Botany: Rate of Photosynthesis Experiment

In the experiment, the rate of photosynthesis is explored. This is a fun experiment, because the first time a little leaf disc rises to the surface there is so much excitement. What is being observed is the rate of photosynthesis determined by how much oxygen is being produced. When enough oxygen has been produced in a leaf disc, the gas will cause the disc to rise to the surface and float. This experiment is a simplified form of the photosynthesis experiment, “Up, Up, and Away,’ found in Chapter 8 of Book 2: Guided Tour of the Living Cell in E.O.  Wilson’s Life on Earth series, a free (through Apple) high school biology text book.

This is a fun experiment, because the first time a little leaf disc rises to the surface there is so much excitement.

In this post you will see

We first performed this experiment with light being our treatment and dark being our control. After this initial experiment, we repeated it using green light and red light as our treatments. This is a great opportunity to introduce children to the scientific principles, and the terms dependent and independent variables, as well as the control of an experiment. The light treatment in our experiment was our independent variable. All other variables were the same. The depended variable was the floating of the discs. Our control was the experiment with no light. All other variables were the same – the leaf discs went through the same process, but no light was administered. This allows us to see that process itself is not responsible for the discs rising, but the light treatment is.

We chose light as our variable to test, but other variables such as temperature could be tested as well. We measured our rate by counting the number of floating discs at intervals of 2-minute over a 10-minute period for each treatment. It may be easier for your child to count at 1-minute intervals to keep them move engaged and to make the graphing easier. 

DISCUSSION AND PREDICTIONS

Before doing this experiment, we spent some time going over the process of photosynthesis. We know that plants take in water and carbon dioxide and use light energy from the sun to change those into glucose (sugar) and oxygen. Oxygen is a gas, so if the leaves are photosynthesizing what should happen to our discs? They should eventually float. How can we know how fast they are photosynthesizing? If they float faster, then the rate of photosynthesis is faster. If they do not float at all, then photosynthesis is very slow or not at all. What can we guess will happen when we expose our leaf discs to the light? What can we guess will happen if they are left in the dark? Why is that? These are all questions that we discussed before we started the initial experiment. 

After each experiment we paused to discuss our observations and reflect on our process and results. We were correct? Did anything happen that we didn’t expect? What was that? Why do you think that was? What would we do differently next time? How do we think that would change the results. 

Right before each additional treatment we also had a discussion and came up with a hypothesis? Did we think the green and red lights would work the same? Why? So it turns out that red light has a high rate of photosynthesis and green light has a much slower rate, equivalent to the dark treatment. This was a result that was not expected, but using the information that we had recently learned in our physics unit (Grade 5 Physics: Light) we hypothesized that green light did not cause photosynthesis because all the light was reflected back, and there was no light to use for energy. We hypothesized that the red light had a faster rate, because all the light was absorbed to use for photosynthesis and it was not reflected back to us by the green color. 

Okay, let’s get started with the experiment.

SUPPLIES

You’ll need:

  • 1/8 tsp Sodium bicarbonate (baking soda)
  • one drop dish detergent
  • 300 ml (approximately 1 1/2 cups) water
  • spinach leaves
  • syringe (creates a vacuumed to expel air from the discs)
  • hole punch
  • 300ml beakers or cups
  • Tin foil to eliminate light from one of the cups
  • Different light sources (We used white, red, and green light)

PREPPING THE DISCS

First make the solution by adding sodium bicarbonate and 1 drop of dish detergent to the water. Stir slowly so that no bubbles are formed. 

Use the hole punch to punch discs from the spinach leaves. We used 9 per experimental treatment, but our intention was to use 10 discs. If you follow our experiment, you’ll need approximately 40 discs.

Divide the solution into the different cups to do your experiment. We did ours in two sessions, so our solution was divided in two. When we finished the first sessions, we removed the disc and used the solution again with a new set of discs. You can just as easily divide the solution into 4 cups or a cup per treatment.

Place 10 discs into the syringe. Expel as much air as possible by squeezing the syringe without squeezing the disc in any way. Draw up some of the solution from a cup. Remove any air from the syringe by pushing the syringe again. Placing the cap or your finger on the top of the syringe, pull the plunger to create a vacuum, leaving for a few seconds. You should feel some definite tension here if the syringe is working. After a few seconds let go, and observe the discs in the syringe. If it worked, the discs should all sink. If they did, place the 10 discs and liquid into the cup you removed the solution from, and cover with tin foil. This will be your control. Repeat this process for your light treatment with a new cup.

CONDUCTING THE EXPERIMENT

At this point, we have two treatments: dark, our control, and light. The dark treatment stays covered with tin foil for the full 10 minutes, and then is checked at the end. The light experiment is treated with white light. This can be from a flashlight or lamp that is close to the cup. We measured the rate of photosynthesis by counting the number of floating discs in 2-minute intervals and charting them on a graph. This happened over a total period of 10 minutes.

Afterwards, we prepped 9 (should have been 10) additional discs and treated them with red light. We again counting the number of floating discs in 2-minute internals over 10 minutes. Following this, our last treatment was green light. We prepped our last 10 discs, and exposed them to green light. We counted the number of floating discs at 2-minute intervals over 10 minutes.

OUR RESULTS

As expected we found that when we exposed the leaf discs to white light, they slowly rose to the top. The first disc rose at about 5 minutes, and they steadily increased through 8 minutes with the last disc rising at 10 minutes. We also found that all of our discs in the control were still at the bottom after 10 minutes. When testing the red light, we found that the discs floated to the top at a faster rate with first one floating up at 2 minutes. That was a big contrast to our white light, where it took the first one 5 minutes to float. The discs being treated with the red light continued to rise more quickly than the white light, but stalled at about 8 minutes. The last two discs did not rise. The green light mimicked the dark treatment in its results – not a single disc floated. 

The following day we graphed the results, and we took some time to talk about why we thought the different colors produced different results. Earlier in the year, we had explored light in our physics unit. She remembered that the color we see is the color reflected back to us. Since the leaf discs were green, the green light was being reflected and no light was being absorbed by the leaf. She deduced that if it had no light absorbed, then it would not be able to photosynthesize. We also had some ideas about why the red had a greater rate of photosynthesis than the white. We talked about what else we could do to explore further.   

Video of our Process

Below is a video of our experiment in real time.

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