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Experiment 2 -- Aerobic Respir ation vs. Photosynthesis:

The purpose of this experiment is to demonstrate the link between photosynthesis and respiration. This

experiment involves breathing into a container of water and a phenol red, a pH indicator. Phenol red

turns yellow in the presence of an acid; when CO2 from exhaled breath goes into solution, it produces an

acid (carbonic acid), thus lowering the pH. A sprig ofElodea is placed in the liquid, and allowed tophotosynthesize and thus take up the CO2. This experiment will examine the effect of light conditions

on the rate at which the Elodea takes up the CO2.

Materials (per group):

400 ml beaker of water

2 large test tubeswhite index card

1 piece of foil (to wrap tube)

1 straw

1 250 ml flaskGoose neck lamp

Materials (per room):

Elodea sprigsPhenol red (0.25 % solution = 0.25 g in 100 ml distilled water)

Transfer pipet for the phenol red

50 ml Graduated cylinder

Procedure:

1. Place 50 ml of tap water into the flask2. Add 1 ml of phenol red solution to the water in the flask. Note the color of the solution.3. Place the straw in the flask andgently blow bubbles in the solution. STOP as soon as the

solution turns yellow/orange! (Phenol red stains if it gets on clothes.) Record this color

in the data table.

4. Fill each test tube full of the now yellow solution5. Add two sprigs ofElodea (about 2-3 inches in length) to each tubemaking sure that it

is fully submerged6. Wrap one test tube with foil and only quickly remove and replace this foil when making

subsequent observations.

7. To control all variables other than the test variable (presence/absence of light), placeboth test tubes in the beaker of water (acts as a heat sink, so as not to cook yourElodea)

8. Shine the lamp onto the beaker of water (very close to the beaker) OR if it is sunnyoutside, place beakers in full sun outdoors or in the greenhouse.

9. Check test tubes every 15 minutes for at least 60 minutes and record the time when theybegin to change color and when the color change is complete.*

OBSERVED COLOR OF TUBES CONTAINING ELODEA SPRIGS

Light Condition

Time (min) Dark (foil) Light

0 (before blowing into

phenol red solution)

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0 (after blowing into

phenol red solution)

15

30

45

60

* Notethe phenol red will begin to turn back to wine or grape red color over time. Swirl the test

tube and place back in the beaker periodically to make sure the reaction is complete and the indicator

turns completely red (same color red as it was when you started [step 2 above]).

Questions:

1. What causes the color change from red to yellow when you blow bubbles into it (be specific)?

2. Why did the yellow color to progress back toward red (be specific)?

3. Explain the source of observed differences between your two test tubes.

4. In addition to the observed color change, what other indication do you have that photosynthesiswas occurring?

5. Why are photosynthesis in autotrophs and respiration in heterotrophs considered to worktogether as a cycle?