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How Is Glucose Made in Photosynthesis?

author image Adam Cloe Ph.D./M.D.
Adam Cloe has been published in various scientific journals, including the "Journal of Biochemistry." He is currently a pathology resident at the University of Chicago. Cloe holds a Bachelor of Arts in biochemistry from Boston University, a M.D. from the University of Chicago and a Ph.D. in pathology from the University of Chicago.
How Is Glucose Made in Photosynthesis?
Sunlight shining on trees in the forest. Photo Credit: alexkich/iStock/Getty Images

Light Absorption

The first step for the formation of glucose from photosynthesis is the absorption of light. According to Estrella Mountain Community College's website, when sunlight hits an organism that can perform photosynthesis (such as a plant), one of three things can happen. The light energy can be given off as heat; it can be re-emitted at a different wavelength (color); or it can set off a chemical reaction. Photosynthetic organisms contain various pigments, such as chlorophyll, that can absorb and harness light to make high-energy molecules. When light gets absorbed by these pigments, it causes the release of a high-energy particle (called an electron), which can then be used to convert the light energy into chemical energy. This portion of photosynthesis is called the light reaction. because it has to occur in parts of the organism that receive sunlight.

ATP and Carbon Dioxide

Once high-energy electrons have been generated, the photosynthetic organism can turn these electrons into sugar. The first step is storing this energy in a more stable form. The electron gets absorbed by molecules in the organism that are able to perform a series of reactions. These reactions use the electron's energy to form a molecule called adenosine triphosphate (ATP). ATP is a molecule that is similar to DNA but is used by organisms as a temporary energy reservoir. In order to turn the energy from ATP into glucose, plants also need carbon dioxide, which they obtain from the atmosphere (or, in the case of plants that live underwater, from the surrounding water).

Glucose Manufacture

According to the University of Cincinnati's biology department, carbon dioxide and ATP are turned into glucose in what is called the dark reaction (because it does not directly require light). Three ATP molecules react with three molecules of carbon dioxide via a series of chemical reactions (called the Calvin cycle) to form an intermediate called glyceraldehyde 3-phosphate. Two of the glyceraldehyde 3-phosphate molecules then are combined to form a glucose molecule. Thus, six molecules of carbon dioxide and light energy are needed to make one molecule of glucose for photosynthesis.

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