Aerobic Respiration Vs

Name ______Date ______Period ______# ______

Aerobic Respiration 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 that has phenol red, a pH indicator, inside of it. 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 of Elodea is placed in the liquid, and allowed to photosynthesize 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 tubes white index card 1 piece of foil (to wrap tube) 1 straw 1 250 mL flask Goose neck lamp

Materials (per room): Elodea sprigs Phenol red (0.25 % solution) Transfer pipet 50 mL graduated cylinder

Procedure: 1. Place 50 mL of tap water into the flask 2. 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 and gently 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 ½ full of the now yellow solution 5. Add two sprigs of Elodea (about 2-3 inches in length) to each tube – making sure that it is fully submerged 6. 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), place both test tubes in the beaker of water (acts as a heat sink, so as not to “cook” your Elodea) 8. Shine the lamp onto the beaker of water (very close to the beaker) OR if it is sunny outside, place beakers in full sun outdoors or in the greenhouse. 9. Check test tubes every 10 minutes for at least 60 minutes and record the time when they begin to change color and when the color change is complete.* Name ______Date ______Period ______# ______

DATA: OBSERVED COLOR OF TUBES CONTAINING ELODEA SPRIGS

Light Condition Time (min) Dark (foil) Light 0 (before blowing into phenol red solution)

0 (after blowing into phenol red solution)

* Note – the 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 in step 2 above).

Conclusions: 1. What causes the color change from red to yellow when you blow bubbles into it?

2. Why did the yellow color progress back toward red?

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 photosynthesis was occurring?

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