|Explore||Photosynthesis is an anabolic process by which the energy of sunlight is captured and used to convert carbon dioxide and water into carbohydrates (glucose-C6H12O6) and oxygen gas. The purpose of this lab is to experimentally determine the effects of three different leaf colors: green, white, and...|
|Research Question||Does leaf color have an affect on the rate of photosynthesis in the Tradescantia zebrina?|
|Predictions||The Tradescantia zebrina plant with the green leaves contains more pigments which allows for an increased absorption of light in the visible spectrum, resulting in a faster rate of photosynthesis compared to that of the Tradescantia zebrina with the purple leaves or that of the Tradescantia...|
|Experimental Design||Obtain 3 plastic cups and label one for each leaf color using the tape and marker. Measure out 100mL of distilled water and add 100mL to each cup. Add a pinch of sodium bicarbonate and 5 drops of dish soap to each cup. Hole punch 10 leaf discs out of each color of leaf, placing them on separate...|
|Conclusion||What claim can we make from our experiment? What are possible explanations for our results? How do the data we collected and our reasoning with scientific ideas support our claim? What future experiments could be done to expand on the results of this experiment?|
You people did a great job in doing experiments and making a appealing interpretation of the data. I congratulate you and your mentor Mary for have a fine tuned scientific conversation on different aspects of the project.
Apart from the species variation, also there are factors which may contribute in getting inconsistent results such variation in amount of baking soda, water, light, temperature etc. Also as far as possible please keep the time constant for each independent experiments like if 40 minutes for one then keep 40 minutes for all.
Altogether, I found your arguments and discussion quite convincing to support you results. I am really impressed by your hard work and sincerity in conducting and reporting the experiments. I also express my thanks to Mary, as she provided you enough hints and ground to build up on. Thank you very much!
This is a great write up of your project. It is very thorough and captures your understanding of the concepts. A few items of note:
There were a lot of bubbles, but no floating - which you attribute to mass. As you mentioned, perhaps measuring the mass of the leaf discs prior to submerging would be a good idea to control for variability.
Secondly, I think that it would be good to emphasize what is being measured and what is being approximated through a "proxy" measurement. Specifically, you measured the time and number of leaves that rose for leaves of different colors. You also measured the number of bubbles (of the same size? each time they formed, or did they persist?) for each leaf disc. Both of these measurements are approximations of what you are interested in - the amount of carbon dioxide taken up, and the amount of oxygen released, through the processes of photosynthesis. Something to keep in mind is that respiration of the mitochondria in plant cells occurs simultaneously - which releases carbon dioxide and takes up oxygen. It is likely the plants with the most photosynthesis will also have the most respiration - so more bubbles overall, even though we dont' know which gas the bubbles are made of!
I am really proud of you - this was great work, and a learning experience. You also are aware of the improvements you would take if you did this again, which is one of the main take-aways of an experiment, and proves how sciences is just a series of ongoing projects to get answers. I am very impressed.
Further, I want to emphasize the fact that learning about photosynthesis isn't just important because "science is cool" or maybe just something you do in class. Photosynthesis drives how much carbon dioxide is in the atmosphere everyday, both in your hometown, and around the world. If humans continue to rely on fossil fuels for energy as we have been doing for 150 years (coal, gas, oil, etc), we are putting more and more carbon dioxide in the atmosphere. We rely on plants (and algae in the ocean) to take up this carbon dioxide through photosynthesis, so that the atmosphere doesn't become so heavily full of carbon dioxide that is affects our health and systems (for examples of this look up Beijing and New Delhi). Knowing more and more about photosynthesis and how it works will help us plan for the future and know how much carbon dioxide plants absorb from the atmosphere. Similarly, photosynthesis is related to growth of plants, and as you know, most (all?) of our food originally comes from plants! Not just fruits and veggies, but corn, flour, soy, and plant products that feed poultry, cows (which then produce milk, etc), so keeping photosynthesis going regularly and understanding how it works is at the backbone of all our diet!!!
Below are the results and conclusion we have written. We would love to hear any feedback that you may have regarding how to make it better. Thanks for you time!
At first, it was thought that the rate of photosynthesis could be determined by timing how long it took for sunken leaves to rise in an aqueous solution. The results of three trials, however, contradicted this initial idea. In the first trial, which was run for only 26 minutes, purple and green leaves did not rise while 7 white leaves rose. In the second trial, which was run for 40 minutes, 6 purple leaves rose, 8 green leaves rose, and 5 white leaves rose. In the final trial, which was also run for 40 minutes, 0 purple leaves rose, 3 green leaves rose and 3 white leaves rose. Not only are the values from the three trials vastly different, they did not support the hypothesis. It was expected that green leaves would undergo more photosynthesis than the other leaf colors, but in trial 1 the white leaves rose much faster than the green leaves. However, a more accurate method of measuring photosynthesis in this experiment was found to be counting the number of air bubbles rather than counting the number of risen leaves. The green leaves from trial 2 collectively had 127 bubbles on them, while the white leaves only had 57 bubbles and the purple leaves only had 18 bubbles. The green leaves from trial 3 collectively had 586 bubbles on them, while the white leaves only had 10 bubbles and the purple leaves only had 4 bubbles. This data supports the hypothesis, and is a more accurate way of measuring the rate of photosynthesis. The original method of counting the floating leaf discs didn’t take into account the mass of the leaves, which may have been the reason why the white leaves rose so quickly and the purple leaves didn't.
Discussion and Conclusion:
The hypothesis was that because they contain more pigments, the green Tradescantia zebrina leaves would perform photosynthesis at a faster rate compared to the The amount of pigment was determined through the use of chromatography, which showed that the green leaves had the most pigments, the purple had the second most, and the white had the least amount of pigments. It was expected that all of the leaves would undergo photosynthesis, since they all displayed pigment. The rate of photosynthesis was measured by determining the amount of time it took, in minutes, for the leaf disk to rise in an aqueous bicarbonate solution due to the accumulation of oxygen bubbles. The original procedure was to count the number of leaves that rose in each color for about 40 minutes. Photosynthesis would be measured by observing the amount of leaf discs that rose and the time it took. The green leaves were expected to rise the fastest and in the shortest amount of time. This was not the case for trial 1, which had 0 green leaf discs after 26 minutes and 7 white discs rise. There was suspected error in trial one, such as the killing of the leaves when sinking them, so it was stopped after 25 minutes. After sterilizing all of the equipment and carefully sinking the leaves, trials 2 and 3 were conducted. The results of trial 2 were more true to the hypothesis, with 8 green leaves rising, 6 purple and 5 white. However, in trial 3 there was minimal rising of the leaves, with only 3 green risen, zero purple, and 3 white. It was unexpected that there was the same number of green and white leaves risen in trial three. Trial three, however, showed many air bubbles on the green leaves. It was then realized that the amount of photosynthesis could be measured by counting the air bubbles on each leaf color. The air bubbles proved that photosynthesis was occurring, because oxygen is a product of photosynthesis, and factors such as the thickness of the green leaves could have prevented them from rising. The white leaves were much thinner, which explains why they continued to rise in each trial. To get a better sense of the amount of photosynthesis occurring, the number of air bubbles on each leaf color was counted and an average was taken. The data shows that in both trials two and three, the green had the highest average of air bubbles, the white had the second highest, and the purple had the least. This better proves the hypothesis in that the green leaves would undergo the most photosynthesis, however it shows that the white underwent more than the purple.
The experiment could be improved by adding a heat sink under the lamps, to rule out heat as a factor affecting photosynthesis. The experiment could also be performed for a longer period of time, allowing all of the leaves to rise. The leaves could have been stored in a better environment instead of being on top of the air blower in the classroom that blows out cold air.
To further research this experiment, leaves from a plant that contains different colors within the same leaf could be used. This would control the thickness and quality of the leaf discs.
Good ideas. I think counting the number of bubbles was a smart thing to do at the time - you can consider this as a "proxy", meaning that you aren't measuring photosynthesis directly, but rather an associated measurable variable that you think is closely related to photosynthesis. I would express some caution though - since you don't know what gas is forming in the bubbles. It could be oxygen, or carbon dioxide - which represent two different pathways, not necessarily photosynthesis.
Another thing you could try graphing is the proportion of leaves that rose -not the number. So if 6/10 rose, you could graph 0.6 or 60%, or if 2/6 rose, you could graph 0.333 or 33%, etc.
Also, we only have pictures of the green leaves for trial one, so we would not be able to count the bubbles on the white and purple for that trial. Should we scrap the data for trial 1 and just use trial 2 abd 3 as our data?
We made three graphs ( one for each trial). The graphs, right now, are based on the number of leaves that rose. However, this is not displaying our results very nicely due to the fact that the leaves did not rise as expected. On trial three we noticed that the leaves were not rising, but had a lot of air bubbles. It occurred to us then that we could measure photosynthesis using the amount of bubbles on the leaves, especially because the leaves are all different thicknesses.
Should we make a bar graph showing the amount of bubbles that each color of leaf had? We would be able to count the bubbles using the pictures that we took
Great work on your trials - and sorry if you are disappointed they didn't go exactly as planned. As young scientists, you are getting some very important experience on methods-testing and troubleshooting if things go wrong. My husband is a graduate student in chemistry and just spent about 4-5 months on an experiment only to find out something was very off when he looked at the final values of a "standard" (meaning a control value). So he's spent a couple days just looking back over his methods, re-running things, etc, and will hopefully find the culprit! It's important to remember that everyone faces this, and honestly, it's a big part of the job is figuring out problems as much as getting answers straight away. That is why some people call Science the "Endless Frontier" - because everytime you get to a point, there are still many more questions to ask and many more items to resolve - there are few fully resolved topics in any field of science. It is a constantly changing scenery, with new challenges, new technologies, new questions - I think that is why I find it so interesting, and I hope you do too!
I think that the difference in leaf discs is very normal, so to speak. In science, trials are often done many times, and the mean and variance of the results are used to represent the outcome. For example, if you had 1 green leaf rise one trial, 4 the next, 3 the third trial, and 6 the final trial (out of the same number of starting discs, let's say, 10), then the average number of leaf discs that rose would be equal to: (1+3+4+6)/4 = 3.5 leaf discs out of those 4 trials. If you present this as a percent (as in this percent of green leaf discs rose out of the total submerged leaf discs), the average percent would be 35% for that same example. You can calculate these from your various trials and then compare, as they are likely to vary from one trial to the next. If you did this experiment 100 times, what would you anticipate being the most likely outcome? A lot of science is based on these types of calculations and inferences about data. As in, most of the time, green discs rose the most/the fastest, and we can't test EVERY single leaf in the world, so we can assume that based on these data, green leaves are likely to rise more/less than other leaf colors.
Does that make sense? Honestly, if you had the same outcome every single time, i would be very surprised! There is so much randomness in the world, to get the same number would almost be more surprising than to get variation.
These pictures are from our third trial that we performed today. We noticed the oxygen bubbles and our teacher thinks this is significant to our data. The leaves took much longer to rise today than yesterday. Today 0 purple leaves rose, and only 3 green and 3 white rose after 40 minutes. The white leaf discs looked very thin after the 40 minutes in solution. Do you think there are any explanations as to why all 3 trials had different number of leaf discs rise?
I will be posting more pictures of the experiment this weekend.
You guys look like total pros! Well done on the replication (both in the chromatography and the floating leaf disc assay)!
As you learned, often experiments do not go as expected (another reason to do them multiple times). Sometimes you know why things went wrong, and sometimes you are left to guess. THis unfortunately does not change with age or experience, and is an ongoing challenge of being a scientist!
Some thoughts on your experiment:
- Warmed temperatures often lead to higher rates of photosynthesis. generally 25˚C is a good temperature for rates, where 15˚C might be too cold and 35˚ might be too warm. This varies based on species and the local environment of the plant.
- The spinach leaves are generally VERY green, meaning they have more chlorophyll and chloroplasts - the pigment and organelles that drive carbon assimilation in leaves. I would guess spinach would have the fastest rates of photosynthesis based on this, and also knowing about spinach. Spinach is generally grown outdoors, and can grow quickly, while the other plant is mainly an indoor plant, used to low-light and thus slow growth - likely having lower rates of photosynthesis.
- The experiment itself may have something to do with the results as well. The leaf discs need to fully absorb the solution to get CO2, and there may be some differences in the leaf structure that limited its absorption. You mentioned the fuzzy outer layer - these are called trichomes and prevent water loss in drier environments, but they may also create a structural barrier to the leaves being fully wet when submerged. Just an idea.
- The chromatography results make sense to me - more green, in the leaf, more green pigment for photosynthesis. I think you are on the right track with the lack of sinking issue. Again, it's VERY common for things not to go as planned, so don't be discouraged!
Today in class we preformed our experiment 2 times. The first trial did not go as expected. No green or purple leaves rose to the surface, and 7 of the white leaf discs rose by the end of the trial. After running this first trial for 26 minutes, we decided to stop and try again.To prepare for the second trial, we hole punched new leaves, sterilized the materials, and made new water solutions. As we were preforming the second trial, one of the green discs rose from the first trial.We ran the second trial for 40 minutes because early on in the experiment at minute 14, one green leaf had risen. This gave us much more hope for the second trial. The first white leaf disc rose at minute 16 and the first purple leaf rose at minute 22. By the end of the 40 minutes, 8 out of 10 green leaves were risen, 6 out of 10 purple leaves, and 5 out of 10 white leaves. We will be running a third trial.
Compared to the spinach leaves, the Tradescantia zebrina leaves took much longer to rise. We think this might be because they are much thinner than the spinach leaves. Also, the plants were kept by the window and air blower in our classroom, which is a fairly cold environment. They were only 2 inches away from the light source in the experiment and did not have a heat sink. Do you think this change in temperature could damage the leaves and slow the rate of photosynthesis? The purple leaves had a fuzzy layer on them, and we thought this also might have had an effect on the rate of photosynthesis. Why do you think the Tradescantia zebrina leaves took so long to rise?
The results of the first trial were very confusing to us, because it contradicted our chromatography and hypothesis. We think the green and purple leaves might have been damaged during the process of sinking, and that the white were not completely sunk. Do you have any ideas on why our trial one results occurred?
These are our chromatography papers from yesterday. The labels are on the top. The labels are the same as last time.