Investigation 5: Photosynthesis Lab #YoungMoola #$$$

Devansh Taori

AP Biology (per. 2)

#Mr #Wong

Introduction

Have you ever felt like you wanted to take a photo, but just didn't know what to synthesize? Well, don't worry. There's something that can help you out: photosynthesis! It's the one process in plants that sustains ecosystems and replenishes the Earth's atmosphere with oxygen. It's arguably essential to life on Earth. While photosynthesis appears extremely complex, we can do experiments to figure more about it. This is one such experiment. Like most enzyme-driven reactions, the rate of photosynthesis can be measured by either the disappearance of a substrate or the accumulation of a product. The accumulation of product in this case is the production of O2. The molecular formula for photosynthesis is:

Because the spongy mesophyll layer in leaves is filled with O2 and CO2, leaves normally float in water. As photosynthesis proceeds, oxygen accumulates in the air spaces of the spongy mesophyll and leaf disks in water will gradually rise to the top. But, cellular respiration is also taking place at the same time, draining oxygen in the leaf. Thus, the net rate of photosynthesis can be measured by the buoyancy of leaf disks.

Objective

The objective of this experiment is to measure the net rate of photosynthesis by testing a variable that affects photosynthesis (leaf disk buoyancy). 

Materials

- baking soda (sodium bicarbonate)

- liquid soap (approx. 5 mL of dishwasher in 250 mL of water)

- 3 plastic syringes without needle (10 mL or larger)

- living leaves (spinach, ivy, etc...)

- hole punch

- 2 plastic clear cups

- timer

- light source

Procedure

1 – Prepare 300 mL of 0.2% bicarbonate solution and 300 mL of 0.5% bicarbonate solution. The bicarbonate will serve as the source of CO2 for the leaf disks while they're in the solutions.

2 – Fill one cup with water, another with 0.2% bicarbonate solution, and another with 0.5% bicarbonate solution. This way, we can test the effect that increasing the concentration of bicarbonate has on the buoyancy of the leaf disks. 

3 – Add one drop of liquid soap to each cup. The soap acts as the "wetting agent" for the leaf, wetting the hydrophobic surface of the leaf and allowing the solution to be drawn into the leaf.

4 – Using a hole-puncher, cut 10 uniform leaf disks for each cup. 

5 – Draw the gases out of the spongy mesophyll and infiltrate the leaves with the sodium bicarbonate solution by doing the following: remove the plunger from the syringes and place 10 leaf disks into each syringe barrel. Push in the plunge until only a small volume of air and leaf disk remain in the barrel. Pull a small volume (5 cc) of the water solution, the 0.2% bicarbonate solution, and the 0.5% bicarbonate solution with soap respectively into the syringes. Suspend the leaf disks in the solution. Make sure no air remains. Then, create a vacuum by holding a finger over the narrow syringe opening while drawing back the plunger. Hold this for about 10 seconds. Then, swirl the leaf disks to suspend them. The solution should infiltrate the air spaces in the leaf disk, causing the disks to sink in the solution. 

6 – Pour the disks and the solution into the appropriate cups. 

7 – Place the cups under a light source and start the timer. Observe and record the rate at which the leaf disks rise to the top.

Hypothesis

I hypothesized that as the concentration of bicarbonate solution increases, the rate of photosynthesis (and thus the rate of leaf disks rising to the top) will increase because there will be more available carbon dioxide. With more available carbon dioxide, photosynthesis can occur faster.

Results/Analysis

Through analysis of the time-lapse videos, we measured the ET-50 for each cup. For the 0.2% bicarbonate solution, the ET was the lowest at 8 minutes (meaning the rate of photosynthesis was the greatest). With the water and 0.2% bicarbonate solution, the ET-50 was extremely similar with 17 and 16 minutes respectively. Because the time-lapse is 30 times faster than real-life, we multiplied the time stamps in the video by 30 to get the time for each ET-50 – 17 minutes for water, 8 minutes for 0.2% bicarbonate solution, and 16 minutes for 0.5% bicarbonate solution. 

Conclusion
The results – contrary to my hypothesis – actually suggest that there's an optimal concentration of CO2 at which photosynthesis performs. When there's no bicarbonate (water), the rate of photosynthesis was slower compared to 0.2% bicarbonate. Likewise, when there was too much bicarbonate (0.5% solution), the rate of synthesis was also slower. Thus, we can see from the results that photosynthesis most likely occurs at its optimal rate when there's a bicarbonate concentration of 0.2%. 

Generalizing this, we can infer that there's a certain threshold of CO2 that makes photosynthesis work the best. While this experiment didn't investigate the reasons for why too much CO2 actually slowed down the process, this would be an interesting topic of further research, and a future experiment that incorporated answers to this question would be useful. 

Ultimately, photosynthesis is the process that sustains all life on Earth. And yet, it's a very delicate process – too little CO2 can hamper it, and too much CO2 can overload it. There's a perfect balance for everything, and that certain applies to photosynthesis as well.

Of course, there's room for experimental error. The leaf disks we cut might have been of different sizes. We might not have effectively placed the disks into a vacuum. We might have had rounding calculations. These errors are things to look out for, but they don't disprove that the results we attained definitely point to the conclusion articulated in this article.

Special shout out to the Wong Doggy Dog for letting us conduct this experiment. It was tons of fun. Go period 2!

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