Elodea Lab

Elodea Lab

Photosynthesis

Background

Plants are autotrophs meaning they make their own food. The food they produce is glucose, C6H12O6.

In order to make their food, plants carry out a process known as photosynthesis. During this process, plants use energy from the sun, water, and carbon dioxide to build glucose molecules. Oxygen is also released from this reaction. Chemically, photosynthesis can be written as:

6CO2 + 6H20 + light energy C6H12O6 + 6O2

In this lab, you will determine how quickly photosynthetic reactions use carbon dioxide. You can measure the carbon dioxide concentration in water environment by measuring pH. Recall that a substance with a pH of 7 is neutral, under 7 is acidic, and over 7 is basic.

When carbon dioxide is dissolved in water, it produces carbonic acid, H2CO3. The presence of carbonic acid causes the pH of a water environment to become more acidic. (lowers the pH)

Bromothymol blue is an indicator solution for acids and bases. As you just read, as carbon dioxide dissolves in the water, carbonic acid is formed. This will alter the pH of the solution to make it acidic. Bromothymol blue turns yellows in an acidic solution.

Safety

1. Bromothymol blue can stain clothing. Use caution when handling this solution. 2. Do not suck any of the liquid into your mouth in step 2 of the procedure. 3. Use caution when handling the lamp. In particular, the bulb will be hot!

Materials

- sprig of Elodea - water - light source - ruler - test tube - beaker

- 1 flask - stop watch/clock - drinking straw - pH probe - bromothymol blue - dropper

Procedure

1. Put 100 mL of water into a beaker. 2. Put the drinking straw into the beaker. Blow gently through the straw into the

water for 3 minutes. This process will add a gas to the water. Throw away the straw. 3. Use a dropper to add 2.5 mL of bromothymol blue to the water. Swirl the beaker to mix. 4. Place a sprig of Elodea, cut end up, into a test tube. 5. Fill the test tube 2/3 full with the solution in beaker. 6. Fill a flask 2/3 full with water from the sink. 7. Use the ruler to position the bulb of your lamp exactly 12 inches away from the flask. 8. Put your test tube into the flask. 9. Carefully insert the pH probe into the test tube. While doing this, do not damage the Elodea sprig. 10.Take the initial pH reading and record in your data table. 11.For the next 30 minutes, take the pH every 5 minutes and record in your data table.

* Your teacher will have a similar set up on a lab counter. The difference is that his/her beaker will not have had a person blowing into it with a straw. As you are making your observations, you also need to make observations on his/her set up. You will record these results in the Data Table B.

Pre- Lab Questions

1. What is the purpose of this lab? 2. What type of gas was blown into the beaker? 3. In this lab, how is the carbonic acid being formed? 4. Bromothymol blue is an indicator solution. In this lab what does it indicate? 5. If the plant uses the carbon dioxide in the water, what will happen to the

amount of carbonic acid in the water? 6. How might the color of the bromothymol blue solution change if the Elodea

undergoes photosynthesis? 7. When you place the Elodea into the test tube, should the cut side be at the

bottom of the tube or at the top of the tube? 8. Why is it necessary to put water in the beaker? 9. What is the purpose of your teacher's set up? 10. Use the procedure to draw a diagram of what your experiment will look like.

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