effect light intensity cyclosis elodea leaf cells

Research Question: What is the effect of light intensity on the speed of movement of chloroplast (rate of cyclosis) in Elodea?

* Independent variable; Light intensity (Lux)

* Dependent variable; time taken for a chloroplast to move a measured distance. (s)

* Controlled variables; temperature (oC), volume of water (mm3), using same leaf, CO2 (ppm.)

Hypothesis: As light intensity exposed on an Elodea cell increases, rate of cyclosis inside the cell increases.

Apparatus used:

* Glass slides to view specimen

* Elodea leaf (fresh from plant)

* Microscope

* Stopwatch

* 3 lamps

Procedures:

One leaf was taken from an Elodea plant and examined under a light microscope at a magnification of around 400x. A cell was then located which performed cyclosis, and was exposed to the light produced by 1 lamp. 5 recordings were taken of the time taken for one chloroplast to move from one end of the cell to the other. This procedure was repeated with 2 lamps and 3 lamps giving 15 readings. The cell length was then measured by finding the length being viewed under the magnification and divided by the number of cells that fit into that length.

Interpretation:

The graph shows a trend where the rate of cyclosis increases (generally) as light intensity increases (i.e. when increasing the number of lamps) because graph lines of ‘speed with 3 lamps’ or speed of chloroplast with light from 3 lamps is higher than with 2 lamps or 1, and the speed of chloroplast is greater with 2 lamps than 1 lamp because its line is above the line for 1 lamp.

Errors:

Random errors: As seen in the graph, the 5th measurement of ‘speed with 3 lamps’ was an extreme value due to a large ‘pileup of chloroplasts inside the cell leading to a very long time taken for the chloroplast to move the required distance. In addition, the temperature at which the experiment took place was not controlled, water supply was not monitored and the CO2 concentration was not taken into account.

Human errors: The 12th reading of 1 lamp may have human attention span/concentration error since the time measured was much shorter than the average time taken by chloroplast (i.e.) by mistaking another chloroplast for the original while measuring, leading to a different measurement. Breathing onto the specimen alters the CO2 concentration and so alters the result. Uneven number of recordings were made due to bad time management.

Systematic errors: A systematic error in the experiment came in the controlled variable, since the brightness of each lamp is not exactly the same and not equal amounts of light of each lamp reaches the sample. The instrument used to measure the length of a cell was also not accurate because, at high magnification the division lines are very thick and so it is impossible to accurately measure the size of the cell.

Improvements:

This investigation can be made to give more accurate results by controlling temperature, water supply, controlling CO2 concentration and by using the same leaf cell for all measurements. To control temperature, the experiment can be carried out in a closed laboratory with no air circulation. Temperature can be measured periodically to make sure temperature stays constant. Water supply can be controlled by keeping a drop of water next to the specimen and using blotting paper to suck water into the slide. CO2 concentration can be controlled by not breathing onto the specimen and keeping it in a closed container. Using the same leaf avoids differences in measurements due to different chloroplast concentration or size of the cell.

Conclusion:

Through this experiment one can conclude that the rate of cyclosis increases as light intensity increases. The graph formed by the collected data shows how the speed at which chloroplast moves in increasing light intensities increases. The data thereby supports the hypothesis as chloroplast moved at an average of 6.36um/s while being exposed to the light of one lamp but 8.96um/s with 3 lamps. Though there were some extreme values, they can be accounted for by the errors explained earlier.

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