When is the DNA replicated during meiosis?
As in mitosis, DNA is replicated during the S phase of the cell, between the G1 and the G2 phases of the cell. The only difference between mitosis and meiosis is that the cell that is dividing for mitosis is a regular somatic cell, while the cell used for meiosis is a specialized germ cell located in the sexual organs whose sole purpose is to grow and divide into 4 gametes.
Are homologous pairs of chromosomes exact copies of each other?
Homologous pairs of chromosomes are not exact copies of one another because one of the homologs is given to the cell from its father, and the other is from its mother. When DNA is replicated, the entire chromosomes are not replicated, only the sister chromatids from each of the parent's genes. There are many genetic variations between the chromosome belonging to the father and the chromosome belonging to the mother, and this in turn promotes genetic variation.
What is crossing over?
Crossing over is a process by which homologous pairs of chromosome form crossing points, called synapse, and exchange sets of DNA between each other. This means that the father's chromosome could trade its portion of DNA that codes for hair color, for example, with the mother's chromosomes gene for hair color. The resulting chromosomes are called recombinant chromosomes. This increases genetic variation because then it means that gametes can have random combinations of their parents DNA, not just be forced to have one or the other parents's genetic information.
What physical constraints control crossover frequency?
The physical constraint for crossing over are where the gene is on the chromosome. Because crossing over could lead to huge problems in a gametes DNA if a gene is spliced in half incorrectly and would therefore become defective, crossing over can only happen when two complete genes are switched. Therefore, only certain sections of the chromosome can swith, and if they do switch, the entire gene is moved.
What is meant by independent assortment?
Independent assortment is the process by which homologous chromosome pairs line up along the metaphase plate during meiosis I. When the chromosomes line up, they are not forced to have all of the maternal DNA on one side and all the paternal DNA on the other. Instead, because the homologs assort independently, there is a random mixture of paternal and maternal DNA on each side of the metaphase plate. Thus, in turn, when the homologs separate, a random mixture of DNA is put into each resulting gamete.
What happens if a homologous pair of chromosomes fails to separate, and how might this contribute to genetic disorders such as Down syndrome and cri du chat syndrome?
When a homologous pair of chromosomes fails to separate, the gametes resulting from that meiosis will either have one less or one extra chromosome. Without the correct number of chromosomes, if this gamete becomes a zygote, the child will face genetic disorders such as downs syndrome, which is having an extra chromosome 20, 21, or 22; or cri du chat syndrome, which is the deletion of the 5th chromosome. The child's DNA is messed up, and therefore their mental capabilities and many of their body's structures are abnormal.
How are mitosis and meiosis fundamentally different?
Mitosis and meiosis are fundamentally different in both their purpose and their products. For mitosis, the purpose is just to produce two identical, diploid cells to either increase the size of an organism as in a teenager going through puberty, or replace dead cells such as on a skinned knee after a bicycle accident. Meiosis, on the other hand, produces 4 haploid cells used for sexual reproduction. Each cell produced is different from the rest of the haploid cells produced because of processes that promote genetic diversity like crossing over and independent assortment.
Link to meiosis video:
https://www.youtube.com/watch?v=OZLBftmBsow
Tuesday, December 8, 2015
Sunday, December 6, 2015
Investigation 7: Part 2
Introduction
Cells, the building blocks of the body, are constantly dying and replacing themselves. Through a process called mitosis, one cell divides itself into two daughter cells that can then grow and divide as well. Though mitosis is the most lively stage of a cell's life cycle, it is also the smallest portion of that cell's life, with large phases of growth and DNA replication occurring alongside mitosis. Pictured below is a diagram representing a typical cell cycle.
As seen above, a cell's life is mostly growth and DNA replication that is punctuated by brief period of mitosis. So, this large time period, called Interphase, is just as important as mitosis, even though mitosis is where the action is at. Returning to the lab, Vikram, Vinay, Shreyan and I were tasked with the seemingly simple mission of counting cells in a growing root tip. In the root tip, because it is growing, there would be many cells performing mitosis and also quite a few in Interphase, as they are all over the organism's body. We had to find out how many cells were undergoing mitosis, and also we counted how many cells were in their Interphase stage.
Unfortunately for us, there isn't an exact indicator on when a cell is undergoing mitosis or not, so we had to use our eyes and make an educated guess on whether or not a certain cell was dividing or not. We also tried to compare the cells we saw under the microscope to diagrams such as the one below, which shows the different stages of mitosis so we could compare the two and determine if the cell in question was dividing or not.
As well as simply counting cells of a regular root tip for our control group, we also had to count cells of a root tip that had been specially treated as an experimental group. We were not told what the chemical was that the cells had been treated with, but we expected an increase in the number of cells undergoing mitosis because we believed that the chemical would mess with the cell cycle, but there was no explanation as to why we believed the rate of mitosis would increase.
Procedure
To begin, the group picked up two microscope slides from Mr. Wong, one regular, unlabeled slide, and a slide that had been treated with the unknown chemical and was labeled "T". Then, using the microscope, we took a picture of a certain part of each slide, and transferred the pictures to our iPads where we expanded the pictures and began labeling whether or not a cell was in mitosis. If the cell was undergoing mitosis, it was labeled "M", if it wasn't, it was labeled "I" for Interphase. After counting all of the cells that we could, we tallied up the number of cells in mitosis and not in mitosis, and then we shared our numbers with the class. The date is below.
Data/Pictures
Here is the table of all of the class's data from groups 1 to 6.
Here is a picture of the cells that we counted for our control group:
Here is the picture of the cells that we counted for our experimental group:
Conclusion
For our conclusion, rather than writing in the regular paragraph form, we were asked to answer the questions associated with our lab in our lab packet, and also some Postlab Review Questions posted on Canvas.
Investigation 7 Part 2
Collect the class data for each group, and calculate the mean and standard deviation for each group. Make a table. These are the observed values for the class.
Table 1
Use the data from Table 1 to calculate the totals using formulas found in Table 2
Table 2
Table 2 Completed
Use the totals from Table 2 to calculate the expected values (e) using the formulas from Table 3.
Table 3
'
Cells, the building blocks of the body, are constantly dying and replacing themselves. Through a process called mitosis, one cell divides itself into two daughter cells that can then grow and divide as well. Though mitosis is the most lively stage of a cell's life cycle, it is also the smallest portion of that cell's life, with large phases of growth and DNA replication occurring alongside mitosis. Pictured below is a diagram representing a typical cell cycle.
 |
| The Cell Cycle |
As seen above, a cell's life is mostly growth and DNA replication that is punctuated by brief period of mitosis. So, this large time period, called Interphase, is just as important as mitosis, even though mitosis is where the action is at. Returning to the lab, Vikram, Vinay, Shreyan and I were tasked with the seemingly simple mission of counting cells in a growing root tip. In the root tip, because it is growing, there would be many cells performing mitosis and also quite a few in Interphase, as they are all over the organism's body. We had to find out how many cells were undergoing mitosis, and also we counted how many cells were in their Interphase stage.
Unfortunately for us, there isn't an exact indicator on when a cell is undergoing mitosis or not, so we had to use our eyes and make an educated guess on whether or not a certain cell was dividing or not. We also tried to compare the cells we saw under the microscope to diagrams such as the one below, which shows the different stages of mitosis so we could compare the two and determine if the cell in question was dividing or not.
As well as simply counting cells of a regular root tip for our control group, we also had to count cells of a root tip that had been specially treated as an experimental group. We were not told what the chemical was that the cells had been treated with, but we expected an increase in the number of cells undergoing mitosis because we believed that the chemical would mess with the cell cycle, but there was no explanation as to why we believed the rate of mitosis would increase.
Procedure
To begin, the group picked up two microscope slides from Mr. Wong, one regular, unlabeled slide, and a slide that had been treated with the unknown chemical and was labeled "T". Then, using the microscope, we took a picture of a certain part of each slide, and transferred the pictures to our iPads where we expanded the pictures and began labeling whether or not a cell was in mitosis. If the cell was undergoing mitosis, it was labeled "M", if it wasn't, it was labeled "I" for Interphase. After counting all of the cells that we could, we tallied up the number of cells in mitosis and not in mitosis, and then we shared our numbers with the class. The date is below.
Data/Pictures
Here is the table of all of the class's data from groups 1 to 6.
|
|
Control
|
Experimental
|
||
|
Group
|
Interphase
|
Mitosis
|
Interphase
|
Mitosis
|
|
1
|
131
|
11
|
147
|
22
|
|
2
|
140
|
9
|
142
|
3
|
|
3
|
155
|
16
|
272
|
18
|
|
4
|
368
|
17
|
162
|
23
|
|
5
|
141
|
5
|
330
|
15
|
|
6
|
234
|
5
|
269
|
21
|
Here is a picture of the cells that we counted for our control group:
Here is the picture of the cells that we counted for our experimental group:
Conclusion
For our conclusion, rather than writing in the regular paragraph form, we were asked to answer the questions associated with our lab in our lab packet, and also some Postlab Review Questions posted on Canvas.
Investigation 7 Part 2
Collect the class data for each group, and calculate the mean and standard deviation for each group. Make a table. These are the observed values for the class.
Table 1
|
|
Control
|
Experimental
|
||
|
|
Interphase
|
Mitosis
|
Interphase
|
Mitosis
|
|
Mean
|
195
|
11
|
220
|
17
|
|
Standard Deviation
|
93
|
5.2
|
80
|
7.5
|
Use the data from Table 1 to calculate the totals using formulas found in Table 2
Table 2
|
|
Interphase
|
Mitosis
|
Total
|
|
Control
|
A
|
B
|
A + B
|
|
Treated
|
C
|
D
|
C + D
|
|
Total
|
A + C
|
B + D
|
A + B + C + D = N
|
Table 2 Completed
|
|
Interphase
|
Mitosis
|
Total
|
|
Control
|
195
|
11
|
206
|
|
Treated
|
220
|
17
|
237
|
|
Total
|
415
|
28
|
443
|
Use the totals from Table 2 to calculate the expected values (e) using the formulas from Table 3.
Table 3
|
|
Interphase
|
Mitosis
|
|
Control
|
[(A + B)(A + C)]/N
|
[(A + B)(B + D)]/N
|
|
Treated
|
[(C + D)(A + C)]/N
|
[(C + D)(B + D)]/N
|
Table 3 Completed
Enter the observed Values from Table 2 and the expected values from Table 3 for each group into Table 4. Calculate the chi-square value for the data by adding together the numbers in the right column.
Table 4
Chi-Square value = 0.473
Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
Postlab Review
What was the importance of collecting class data?
The importance of collecting class data was to create a large enough sample size to calculate a valid chi-square value. If the sample size used for this value was extremely small, as it would have been had the values used in calculations had only come from one lab group, then the chi-square value would be less significant. This is because the single lab group could have been a fluke and then the rest of the class could have gotten completely different sets of data.
Was there a significant difference between the groups?
There was not a significant difference between the group. Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
Did the fungal pathogen lectin increase the number of root tip cells in mitosis?
If this slides in this experiment were treated with lectin, I think we might have seen a change in the number of root tip cells in mitosis, but Mr. Wong revealed to us that the "treated" slides were not in fact treated with anything. Rather, Mr. Wong just labeled some control slides "T" and let our minds do the rest. During the experiment, as groups began reading off data, Mr. Wong actually told some groups, including mine, to recount their cells because their data was wildly off in that it showed a large increases in the number of cells in mitosis in the treated group as compared to the control. This is actually a phenomenon studied in psychology called observer bias, or the tendency of researchers to see data in a light that is favorable to their hypotheses. This is a common human error that the entire class made for this experiment.
What other experiments should you perform to verify your findings?
Other experiments that could be performed would be to look at more root tip cells or cells of a different plant and then counting cells and seeing if the chi-square value is different for those. Or we could actually use slides treated with something to determine the effects an environment can play on cells.
Does an increased number of cells in mitosis mean that these cells are dividing faster than the cells in the roots with a lower number of cells in mitosis?
An increased number of cells in mitosis does in fact mean that the cells are dividing at a faster rate than in roots with lower numbers of dividing cells because all cells are on their own specific timers of when to divide and when to not divide. The cells are not synced up at all, because that would be catastrophic if all of our cells divided at one time. Therefore, when the rate of all cells dividing speeds up, more cells are brought to mitosis around the same time, so during a snapshot look into the lives of the cells, as in the treated root tip slides, more cells would be undergoing mitosis than in a regular root tip.
What other way could you determine how fast the rate of mitosis is occurring in root tips?
Another way to measure the rate of mitosis in root tips is to measure the growth of the root tip as a whole. As cells divide and grow, more space is taken up and the tip of the root is pushed farther and farther along. So, as the tip expands, there are more cells performing mitosis.
|
|
Interphase
|
Mitosis
|
|
Control
|
192.98
|
13.02
|
|
Treated
|
222.02
|
14.98
|
Enter the observed Values from Table 2 and the expected values from Table 3 for each group into Table 4. Calculate the chi-square value for the data by adding together the numbers in the right column.
Table 4
|
Group
|
Observed (o)
|
Expected (e)
|
(o - e)
|
(o - e)2
|
(o – e)2/e
|
|
Control I
|
195
|
192.98
|
2.02
|
4.0804
|
0.021
|
|
Control M
|
11
|
13.02
|
-2.02
|
4.0804
|
0.167
|
|
Treated I
|
220
|
222.02
|
-2.02
|
4.0804
|
0.018
|
|
Treated M
|
17
|
14.98
|
2.02
|
4.0804
|
0.267
|
Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
Postlab Review
What was the importance of collecting class data?
The importance of collecting class data was to create a large enough sample size to calculate a valid chi-square value. If the sample size used for this value was extremely small, as it would have been had the values used in calculations had only come from one lab group, then the chi-square value would be less significant. This is because the single lab group could have been a fluke and then the rest of the class could have gotten completely different sets of data.
Was there a significant difference between the groups?
There was not a significant difference between the group. Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
If this slides in this experiment were treated with lectin, I think we might have seen a change in the number of root tip cells in mitosis, but Mr. Wong revealed to us that the "treated" slides were not in fact treated with anything. Rather, Mr. Wong just labeled some control slides "T" and let our minds do the rest. During the experiment, as groups began reading off data, Mr. Wong actually told some groups, including mine, to recount their cells because their data was wildly off in that it showed a large increases in the number of cells in mitosis in the treated group as compared to the control. This is actually a phenomenon studied in psychology called observer bias, or the tendency of researchers to see data in a light that is favorable to their hypotheses. This is a common human error that the entire class made for this experiment.
What other experiments should you perform to verify your findings?
Other experiments that could be performed would be to look at more root tip cells or cells of a different plant and then counting cells and seeing if the chi-square value is different for those. Or we could actually use slides treated with something to determine the effects an environment can play on cells.
Does an increased number of cells in mitosis mean that these cells are dividing faster than the cells in the roots with a lower number of cells in mitosis?
An increased number of cells in mitosis does in fact mean that the cells are dividing at a faster rate than in roots with lower numbers of dividing cells because all cells are on their own specific timers of when to divide and when to not divide. The cells are not synced up at all, because that would be catastrophic if all of our cells divided at one time. Therefore, when the rate of all cells dividing speeds up, more cells are brought to mitosis around the same time, so during a snapshot look into the lives of the cells, as in the treated root tip slides, more cells would be undergoing mitosis than in a regular root tip.
What other way could you determine how fast the rate of mitosis is occurring in root tips?
Another way to measure the rate of mitosis in root tips is to measure the growth of the root tip as a whole. As cells divide and grow, more space is taken up and the tip of the root is pushed farther and farther along. So, as the tip expands, there are more cells performing mitosis.
Monday, November 23, 2015
Mitosis: The Movie Blog Entry
- If a cell contains a set of duplicated chromosomes, does it contain any more genetic information than the cell before the chromosomes were duplicated?
I believe that the cell does not contain any more genetic information after DNA has been duplicated than the cell prior to the duplication because there is no more content to the DNA, only two exact copies of the same coding for genes. This can be compared to a class reading a paper. Though there will be many copies of the paper, the words on the page and information stored within that page is the exact same for everyone and does not increase or decrease in value depending on how many copies of it there are.
- What is the significance of the fact that chromosomes condense before they are moved?
The fact that chromosomes condense before they move is significant because it is the condensing of the chromosomes that makes the splitting of the sister chromatids so much easier. If the strands were lined up in the middle of the cell during metaphase as the long strings that they normally exist in, and then tried to condense, there would be nothing for the spindle fibers to grab onto and it would also impede the fibers' process.
- How are the chromosome copies, called sister chromatids, separated from each other?
Because the chromatids are linked by the centromere and brought to the middle of the cell by the spindle fibers, they are held tightly in place in the metaphase plate. When it is time to split, the enzyme separatase is activated, cutting the centromere bonds of the two chromatids and immediately enzymes within range of the spindle fibers on the chromosomes began to eat the fibers, thus drawing each separate chromatids closer to its respective ends.
- What would happen if the sister chromatids fail to separate?
IF the sister chromatids failed to separate, then one cell would have a chromosome more than normal, and the other would have one less than normal. Because they store the genetic material for life on these chromosomes, cells cannot function well without even a single chromosome. They will produce incorrect proteins and could present horrible defects for the entire organism, especially if the sex cell that created the organism had one more or one less chromosome. An example of this would be down syndrome in humans. For people with down syndrome, when their parents sex cells came together, one had an extra copy of chromosome 21. If one of the first 15 or so chromosomes in the human genome have an extra copy or are missing one of the air, the cell will die because their cells just cannot function with that inherent problem.
- What events could promote genetic variation during mitosis?
There are not many events that could promote genetic variation directly during mitosis because the cell has already replicated the DNA and encoded mutants, which is the primary cause for genetic variation. But during mitosis, the splitting and replication of mutant cells promotes genetic variation because then the mutation spreads through the body or to other tissues and if it it compatible and advantageous, could be kept in the organism.
- What problems could occur with a loss of cell cycle control?
The biggest problem that occurs with a loss of cell cycle control is cancer. With cancer, the cells never stop dividing and spreading, causing major harm to the host and using up a lot of the bodies resources. But even if the process went the other way and one's body couldn't get its cells to replicated to replace tissues lost due to damage or the cells coming to the end of their life span, then the body would die because eventually all the cells in the body would perish and there would be nothing to replace them with.
Link to my group's mitosis movie:
https://www.youtube.com/watch?v=iRTmGbMQ4A0&feature=youtu.be
Thursday, November 19, 2015
Investigation 5: Photosynthesis
Introduction
How does the oxygen that we breathe enter the air? And how does the carbon dioxide that we exhale get taken out of the air so that we do not suffocate on it? The answer to both of these questions is photosynthesis, a chemical process for creating energy and sugars within a plant. Photosynthesis takes place in the leaves of a plant, and within the leaves' cells, in the chloroplast. In the chloroplast are specialized molecules called chlorophyll that absorb light and this absorption excites an electron that then goes through a complex series of reactions to form NADPH, an electron carrier, and ATP, an energy source for the cell.
However, the chlorophyll only absorb a certain frequency of light, and it only absorbs light from the visible light spectrum, so colored light. The chlorophyll do not even absorb the entire spectrum of visible light, usually only absorbing one or two colors of light between a certain wavelength very well, and reflecting the rest. The plant has developed two different types of chlorophyll to account for that, chlorophyll a and chlorophyll b, as well separate compound called carotenoids.
On Friday, November 6th, Vikram, Vinay, Shreyan and I entered the lab again to learn more about photosynthesis. We decided to test the rate of the reactions of photosynthesis under different colored light so that we could see to what affect the absorption of light by the chlorophyll and carotenoids affected the overall reaction. We hypothesized that the rate of reaction would slow when under green light because the chlorophyll and the carotenoids have very low absorption rates in green light as seen above. Also, leaves reflect green light, as seen in their green color.
Our first problem is finding a method by which we could measure the rate of photosynthesis in a plant. Because it is a minuscule process, we couldn't observe it directly through a microscope, but instead would have to find evidence that it is occurring. Here is the chemical equation for photosynthesis:
To find the rate of photosynthesis, we could track the production of one of the products above, or the diminishing of one of the products. Carbon dioxide is a possibility, but would be difficult to measure in an open environment. Water is likewise hard to track, as well as glucose, because we cannot directly see the number of molecules. This leaves us with oxygen, which is also difficult to track individual molecules. Instead of doing that, however, we decided to use the buoyant properties of oxygen in water to measure the rate of reaction. Because as oxygen is produced in leaves during photosynthesis it is kept in the center of the leaf before being expelled through the stomata, the leave actually becomes quite airy. So we could put the leaves in water and watch them float to the top. But to photosynthesize, there must be a source of light, a source of H2O and a source of CO2. If the leaves were submerged, they would not have access to CO2 and would therefore not photosynthesize. Therefore we submerged the leaves in a bicarbonate solution, which would put CO2 into the water for the plant to grab and use for photosynthesis.
The problem with this plan was that the leaves already contained oxygen, so they would float on their own. The only way around this was to evacuate the oxygen from the leave by sucking it out using pressure. At a lower pressure, the oxygen will expand and fill the space, leaving the leaf for the most part and entering the surrounding space around the leaf. Doing this evacuation on a leaf would require larger syringes and tools than we have available so instead we decided to cut the leaves into small dots so they'd fit in the syringe and it would even make the submersion in water easier because then we would not have to find a large tank to submerge full leaves in.
For our experimental variable, which is the color of the light that the plant would be exposed to, we figured we could change this using different colored cellophane. The cellophane would absorb all of the light that wasn't its own color, and let that color through to hit the leaf-dots below. For our controls we performed an experiment with clear cellophane that would supposedly let all light through, but we also did an experiment without cellophane to see if the clear stuff did in fact absorb even a tiny amount of light.
Because of the time consuming nature of the reaction and its effects on the leaf-dots, and the limitations of our equipment, the lab group joined forces with a lab group consisting of Christos, Jorgos and Callen. One group would do three cups with different variables, and the other would do three different cups with different variables, and the data would be shared among us all. Vikram, Vinay, Shreyan and I did the experiments with blue cellophane, yellow cellophane and no cellophane at all.
With this plan of attack, we began our experiment.
Procedure
First, the lab group went outside to pick two fresh ivy leaves to perform our experiment with. We needed to get still living leaves so that they would perform photosynthesis. Next, using the apparatus shown below, I hole-punched 30 dots of leaf out of the leaves and split them into 3 piles. These would be later used for our experiment.
Next, the group created 300mL of a 0.2% bicarbonate solution to submerge the leaves in. Then we evenly separated the solution into three cups and cut three pieces of cellophane for the cups; one blue, one yellow and one clear. Then a drop of dilute soap was added to each cup. The soap acts as a wetting agent to force the leaf dots to allow water from the solution into the cell and sink. This also helps the leaves get their source of CO2 from the solution.
Next we evacuated the leaf dots of all air. To do this, we put 10 dots at a time into a syringe with a stopper and sucked up 10 mL of the bicarbonate solution into the syringe. Then we upended the syringe and squeezed the stopper so all of the air in the solution was evacuated. Once this was done, I put my finger on the opening of the syringe and pulled back on the stopper, effectively creating a vacuum in the syringe chamber, for 10 seconds. I then released the stopper and poured out the 10 leaf dots into one of the cups. This was repeated for each separate group of leaf dots.
Once the first set of leaf dots was evacuated, we immediately started our experiment so as to avoid sunlight causing the leaves to begin their photosynthesis while we evacuated the rest of the dots. This would've caused a skew in our data, but luckily we removed that confounding variable. The experiment consisted of a piece of colored (or clear) cellophane covering the cup and the cup was then placed under a lamp. The light from the lamp then caused the cells in the leaf dots to photosynthesize. We also started a separate timer for each experiment group so we could accurately keep time.
After all of the dots were in their cups and the cups were placed under the lamp, we observed the number of leaf dots that had risen completely to the top and recorded this data. After all the dots had risen, or 30 minutes, the experiment was stopped, and the dots were poured out.
Data/Tables
Here are the results of trials with the blue, yellow and lack of cellophane.
And a picture of the experiment in action:
Here is the data received from our lab group partners Jorgos, Christos and Callen, who experimented with green cellophane and a control cup of no cellophane.
Here is a graph of the results of both experiments combined:
Conclusion
Looking at the results of our experiment, I'd say that the lab was a relative success. As expected, when the plant was exposed to all types of light while under the clear cellophane, the rate of photosynthesis was fastest because all 10 dots floated to the top in the shortest amount of time when compared to single-color lights. However, a confusing piece of data is the extremely low number of dots, only 2, that floated to the top without any cellophane in Christos, Jorgos and Callen's experiment. Plants do not need cellophane to photosynthesize because cellophane is not found in nature, so that cannot be the reason why the dots didn't rise. I believe that there could have been some error with the evacuation of the cells because if one evacuates the cell too hard or for too long, the cell's chloroplasts will die and it will not photosynthesize. This could have happened for our compatriots. Other points of data, however, are quite normal. We did not expect the dots to photosynthesize much while under green light because leaves do not absorb much green light, if any. I was somewhat surprised at our data for blue light because I had thought that the rate of absorption hit a peak for chlorophyll when under blue light, but only 3 dots rose to the top of the water in that group. The results of the yellow group are also predictable because some of the peak absorption of leaves happens near red light, which is right next to yellow light on the visual light spectrum. Therefore, plants should also be able to absorb a lot of yellow light.
Due to the evacuation error mentioned above, as well as other fixes for the lab, the experiment could be improved in a variety of ways. First of all, all of the leaf dots should be evacuated together so that they all have the same level of evacuation when starting the experiment. This would also make sure that different groups using different syringes and applying different pressures wouldn't skew the data as it did for Jorgos' group. Another way in which the lab could be improved is, if the lab group had a long time to spend doing the experiment, we could use the same lamp and place the cup in the same spot each time so that we can ensure that the dots in the cup are receiving the same amount of light from the lamp and that they aren't missing out on light due to position.
I believe that this can be considered a success because we got data that we expected to get, our hypothesis was proved true, but also because we made mistakes. These mistakes helped us to learn ways in which we could improve our lab for next time as well as how to look out for confounding variables in coming labs we have not performed yet.
How does the oxygen that we breathe enter the air? And how does the carbon dioxide that we exhale get taken out of the air so that we do not suffocate on it? The answer to both of these questions is photosynthesis, a chemical process for creating energy and sugars within a plant. Photosynthesis takes place in the leaves of a plant, and within the leaves' cells, in the chloroplast. In the chloroplast are specialized molecules called chlorophyll that absorb light and this absorption excites an electron that then goes through a complex series of reactions to form NADPH, an electron carrier, and ATP, an energy source for the cell.
 |
| An Overview of Photosynthesis |
However, the chlorophyll only absorb a certain frequency of light, and it only absorbs light from the visible light spectrum, so colored light. The chlorophyll do not even absorb the entire spectrum of visible light, usually only absorbing one or two colors of light between a certain wavelength very well, and reflecting the rest. The plant has developed two different types of chlorophyll to account for that, chlorophyll a and chlorophyll b, as well separate compound called carotenoids.
 |
| Absorption Rates of Chlorophyll and Carotenoids |
On Friday, November 6th, Vikram, Vinay, Shreyan and I entered the lab again to learn more about photosynthesis. We decided to test the rate of the reactions of photosynthesis under different colored light so that we could see to what affect the absorption of light by the chlorophyll and carotenoids affected the overall reaction. We hypothesized that the rate of reaction would slow when under green light because the chlorophyll and the carotenoids have very low absorption rates in green light as seen above. Also, leaves reflect green light, as seen in their green color.
Our first problem is finding a method by which we could measure the rate of photosynthesis in a plant. Because it is a minuscule process, we couldn't observe it directly through a microscope, but instead would have to find evidence that it is occurring. Here is the chemical equation for photosynthesis:
To find the rate of photosynthesis, we could track the production of one of the products above, or the diminishing of one of the products. Carbon dioxide is a possibility, but would be difficult to measure in an open environment. Water is likewise hard to track, as well as glucose, because we cannot directly see the number of molecules. This leaves us with oxygen, which is also difficult to track individual molecules. Instead of doing that, however, we decided to use the buoyant properties of oxygen in water to measure the rate of reaction. Because as oxygen is produced in leaves during photosynthesis it is kept in the center of the leaf before being expelled through the stomata, the leave actually becomes quite airy. So we could put the leaves in water and watch them float to the top. But to photosynthesize, there must be a source of light, a source of H2O and a source of CO2. If the leaves were submerged, they would not have access to CO2 and would therefore not photosynthesize. Therefore we submerged the leaves in a bicarbonate solution, which would put CO2 into the water for the plant to grab and use for photosynthesis.
The problem with this plan was that the leaves already contained oxygen, so they would float on their own. The only way around this was to evacuate the oxygen from the leave by sucking it out using pressure. At a lower pressure, the oxygen will expand and fill the space, leaving the leaf for the most part and entering the surrounding space around the leaf. Doing this evacuation on a leaf would require larger syringes and tools than we have available so instead we decided to cut the leaves into small dots so they'd fit in the syringe and it would even make the submersion in water easier because then we would not have to find a large tank to submerge full leaves in.
 |
| As you can see, a leaf consists of cells but also quite a bit of empty space to hold gases like CO2 and O2 |
For our experimental variable, which is the color of the light that the plant would be exposed to, we figured we could change this using different colored cellophane. The cellophane would absorb all of the light that wasn't its own color, and let that color through to hit the leaf-dots below. For our controls we performed an experiment with clear cellophane that would supposedly let all light through, but we also did an experiment without cellophane to see if the clear stuff did in fact absorb even a tiny amount of light.
Because of the time consuming nature of the reaction and its effects on the leaf-dots, and the limitations of our equipment, the lab group joined forces with a lab group consisting of Christos, Jorgos and Callen. One group would do three cups with different variables, and the other would do three different cups with different variables, and the data would be shared among us all. Vikram, Vinay, Shreyan and I did the experiments with blue cellophane, yellow cellophane and no cellophane at all.
With this plan of attack, we began our experiment.
Procedure
First, the lab group went outside to pick two fresh ivy leaves to perform our experiment with. We needed to get still living leaves so that they would perform photosynthesis. Next, using the apparatus shown below, I hole-punched 30 dots of leaf out of the leaves and split them into 3 piles. These would be later used for our experiment.
Next, the group created 300mL of a 0.2% bicarbonate solution to submerge the leaves in. Then we evenly separated the solution into three cups and cut three pieces of cellophane for the cups; one blue, one yellow and one clear. Then a drop of dilute soap was added to each cup. The soap acts as a wetting agent to force the leaf dots to allow water from the solution into the cell and sink. This also helps the leaves get their source of CO2 from the solution.
Next we evacuated the leaf dots of all air. To do this, we put 10 dots at a time into a syringe with a stopper and sucked up 10 mL of the bicarbonate solution into the syringe. Then we upended the syringe and squeezed the stopper so all of the air in the solution was evacuated. Once this was done, I put my finger on the opening of the syringe and pulled back on the stopper, effectively creating a vacuum in the syringe chamber, for 10 seconds. I then released the stopper and poured out the 10 leaf dots into one of the cups. This was repeated for each separate group of leaf dots.
Once the first set of leaf dots was evacuated, we immediately started our experiment so as to avoid sunlight causing the leaves to begin their photosynthesis while we evacuated the rest of the dots. This would've caused a skew in our data, but luckily we removed that confounding variable. The experiment consisted of a piece of colored (or clear) cellophane covering the cup and the cup was then placed under a lamp. The light from the lamp then caused the cells in the leaf dots to photosynthesize. We also started a separate timer for each experiment group so we could accurately keep time.
After all of the dots were in their cups and the cups were placed under the lamp, we observed the number of leaf dots that had risen completely to the top and recorded this data. After all the dots had risen, or 30 minutes, the experiment was stopped, and the dots were poured out.
Data/Tables
Here are the results of trials with the blue, yellow and lack of cellophane.
|
Time (min)
|
# of dots risen (clear)
|
# of dots risen (blue)
|
# of dots risen (yellow)
|
|
1
|
0
|
0
|
0
|
|
2
|
0
|
0
|
0
|
|
3
|
0
|
0
|
0
|
|
4
|
0
|
0
|
0
|
|
5
|
0
|
0
|
0
|
|
6
|
0
|
0
|
0
|
|
7
|
0
|
0
|
0
|
|
8
|
0
|
0
|
0
|
|
9
|
0
|
0
|
0
|
|
10
|
0
|
0
|
0
|
|
11
|
0
|
0
|
0
|
|
12
|
0
|
0
|
0
|
|
13
|
0
|
0
|
0
|
|
14
|
0
|
0
|
1
|
|
15
|
0
|
0
|
2
|
|
16
|
0
|
0
|
4
|
|
17
|
0
|
0
|
5
|
|
18
|
2
|
0
|
5
|
|
19
|
3
|
0
|
7
|
|
20
|
6
|
0
|
7
|
|
21
|
8
|
0
|
7
|
|
22
|
10
|
0
|
7
|
|
23
|
-
|
0
|
8
|
|
24
|
-
|
0
|
8
|
|
25
|
-
|
0
|
9
|
|
26
|
-
|
1
|
9
|
|
27
|
-
|
1
|
9
|
|
28
|
-
|
1
|
10
|
|
29
|
-
|
2
|
-
|
|
30
|
-
|
3
|
-
|
And a picture of the experiment in action:
Here is the data received from our lab group partners Jorgos, Christos and Callen, who experimented with green cellophane and a control cup of no cellophane.
|
Time (min)
|
# of dots risen (no cellophane)
|
# of dots risen (green)
|
|
1
|
0
|
0
|
|
2
|
0
|
0
|
|
3
|
0
|
0
|
|
4
|
0
|
0
|
|
5
|
0
|
0
|
|
6
|
0
|
0
|
|
7
|
0
|
0
|
|
8
|
0
|
0
|
|
9
|
0
|
0
|
|
10
|
0
|
0
|
|
11
|
0
|
0
|
|
12
|
0
|
0
|
|
13
|
0
|
0
|
|
14
|
0
|
0
|
|
15
|
0
|
0
|
|
16
|
0
|
0
|
|
17
|
1
|
0
|
|
18
|
1
|
0
|
|
19
|
1
|
0
|
|
20
|
1
|
0
|
|
21
|
1
|
0
|
|
22
|
1
|
0
|
|
23
|
1
|
0
|
|
24
|
1
|
0
|
|
25
|
1
|
0
|
|
26
|
1
|
0
|
|
27
|
2
|
0
|
|
28
|
2
|
0
|
|
29
|
2
|
0
|
|
30
|
2
|
0
|
Here is a graph of the results of both experiments combined:
Conclusion
Looking at the results of our experiment, I'd say that the lab was a relative success. As expected, when the plant was exposed to all types of light while under the clear cellophane, the rate of photosynthesis was fastest because all 10 dots floated to the top in the shortest amount of time when compared to single-color lights. However, a confusing piece of data is the extremely low number of dots, only 2, that floated to the top without any cellophane in Christos, Jorgos and Callen's experiment. Plants do not need cellophane to photosynthesize because cellophane is not found in nature, so that cannot be the reason why the dots didn't rise. I believe that there could have been some error with the evacuation of the cells because if one evacuates the cell too hard or for too long, the cell's chloroplasts will die and it will not photosynthesize. This could have happened for our compatriots. Other points of data, however, are quite normal. We did not expect the dots to photosynthesize much while under green light because leaves do not absorb much green light, if any. I was somewhat surprised at our data for blue light because I had thought that the rate of absorption hit a peak for chlorophyll when under blue light, but only 3 dots rose to the top of the water in that group. The results of the yellow group are also predictable because some of the peak absorption of leaves happens near red light, which is right next to yellow light on the visual light spectrum. Therefore, plants should also be able to absorb a lot of yellow light.
Due to the evacuation error mentioned above, as well as other fixes for the lab, the experiment could be improved in a variety of ways. First of all, all of the leaf dots should be evacuated together so that they all have the same level of evacuation when starting the experiment. This would also make sure that different groups using different syringes and applying different pressures wouldn't skew the data as it did for Jorgos' group. Another way in which the lab could be improved is, if the lab group had a long time to spend doing the experiment, we could use the same lamp and place the cup in the same spot each time so that we can ensure that the dots in the cup are receiving the same amount of light from the lamp and that they aren't missing out on light due to position.
I believe that this can be considered a success because we got data that we expected to get, our hypothesis was proved true, but also because we made mistakes. These mistakes helped us to learn ways in which we could improve our lab for next time as well as how to look out for confounding variables in coming labs we have not performed yet.
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