January 23, 2017

Egeria densa chloroplasts VS Arduino photoreceptor LDR.

Jeunes Scientifiques Cools - Julie, Simon, Clément
@JSC_Biosensors, @FdvJulie, @SimoonFR, @CaporalClement

Biological and electronic sensors:
How light intensity is perceived by Egeria densa chloroplasts and photoreceptor LDR ?

    To feed ourselves, we eat nutrients contained in food, and our body uses these different components to stay alive. Plants also need elements to grow and develop during time, and their main source of energy is… light.
Indeed,  with a process called photosynthesis, the light emitted by the sun is transformed by little organelles (a cell sub-unit) called chloroplasts, into chemical energy.
This chemical energy is stored by the plant to grow. Creating energy with chloroplasts release heat. Too much heat can be dangerous for the cell. When there is too much light, scientists have shown that chloroplasts move away from the light to decrease the production of chemical energy and heat. We call this phenomena light avoidance or negative phototaxis. We considered this movement as a response to light for the cell.
We want to compare this biological sensor with a mechanical sensors. In our researches, we confronted chloroplast characteristics and a light dependent resistor of arduino.


   

  First Candidate - the aquatic plant Egeria densa
Egeria densa - an aquatic plant, usually used to produce oxygen in aquarium, grows fast and as any plant has many chloroplasts ready to produce energy.
Our research aim is to quantify the movement of chloroplasts in cells of Egeria densa in function of the light intensity. Then, compare the time response and the range with a light dependent resistor with Arduino.
  To observe the leaf under the microscope and see the chloroplasts in it, we have to get only one layer of cell. We tried many techniques, but the better one is scratching the leaf with a scalpel. If you want to know more how to do a microscope slide of an aquatic plant and use a microscope, look this lesson, made by a biology teacher. The slides are sequentially reserve in a dark place during 20 minutes with aluminum.
In scientific research, it is important to do several times the same experiment. :  it is called replication.
So we observe six different slides. We tested three different light intensity : low, medium and high. (We used the microscope light). We expose the slide during 10 minutes, taking picture every minute. At the end we can compile all our pictures into a time lapse as shown here.

output_APwkL0.gif
   Figure 1 : This time lapse presents the observation of Egeria densa cell under microscope x63 at medium light during 10 min. We can see some chloroplasts slightly moving in the cell. But almost none of them move themselves.


Our study shows that most of chloroplasts didn’t move after 10 minutes of light exposure.
We imagine two hypothesis to explain this non-movement :
  • the time response of chloroplasts is very slow, so no movement was able to be seen in only 10 minutes.
  • The light was not enough intense to activate the movement mechanism.
In conclusion, chloroplasts seems to have a very long time response and a range of detection quite high.

   Second Candidate - the arduino device Light dependent resistor
Light Dependent Resistor (LDR) are electronic sensor which modifies the voltage in electric cable when exposed to light. The voltage variation is interpreted by an Arduino device and printed on a computer. Let’s look at our Arduino arrangement. 
We exposed the LDR to the same condition than the layer of cell : low, medium and high light intensity.
 We have shown in our results that the electrical response is extremely sensitive at very low light intensity. However, the LDR sensor seems to be saturated in high intensity of light.
It means that the sensor is dazzled and can’t quantify accurately the light intensity. In addition, we made two replicates of the measurement. We noticed that the two were quite different, up to 20% of variation.
LDR is sensible to low light but get saturated quickly. It is also not very precise, giving high variation.


Final verdict :
  After observations and data analysis, we can conclude several statements :
  • chloroplast reactions to light are much more unpredictable than we thought.
The light intensity doesn’t seem to increase significantly chloroplast movements. Other phenomenons occur inside on cells, particularly in aquatic plant ones which complicate the observations of chloroplasts.
  • LDR photo-receptor on the other hand is very sensitive on low light intensity, but saturated
quickly. But it is definitely more constant than chloroplasts if your only goal is to quantify light intensity changes.
Further experiments should be done to explore chloroplast potential. Look this video to see movement of chloroplasts in another aquatic plant Elodea canadensis.
But before them, we officially declare LDR photo-receptor winner of the tense competition, in which biology competed electronic.
We made a beautiful power point, if you need more information on the protocol we used to compare these two sensors. To go deeper on what we did, go on GitHub ! And finally, we tweeted all among our experiments, get a look on our storify.



Bibliography :
Videos
Scientific articles

  • Kasahara, Masahiro, Takatoshi Kagawa, Kazusato Oikawa, Noriyuki Suetsugu, Mitsue Miyao, and Masamitsu Wada. “Chloroplast Avoidance Movement Reduces Photodamage in Plants.” Nature 420, no. 6917 (December 19, 2002): 829–32. doi:10.1038/nature01213.




  • « Induction of Cytoplasmic Streaming and Movement of Chloroplast Induced by L-Histidine and its Derivatives in Leaves of Egeria densa ». Plant and Cell Physiology 32, no 2 (1 mars 1991): 253‑60. doi:10.1093/oxfordjournals.pcp.a078071.

You can see our other document in the Github of Biosensors : https://github.com/learningthruresearch/Biosensors2017
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