Experiment #5: Photosynthesis

[Pages:6]Experiment #5: Photosynthesis

Objective

Can we measure how fast plants work? In this experiment, we are going to measure the photosynthesis rate of a water plant under different intensities and colors (wavelengths) of light.

Theory

Photosynthesis Photosynthesis is the process by which plants, some bacteria, and some protistans use the energy from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by all living things. The conversion of sunlight energy into usable chemical energy, is associated with the actions of the green pigment chlorophyll.

Figure 1: The inputs and outputs of the photosynthetic process. We can write the overall reaction of photosynthesis as: 6H2O + 6CO2 Light C6H12O6+ 6O2 The above chemical equation can be written in words as: Water + Carbondioxide Light Sugar + Oxygen

The primary molecule involved in photosynthesis, chlorophyll, can absorb red and blue light. The energy that falls to the plant as sunlight is white light. It contains the mixture of all colors in the visible spectrum. When the leaf absorbs blue and red colors of light from the Sun, green light is reflected. This is the reason why plants appear green. Number of secondary molecules involved in photosynthesis tend to absorb blue light, so they appear red and orange. The colors of secondary molecules are masked as the amount of chlorophll molecule in plant leaves is much greater. However, during autumn, when the plant dies and the chlorophyll is no longer produced, the secondary colors can be seen. Photosynthesis and Respiration Some of the sugar produced during photosynthesis is used by the plant for its life processes (such as growing and reproducing); the excess is converted mainly to starch and stored in various plant parts which may be used as food by animals and humans. The reaction of respiration: C6H12O6 + 6O2 = 6CO2 + 6H2O + energy. Sugar + Oxygen Carbon dioxide + Water + Energy

Figure 2: The cycle of Photosynthesis and respiration

Oxygen produced during photosynthesis replenishes the oxygen that was used up by living things during respiration. This cycle of photosynthesis and respiration maintains the balance of carbon dioxide and oxygen on earth.

Rate of Photosynthesis The rate of photosynthesis means how fast photosynthesis takes place. This can be measured by the amount of glucose or oxygen produced by a plant over a given time. By understanding the factors that affect the rate of photosynthesis scientists can do work to try and increase the rate of photosynthesis to increase the yield of a crop.

The three main things affecting the rate of photosynthesis are, Light, Temperature and Carbon dioxide.

The rate of photosynthesis increases linearly with increasing light intensity. Gradually the rate falls of and at a certain light intensity the rate of photosynthesis stay constant. After that point, the rise in light intensity has no effect on the rate of photosynthesis as the other factors such as temperature and carbon dioxide become limiting.

Photosynthesis is a chemical reaction and the rate of most chemical reactions increases with temperature. However, for photosynthesis at temperatures above 40?C the rate slows down. This is because the enzymes involved in the chemical reactions of photosynthesis are temperature sensitive and destroyed at higher temperatures.

The rate of photosynthesis increases linearly with increasing carbon dioxide concentration. Gradually the rate falls of and at a certain carbon dioxide concentration the rate of photosynthesis stays constant. After that point, the rise in carbon dioxide levels has no effect on the rate of photosynthesis.

In our experiment, we are going to determine the rate of photosynthesis by measuring oxygen production of a water plant, elodea under bright light, different colors of light and dim light.

We are going to use two different method in measuring the oxygen production. The first method is to count the oxygen bubbles prodused in a given time. Second method is to measure the chance in the water volume in the capillary tube.

Safety Considerations:

Wear goggles Be cautious with glassware; report breakage to your lab instructor immediately! The spotlights and the metal shades become extremely hot and can cause serious burns

Equipment

-Elodea (green water plant) -250 ml beaker -Funnel -1 ml measuring pipette -Tubing - Syringe -Desk lamp -Red and green filters -Stopwatch

Figure 3: Experimental Set up

Procedure

1- Obtain a sprig of elodea. Remove several leaves from around the top end of the stem. 2- Cut off a piece of the stem near the top end at an angle and lightly crush around the cut. 3- Add about 100 ml water and 5 ml of baking soda to a beaker. 4- Place the sprig in the beaker, the cut side up. 5- Place a funnel on top of the plant in the beaker.

Make sure that all the plant is covered with the funnel and there is no air bubble left in the funnel. 6- There is a 1 ml pipette and tubing connected to the funnel. Connect a syringe to the open end of the tube. 7- Slowly increase the inner volume of the syringe. This will create less pressure in the tubing and the atmopheric pressure will push the water in the funnel up. 8- Let the water level rise to middle of the pipette. 9- Place a desk lamp next to the beaker.

The water in the beaker will help to absorb the heat from the light, helping to keep the temperature constant. 10- Turn on the desk lamp, the source of bright white light. 11- As soon as you see small bubbles coming from the cut end of the stem. Start timing the photosynthesis reaction for 4 minutes. If you do not see bubbles for two minutes, cut the stem again and recrush the plant. 12- One group member will observe the volume change in the pipette as with the increasing gas volume in the tubing, the water level will drop. 13- Read the initial volume in the pipette and record it in table 1. 14- Read final volume in the pipette after 4 minutes and record it in table 1. 15- One group member will count the number of oxygen bubbles given of by the plant for 4 minutes and record the result in table 2. 16- Place a red filer between the plant and the desk lamp for a source bright red light. Repeat steps 13-15 and record your results in table 1 and table 2. 17- Place a green filter. Repeat the steps 13-15 and record your results in table 1 and table 2. 18- Trun off the dek lamp. Repeat the steps 13-15 and record your results in table1 and table2.

Table 1

Bright white light Bright red light Bright green light Dim light

Initial volume in the pipette (ml)

Final volume in the pipette after 4 minutes (ml)

Change in volume in 4 minutes (ml)

Change in volume per minute (ml)

Table 2

Bright white light Bright red light Bright green light Dim light

Bubbles in 4 minutes

Bubbles per minute

Data Analysis

1- Calculate the oxygen production of the plant under white light, red light, green light and dim light in volume change per minute. Record your results in table 1. 2- Calculate the oxygen production of the plant under white light, red light, green light and dim light in bubbles per minute. Record your results in table 2. 3- On a graph paper, draw a graph to illustrate the effect of different light sources on the oxygen production of the plant.

Questions

1- Compare the two methods you used to measure the rate of photosynthesis? 2- How did the rate of photosynthesis change with the light intensity? Did the result agree with your prediction? 3- How did the photosynthesis rate change by changing the color of the incident light? Did the result agree with your prediction? 4- What are the variables in this experiment and what did you do to eliminate them?

Reference

1- Trefil, J., M. Hazen, R., Sciences, An Integrated Approach, 6. Ed, 2010, ISBN: 978-0-47050581-6

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