Coupled reactions: How photosynthesis takes place

[Bernard NebelKeymaster]

It is conspicuous that shining light on a glass of water, which has atmospheric carbon dioxide dissolved in it, does not produce sugar as the chemical formula for photosynthesis suggests. Let kids try it if they wish. Actual photosynthesis  is far more complex. With kids using models, emphasize how photosyntheses involves breaking the low potential energy   C = O bonds of CO₂ and replacing them with single high energy bonds to hydrogen or other carbon atoms. 

This represents a huge leap from low to high potential energy. It is too much for nature (or humans) to accomplish in any simple one-step chemical reaction. However, nature has developed a way that is analogous to building a ramp. Numerous linked chemical reactions accomplish the task in incremental steps as portrayed in the following video. 

The detail shown in the video is likely to overwhelm kids. Keep reminding them that the overall concept is what is important to grasp. This is: To accomplish the chemically huge energy jump from carbon dioxide and water to glucose, nature has developed a multistep “ramp” which accomplishes the jump in a number of incremental steps. The energy for the gradual push up the ramp comes from light energy absorbed by chlorophyll within the chloroplasts of the cell. 

Remind kids of their first-hand observation of cells and chloroplasts  (Lesson B – 13) or, type into your browser: chloroplast images/. Take time to observe and draw kids to appreciate the the amazing structural design of chloroplasts, microscopic solar cells. 

Note that chlorophyll, the light absorbing pigment within cloroplasts, absorbs red and blue light and reflects green giving plants their green color. Also, let kids know that this detail of photosynthesis was far from self-evident; it is the product of many of scientists spending their careers in research. 

 

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