Friday, January 5, 2018

Homeschool Science: Demonstrate Carbon Dioxide Uptake and the Necessity of Light in Photosynthesis

The kids and I are using CK-12's 9th/10th grade Biology textbook as the spine for this year's biology curriculum--for Will, who is in the eighth grade but who is taking high school-level coursework, this will be recorded as Honors Biology on her transcript.

In addition to that textbook, we're using The Illustrated Guide to Home Biology Experiments as our lab manual, and of course we've got a plethora of other reading/viewing/listening resources and hands-on activities to enrich our study.


Time for more photosynthesis! After reading the chapter on photosynthesis in their textbook, exploring some other reading/viewing resources (more on that another time), and successfully modeling the chemical process of photosynthesis, the kids were well-prepared for a science lab that would allow them to see photosynthesis in action.

If you're following along with us in The Illustrated Guide to Home Biology Experiments, which makes a great lab manual for CK-12's 9th/10th grade Biology textbook, this is Procedure IV-1-1: Observing Carbon Dioxide Uptake. To perform this demonstration, we needed bromothymol blue, hydrochloric acid, several test tubes with rubber stoppers, and several sprigs of elodea, a water plant. If you're not doing this in the late autumn or winter, you can likely find elodea, or a similar water plant species, just by hiking to a local pond, but I bought ours from AquariumPlants on etsy. I've actually purchased elodea from this shop twice, because Will also wanted to use elodea for the experiment that she had to design and perform as part of her Space Camp academic scholarship application.

To set up this demonstration, we must turn the kitchen table into our science laboratory. That involves removing everything from the table, wiping it down, and laying newspapers down.

Boom! We have a science lab!

Syd measures out the correct amount of distilled water into a beaker:

Will adds bromothymol blue until its color is distinct:

Syd adds the blue-tinted water to select test tubes, some of which have sprigs of elodea already in, and some of which are going to be left empty:

The bromothymol blue solution in those test tubes will be turned slightly acidic not with carbon dioxide, which we know is required for photosynthesis, but with hydrochloric acid, which Will is adding a teensy drop of to each tube (notice the appropriate protective gear for once!):


Now we have some acidic solutions, but we expect that the bromothymol blue indicator will not change color when this plant is exposed to light:

Next, Syd introduces carbon dioxide to the remaining bromothymol blue solution, by the expedient means of blowing into it through a straw:



Science IS magic!!!

Now all Will has to do is fill the remaining test tubes with that solution--



--and here we have two springs of elodea, both in acidic solutions:

The kids put a test tube with elodea and bromothymol blue solution with carbon dioxide; a test tube with elodea and bromothymol blue solution with hydrochloric acid, and a test tube with only bromothymol blue solution with carbon dioxide into a dark room, and another set of the same in a sunny window:


They visit all of the test tubes every ten minutes to make observations. Within the hour, however, their experiment has a positive result:

See that test tube on the left? Its bromothymol blue is turning from yellow back to blue, and doing so first in the vicinity of the elodea sprig. This tells us that the solution is changing from acid to base, but the only thing added to that solution to make it acidic was carbon dioxide. The test tube with the hydrochloric acid solution is not changing. This tells us that the carbon dioxide is being depleted from that solution, but the only thing added to that test tube was elodea. The empty test tube with carbon dioxide added is not changing. 

This tells us that the elodea is depleting carbon dioxide from the solution. It's only doing so in the test tube that's in the sunny window; the matching test tube in the dark room shows no change.

This, then, tells us that photosynthesis takes carbon dioxide and requires light. 

And that was a good afternoon of science!

I'm slowly writing up our complete lesson plans for each chapter in CK-12 9th/10th grade Biology; here's chapter one.

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