used to make glucose, oxygen, and water.


  • Photosynthesis is not a single chemical reaction, but rather a set of chemical reactions. The overall reaction is:

     6CO2 + 6H2O + light → C6H12O6 + 6O2
  • The reactions of photosynthesis can be categorized as light-dependent reactions and dark reactions.


  • Chlorophyll is a key molecule for photosynthesis, though other cartenoid pigments also participate. There are four (4) types of chlorophyll: a, b, c, and d. Although we normally think of plants as having chlorophyll and performing photosynthesis, many microorganisms use this molecule, including some prokaryotic cells. In plants, chlorophyll is found in a special structure, which is called a chloroplast.


  • The reactions for photosynthesis take place in different areas of the chloroplast. The chloroplast has three membranes (inner, outer, thylakoid) and is divided into three compartments (stroma, thylakoid space, inter-membrane space). Dark reactions occur in the stroma. Light reactions occur the thylakoid membranes.


  • There is more than one form of photosynthesis. In addition, other organisms convert energy into food using non-photosynthetic reactions (e.g. lithotroph and methanogen bacteria)

    Products of Photosynthesis

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