Chemical Pathways of Photosynthesis

By Timothy Banas
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Photosynthesis is the process by which plants store the light energy from sunlight as chemical energy in sugar molecules. The overall reaction is: 6CO2 + 6H2O (+ light energy) --> C6H12O6 (sugar) + 6O2 There are two main steps in this process. One is a set of chemical reactions known as light reactions, in which light energy is captured and sent to another chemical pathway in the form of high-energy electrons and the molecule that is the universal energy currency in cells, adenosine triphosphate (ATP). The second set of reactions is known as dark or light-independent reactions. In these reactions the energy from light reactions is used to build sugar molecules from carbon into carbon dioxide.

Where the Chemical Pathways for Photosynthesis Exist

All the reactions for photosynthesis in plants occur in tiny cell organelles called chloroplasts, which are thought to be descended from symbiotic photosynthetic bacteria. The chloroplasts have specific internal structures in which the two stages of photosynthesis take place. These structures include an inner and outer membrane with an inter-membrane space, the stroma, which is the general interior of the chloroplast, and the stacks of disc-like thylakoids called grana. The light reactions occur in the membranes of the disc-shaped thylakoids because this is where the photosynthetic pigments, chlorophylls a and b, are located. The light-independent reactions occur in the stroma.

Light Reactions

The overall equation for the light reactions in photosynthesis is:

2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light ? 2 NADPH + 2 H+ + 2 ATP + O2

In these reactions, light excites an electron in a collection of chlorophyll molecules called photosystem II. The electron gets passed to the reaction center of the photosystem, where an electron acceptor picks it up and passes it to photosystem I. The electron is replaced in photosystem II by the splitting of water. This process repeats itself many times, creating a flow of electrons called the electron transport chain (ETC).

Electrons lose energy as they pass through the electron transport chain. This energy that they lose is used to set up a proton gradient, which drives an enzyme called ATP synthase to build ATP molecules as the gradient resolves itself through passive diffusion.

As the electrons are passed to photosystem I--another complex of chlorophyll molecules--they are re-excited and passed in their high-energy states to waiting molecules of nicotinamide adenine dinucleotide phosphate (NADP+), which occur then in their reduced form as NADPH.

The molecules of ATP and NADPH migrate out of the thylakoid membranes and into the stroma, where they provide energy and reduction potential, respectively, for the light-independent reactions.

Light-Independent Reactions

The overall equation for the light independent reactions is:

3 CO2 + 9 ATP + 6 NADPH + 6 H+ ? C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

The light-independent reactions together comprise a cyclical chemical pathway known as the Calvin Cycle. As the reaction equation suggests, the Calvin Cycle involves the use of ATP and NADPH from the light reactions to power the synthesis of carbon dioxide molecules with 5-carbon ribulose sugars. An enzyme called Rubisco performs this crucial synthesis.

The end results of one turn of the Calvin Cycle are 3-carbon carbohydrate molecules called glyceraldehyde-3-phosphates (G3P), and the \"spent\" energy molecules adenosine diphosphate (ADP) and NADP+. The molecules of G3P go through further modification to form 6-carbon carbohydrates such as glucose, and ADP and NADP+ migrate back to the thylakoid membranes to be \"recharged\" by the actions of high-energy electrons generated by the light reactions.

About the Author

Timothy Banas has a master's degree in biophysics and was a high school science teacher in Chicago for seven years. He has since been working as a trading systems analyst, standardized test item developer, and freelance writer. As a freelancer, he has written articles on everything from personal finances to computer technology.