The Three Stages of Photosynthesis

••• Jerzy Opioła: Wikicommons.com, Public Domain: Wikicommons.com

Plants and algae act as the food bank of the world thanks to their amazing photosynthetic powers. In the process of photosynthesis, sunlight is collected by living organisms and used to produce glucose and other energy-rich, carbon-based compounds.

Scientists find the three stages of the process intriguing, and the Center for Bioenergy and Photosynthesis at Arizona State University even argues for photosynthesis' importance relative to other biological processes.

TL;DR (Too Long; Didn't Read)

The process of energy exchange in photosynthesis is expressed as 6H2O + 6CO2 + light energy → C6H12O6 (glucose: a simple sugar) + 6O2 (oxygen).

What Is Photosynthesis?

Photosynthesis is a complex process that can be divided into two or more stages, such light-dependent and light-independent reactions. The three-stage model of photosynthesis starts with absorption of sunlight and ends in the production of glucose.

Plants, algae and certain bacteria are classified as autotrophs, meaning they are capable of meeting their nutritional needs through photosynthesis. Autotrophs are at the bottom of the food chain because they produce food for all other living organisms. For instance, plants are eaten by grazers who may eventually be a food source for predators and decomposers.

Food is not the only contribution of photosynthesis. Stored energy in fossil fuels and wood is used to heat homes, businesses and industries. Scientists study the stages of photosynthesis to learn more about how autotrophs use solar energy and carbon dioxide to produce organic compounds. Research findings could lead to new methods of crop production and increased yields.

The Photosynthesis Process: Stage 1: Harvesting Radiant Energy

When a beam of sunlight hits a green, leafy plant, the process of photosynthesis is set in motion.

The first step of photosynthesis occurs in the chloroplasts of plant cells. Light photons are absorbed by a pigment called chlorophyll, which is abundant in the thylakoid membrane of each chloroplast. Chlorophyll appears green to the eye because it does not absorb green waves on the light spectrum. It reflects them instead, so that's the color you see.

Plants take in carbon dioxide through their stomata (microscopic openings in tissue) for use in photosynthesis. Plants transpire and replenish oxygen in the air and ocean.

Stage 2: Converting Radiant Energy

After radiant energy from sunlight is absorbed, the plant converts light energy into a usable form of chemical energy to fuel the plant’s cells.

In light-dependent reactions occurring during the second stage of the process of photosynthesis, electrons get excited and split off from water molecules, leaving oxygen as a by-product. The hydrogen electrons of the water molecule then move to a reaction center in the chlorophyll molecule.

In the reaction center, the electron passes along a transport chain, aided by the enzyme ATP synthase. Energy is lost as the excited electron drops to lower energy levels. Energy from electrons is transferred to adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), commonly referred to as “energy currency” of cells.

Stage 3: Storing Radiant Energy

The last stage of the photosynthesis process is known as the Calvin-Benson cycle, in which the plant uses atmospheric carbon dioxide and water from soil to convert ATP and NADPH. The chemical reactions that make up the Calvin-Benson cycle occur in the stroma of the chloroplast.

This stage of the process of photosynthesis is light-independent and can happen even at night.

ATP and NADPH have a short shelf life and must be converted and stored by the plant. Energy from ATP and NADPH molecules enables the cell to use or “fix” atmospheric carbon dioxide, resulting in the production of sugar, fatty acid and glycerol in the third stage of photosynthesis. Energy that the plant doesn’t need immediately is stored for later use.

References

About the Author

Dr. Mary Dowd studied biology in college where she worked as a lab assistant and tutored grateful students who didn't share her love of science. Her work history includes working as a naturalist in Minnesota and Wisconsin and presenting interactive science programs to groups of all ages. She enjoys writing online articles sharing information about science and education. Currently, Dr. Dowd is a dean of students at a mid-sized university.

Photo Credits

  • Jerzy Opioła: Wikicommons.com, Public Domain: Wikicommons.com

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