PHOTOSYNTHESIS & CELLULAR RESPIRATION



♫ THE CYCLE OF LIFE! ♪

The purpose of this lab is to gain a better understanding of what Elton John was singing about in the song, “The Cycle of Life” . . . jk! ( Actually we’re looking at the relationship between photosynthesis and cellular respiration. We will use plants to observe both pathways because they carry out both cellular respiration and photosynthesis when conditions are favorable. Both processes can be monitored by observing the accumulation or disappearance of carbon dioxide. The indicator, bromothymol blue, is a pH indicator . . . a color change occurs in response to a pH change.

Pre-Lab:

• In your lab notebook, record the title of this lab.

• Construct a data table similar to the one shown. Be sure to allow enough room to record a general description of the appearance of both tubes on Monday.

| | |Day 1 |Day 2 |

|Test Tube |Set-Up | | |

| |Color |Interpretation

(CO2 + or -) |Color |Interpretation

(CO2 + or -) |Observations | |A |Elodea stem | | | | | | |B |Radish seeds | | | | | | |

Materials:

Elodea plant radish seeds

2 small test tubes with cap light source

straw bromothymol blue

scissors marker

ruler glass rod

Procedure:

Day 1 - Friday

1. Use a small piece of masking tape to label each test tube with your period number - table number. Label one test tube A & the other,

B. Place the piece of tape on the cap of the tube.

2. Add 8 mls of tap water to each test tube.

3. Add 4 drops of bromothymol blue indicator to each test tube. Mix contents. Record the color of the water in the data table provided.

This is the color when carbon dioxide is not present, so this is considered a negative test reaction.

4. Set up the test tubes according to the directions below.

• Test Tube A – Light Incubation:

o Obtain a straw, submerge it in the water, and gently blow for approximately 2 minutes. Be careful not to inhale and draw water up through the straw! You may take a breath; just remove your mouth from the straw so you don’t suck in the water! A color change should appear. If no color change is observed, continue blowing. Record the color change on your worksheet.

o Measure an approximately 2 cm piece of the Elodea stem with leaves, cut with scissors, and place the plant in the water.

o Use the glass rod provided to submerge the plant completely.

o Place the cap on the tube.

• Test Tube B – Dark Incubation:

o Obtain 10 radish seeds and a small cotton ball.

o Place the cotton ball in the tube so it is touching the water and damp, but not submerged in the water.

o Drop the radish seeds onto the cotton ball.

o Place the cap on the tube.

5. Place tubes in appropriate class test tube racks to incubate.

Day 4 - Monday

1. Record observations in both tubes.

2. Discard Elodea, seeds, and cotton ball in trash.

3. Clean tubes with test brush and soap.

Analysis: Please answer in complete sentences.

1. Compare the appearance of test tube A on Day 1 to the appearance of test tube A on Day 5. Explain your results.

2. Compare the appearance of test tube B on Day 1 to the appearance of test tube B on Day 5. Explain your results.

3. Which pathway (cellular respiration or photosynthesis) is exergonic? Endergonic? Which is catabolic? Anabolic? Include an explanation of these

terms.

4. In the equation for cellular respiration, what reactant is reduced? Oxidized? In the equation for photosynthesis, what reactant is reduced? Oxidized?

5. Explain why a seed does not carry out photosynthesis. What must it rely on for ATP production? How is this possible?

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