1. Background

In this lab, you will use phenolred as an indicator to show whether CO2 is being consumed or produced in a reaction within plants.

It is well known that in the presenceof light plants perform photosynthesis. It is less well understood that at thesame time plants are also performing cell respiration. Todemonstrate this we will determine whether CO2 is consumed or produced as elodea(anacharis) is placed in either a light or dark environment. The change in CO2 will be detectedby the pH indicator phenol red.

Think about the chemical equationsfor respiration and photosynthesis. Which one releases CO2 causing an increaseand which one uses up CO2which will cause a decrease?

Phenol red is yellow under acidicconditions (high H ion concentration), pink under basic or alkaline conditions(low H ion concentration) and orange under neutral conditions. Achange in CO2 will cause a directly proportional change in H ion.

If the CO2 concentrationdecreases, the H ion concentration will also decrease and the solution willchange to pink, becoming basic.If the CO2 concentrationincreases, the H ion concentration will also increase and the solution willchange to yellow, becoming acidic. Neutral solutions of phenolred will be orange.

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2. Preparation

Create a solution of phenol red byadding concentrated phenol red to about 100 ml of water. The phenol red maychange color as a result of adding water (depending on how acidic your tap wateris). Your goal is to make your solution a neutral orange color. You can do thisby gently blowing into the solution with a straw. Once you have the solutionat an orange color, transfer it to 4 test tubes (they should be filled about2/3 full with your orange solution).

Place a cut piece of elodea (cutend up) into two of the four tubes. The other two test tubes will not have elodeaand will serve as controls. One set will be placed in the light, and one setin the dark (see data table).

Aluminum foil can be used to sealthe test tubes to be set in the dark. All test tubes should be stoppered orcovered with parafilm to minimize reactions with the air.

3. Data

Record the colorsof the solutions in the testtubes after 1-2 days.

Elodea+ PR/ dark Elodea+ PR / light PR / dark(control) PR / light(control)

4. Analysis

Answer the experimental question (Do Plants Consume or Release CO2? ).


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Explain what happened in this lab, use complete sentences and be sure to provide supporting data or statements.