Asa Ware, Bobby Strauss, Lauren Maggiore

CU Boulder, Fall 2004

Our group tested the effects that plasmolyzed Elodea cells have on the rate of photosynthesis. From experiment two (in class), we learned that in the presence of a hypertonic solution (relative to plant cells), elodea cells shrivel. They loose water to the surrounding environment as the plasma membrane pulls away from the cell wall in a process called plasmolysis. Using this background information along with the knowledge that photosynthesis takes place in plant chloroplasts, we made our predictions and derived a hypothesis. We measured the rate of photosynthesis (via oxygen bubbles produced per minute per gram) when Elodea samples were placed in different molarities of NaCl solution. Since the ability for the Elodea cells to perform photosynthesis is affected when in a plasmolyzed state, we hypothesized that the number of oxygen bubbles (rate of photosynthesis) would decrease linearly as molarity of the solution (hypertonic to the Elodea cells) increased.

To test this hypothesis we prepared three different solutions of NaCl (0.0M, 0.2M, and 0.4M). We also made the solutions with 1% Sodium Bicarbonate (NaHCO₃), which stimulated the process of photosynthesis. Each Elodea sample was weighed and the stalks were cut at an extreme angle to maximize the surface area subjected to the Sodium Bicarbonate. One Elodea plant was placed in each solution for about 5 minutes, which allowed the cells to be affected by their environment. Then each sample was placed upside down in their respective solution cuvettes and set up lamps to assist the process of photosynthesis. We timed each sample for 5 minutes, counting how many oxygen bubbles arose. Our group ran the experiment twice, keeping conditions as similar as possible. We predicted that the number of oxygen bubbles produced would decrease linearly as the molarity of the NaCl solution increased.

Our results indicated that there was a significant decrease in the number of oxygen bubbles produced per minute per gram as the molarity increased (Trial 1: 0.0M–89.1 bubbles; 0.2M–18.8; 0.4M–2.7 and Trial 2: 0.0M–22.7 bubbles; 0.2M–3.5; 0.4M–2.7) (Trial 1: R² = 0.996 and Trial 2: R² = 0.8399).

Our results were somewhat inconsistent with our hypothesis. Instead of the number of oxygen bubbles produced having a relationship that showed a linear decrease as molarity of the NaCl increased, there was an inverse exponential decrease. However, this still supports our prediction that the rate of photosynthesis decreases when cells are in a plasmolyzed state. One of the problems we encountered was a discrepancy in bubble size between the two trials. The bubbles in the second trial were slightly larger than those of the first trial, whether this was because of the Elodea sample, cut, or stalk size. Despite this, the two trials still demonstrated the same trend. The R² values for both trials are extremely close to 1.0 for an exponential trend line. A way to potentially fix the experiment would be to measure displacement of 02 produced using a pipette instead of counting bubbles. This would eliminate more error. We would also want to test other molarities of NaCl solution (or another possible solution that would cause plasmolysis in plant cells) as well as other plants to see if the same results surface. After looking at the CABLE website, we determined that none of the experiments resembled our project enough to draw comparisons between experiments. However, our results were confirmed by what Dr. Basey initially told us to expect (bubble production would decrease with increasing molarity). Also, our results reflected what the Campbell Biology book said about plasmolyzed cells leading to plant death from the inability of the plant to function properly.