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Photosynthesis: How Plants Transform Energy from the Sun

11 Jun Posted by in Biology | Comments

Overview:  What Is Photosynthesis?
Photosynthesis is the process that plants use to produce energy from light.  It takes place in a series of chemical reactions involving chlorophyll, using carbon dioxide, water, other nutrients, and enzymes.  The end result, glucose, is stored and used as a food source.

Light from the sun carries energy that is converted into chemical energy.  The best range for photosynthesis is between 200C (680F) and 350C (950 F), although some photosynthesis occurs both above and below those temperatures.  Green plants reflect the green portion of the light spectrum, while absorbing the other wavelengths of light to use in energy transfer.

Chlorophyll and other pigments in plants trap the energy from sunlight.  Special chloroplasts within cells contain the substances necessary for those reactions.  Flattened structures called grana house some of the reactions.  They look like dots through an electron microscope, and a fluid called stroma surrounds the structures.  Since chlorophyll reflects green light, most leaves appear green.  Xanthophylls and carotenes are related pigments that reflect yellow and orange light.  When leaves stop producing chlorophyll in the fall, the xanthophylls, carotenes, and other pigments give leaves their autumn colors.

The Light Reactions
Reactions that depend on light occur during the first phase of the process of photosynthesis.  These processes take place in the grana that house chlorophyll.  Molecules of chlorophyll absorb light.  Energy from the absorbed light causes reactions that result in the formation of a substance called ATP.  Oxygen is released into the atmosphere.  In fact, the largest source of free oxygen in the atmosphere is as a byproduct from photosynthesis.

The Dark Reactions
The second phase of the process of photosynthesis can occur in light or darkness.  Energy stored in ATP made during the first phase of photosynthesis combines with carbon from carbon dioxide, as well as hydrogen, to eventually form glucose.  Excess glucose is stored by some plants as forms of starch, and cellulose is also made from glucose.  Chemical shorthand for photosynthesis can be written as:

6CO2 + 12H2O  — mediated by light, enzymes and chlorophyll –> C6H12O6 + 6O2 + 6H2O, as the overall process.  In other words, 6 molecules of carbon dioxide, mediated by light, enzymes, and chlorophyll, will combine to form 1 glucose molecule plus 6 oxygen molecules and 6 molecules of water.

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