Teaching a grade nine applied science class photosynthesis and cellular respiration and how these two reactions of life are opposites can be challenging, even when only using word equations. Last week I experienced a Eureka moment when some of my students got it!
Carbon dioxide + water + sun’s energy ® glucose + oxygen
Glucose + oxygen ® carbon dioxide + water + usable energy
We started out simply exploring the word “opposite” which I felt was an essential place to start since 1/3 of my students are not native English speakers, 1/3 have learning disabilities and all this happens during the last period of the day! So we reviewed common opposites, which was much more meaningful than reading a definition of the word “opposite”. I gave each child, in turn; a word and they had to tell me the opposite of the word, ie. Black–white, up–down, in–out, etc. This was easy for most of them and so they were proud of their success and eager to experience more!
We looked at a comparison chart of the two reactions and noted that the reactants of one were the products of the other and vice versa. We also talked about how there are different forms of energy and the fact that the sun’s energy was essential to drive photosynthesis and usable energy was produced by cellular respiration.
We watched a short, simple video explaining the concept. We used the smartboard to look at the word equation with symbols to represent each of the chemicals and types of energy. We moved the reactants and products from one side of one equation to the opposite side of the other equation. We drew a picture and colour coded the chemicals. I even made a cut and paste activity where the students had to physically cut out the appropriate chemicals (there were two of each) and glue them onto a diagram.
Finally it hit me! I enlarged all the terms in both equations, including the plus and yield signs, and printed them out on coloured paper, using a one colour for each part of the photosynthesis equation and a different colour for the cellular respiration. Each child was given one word or symbol. (The paper that had Sun’s energy on one side had usable energy on the other side. Otherwise all the papers were single sided.)
The students broke into two groups according to the colour of their paper. They were then instructed to line up, holding their signs, so they made the equation for photosynthesis. (The equations were prominently displayed at the front of the room.)
Next they were told to rearrange themselves into the order of cellular respiration. This process was repeated several times with less and less chaos each time. Eventually a student shouted out, “Everyone stay where you are. The person holding the arrow (yield sign) just has to point it in the other direction!” Eureka! The light bulb moment! Slowly the students started to understand, they explained it to one another while the student holding the arrow kept changing its direction and saying, “Look photosynthesis!” Flipped the arrow. “Now its cellular respiration!” flip the arrow. “Now its photosynthesis again!” flip the arrow. “Now its cellular respiration!”
By this time I was euphoric but exhausted. To consolidate the activity we taped our signs onto the wall. One set read the equation for photosynthesis and the other read the equation for cellular respiration.
Try it! It works!