Photosynthesis Lab (Design only)
Research Question:
How does light intensity affect the rate of photosynthesis on the pondweed Elodea canadensis?
Hypothesis:
The rate of photosynthesis of the Elodea canadensis will increase, as light intensity increases.
Background Information:
Photosynthesis is the process that plants use to convert available chemicals into compounds used as food. It’s the most important endothermic reaction on earth for life to exist. Although photosynthesis consists of a series of reactions, this is the overall equation:
6CO2 + 6H2O + Energy à C6H12O6 + 6O2
Carbon dioxide from the air is absorbed within the air spaces in the leaf, having entered through the stoma, tiny pores in the leaf. Water is taken in by the roots by osmosis, and light is absorbed by the green pigment in the leaves known as chlorophyll. The carbon dioxide and the water are converted to glucose, which is used by the plant in various forms or stored as starch, while oxygen gas is released through the stoma.
Photosynthesis occurs in two stages; the light dependent reaction and the light independent reaction. The light dependent reaction uses the direct energy of the light absorbed to create “energy carrier molecules” which are then used to form covalent bonds of carbohydrates (glucose) in the light independent reaction. Both of these reactions occur in the chloroplasts in the leaves.
In the light dependent reaction, the energy from light produces ATP and NADPH through a series of reactions of electrons, an electron transport process, and water is split into H+ and O- ions. The oxygen is released as a gas, and the ATP, NADPH, and hydrogen are used in the light independent reaction, along with the carbon dioxide.
The light independent reaction do not depend as much on light, although it is thought that the enzymes needed to trigger them are triggered by light. The H+ ions combine in a series of carbon fixing reactions with the CO2 in the cells of the leaf, powered by the energy compounds, ATP and NADPH. This forms PGAL, and then two of these molecules can combine to form glucose.
In order to investigate the effect of light intensity on the rate of photosynthesis, the pondweed, Elodea canadensis was used. This is a plant with green leaves that grows in fresh water and reproduces asexually. The rate of photosynthesis will be able to be measured by calculating the amount of gas given off by the plant at different light intensities. This plant is ideal because it has a large surface area to absorb light due to its many leaves. Also, since it photosynthesizes underwater, it will be possible to count the number of bubbles of gas that are released.
Light intensity is one of the abiotic factors of photosynthesis as the light energy absorbed by the plant fuels the process. Within this investigation, the rate of photosynthesis of the Elodea canadensis will be measured at various light intensities. Also, the rate of photosynthesis will not be able to be measured as a unit; however, the results will be used to show how it varies. Also, light intensity will not be able to be measured in units, but the rate of photosynthesis will be measured against the distance between a lamp and the plant. Thus, the rate of photosynthesis can be plotted against light intensity by using the following formula: light intensity = 1/d2. With these results, it will be easier to establish how light intensity affects the rate of photosynthesis.
Works Cited:
BBC News. BBC, n.d. Web. 05 Nov. 2012. .
“Photosynthesis in Action.” Photosynthesis in Action. N.p., n.d. Web. 05 Nov. 2012. .
Factors:
Structural factors:
- · Leaf surface area
- · Chlorophyll concentration
- · Stomatal number and size
- · Leaf thickness
The investigation only looks at external factors, thus these factors can be held constant by using the same piece of Elodea canadensis throughout the experiment.
External factors:
- · Light intensity
- · Temperature
- · Carbon dioxide concentration
The investigation is only concerned with light intensity; therefore the two other factors will try to be held constant. Temperature will try to be held constant by placing the test tube with the elodea in a beaker full of water which would minimize temperature change. Carbon dioxide concentration will try to be held constant by using 50% carbonated water. Also, the same water will be used for each reading to keep the CO2 concentration constant. Although the plant will use some of the carbon dioxide in the water in photosynthesis, so there will be a slightly lower concentration for later readings, it should not affect the results as there will be an excess of CO2. 50% of carbonated water should be high enough to supply the elodea with CO2 but low enough that it will still be possible to count the number of bubbles. The light intensity will not be completely accurate, as there will definitely be some light present in the laboratory. But to minimize this, the experiment will be conducted with the lights off and the curtains and doors closed.
Independent variable: light intensity
Dependent variable: rate of photosynthesis
The rate of photosynthesis will take a few minutes to adjust to the new environment, because the process of photosynthesis consists of multiple stages, and would even continue in the dark. For this reason, at each level of light intensity, wait at least 3 minutes before beginning to measure the rate of photosynthesis.
Methods:
- 1. Use a piece of elodea with the stem sliced obliquely and place it upside down in a test tube that contains 50% carbonated water and 50% percent tap water. Use a paperclip to hold it firmly in place. The reason for this is because the leaves have a greater number of stomata on the underside of their leaves, so gas will be released more readily.
- 2. Place the test tube in a beaker of water to help minimize temperature change. Also place a thermometer in the beaker to record the temperature for each trial in case temperature change occurs.
- 3. Turn off the room lights and cut off any other major light source, so that the light intensity is entirely controlled. Place lamp at the following distances: 1 cm, 10 cm, 20 cm, 30 cm, and 40 cm. Turn on the lamp, and leave it on for 3 minutes before starting to record data.
- 4. Carefully count the number of bubbles of oxygen released in one minute at each distance, timed carefully. Repeat this step 5 times at each distance in order to have 5 trials to average. This will help improve the accuracy of the results. Record all data in the table down below.
Table 1: Rate of Photosynthesis – Bubbles per minute at various light intensities
Distance d (cm)
Bubbles per minute
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5
Average Bubbles per minute
Temperature
(degrees C)
Light Intensity
(1/d2)
1
1
1
2
10
0.01
3
20
0.0025
4
30
0.00111
5
40
0.000625
