Experiment 2 – Light Independent Reaction
After the light dependent reaction, the light independent takesplace. During this phase, also known as the Calvin Cycle, theenergy generated in the light independent phase is used to fixatethe carbon atoms from carbon dioxide and glucose is produced.
In this experiment you will use baby spinach leaves todemonstrate the utilization of carbon dioxide during the lightindependent reaction of photosynthesis. Phenol red is an organicdye that undergoes a color change according to pH . The color ofthis compound will appear as follows:
Because phenol red can be used to detect changes in pH, it canalso be used as an indirect method of detecting changes in theamount of CO2 dissolved in a solution. When CO2 is added to asolution, some of it reacts with water to produce an acid calledcarbonic acid. In turn, some of the carbonic acid dissociates toincrease the [H+] in the solution. Therefore, if theamount of CO2 in a solution is increased, the [H+]increases (pH is lowered) and the solution becomes more acidic. Italso follows that if the amount of CO2 in a solution is lowered(e.g., by removal of CO2 from the solution), the [H+]will decrease (pH increases) and the solution becomes more basic.The chemical reactions involved are shown below.
To detect the process of carbon fixation (CO2 reduction), youwill use this pH indicator to detect changes in the CO2 level.
Materials
Fresh spinach leaves
2 glass test tubes
2 - 250 ml beakers
Phenol red solution
Permanent marker or wax pencil
Water *
Sunshine or bright light source *
Stopwatch/timer
Metric ruler
Procedure
Add 20 ml of phenol red solution to 2 glass test tubes.
With your straw , blow bubbles over the top of the solutionuntil it turns yellow. The color change occurs when the water inthe phenol red solution and the CO2 are combined forming carbonicacid . Make sure that both tubes are the same shade of yellow.
Add a small leaf of baby spinach to tube 2 only
Place the 2 test tubes into sunshine or a bright light at timefor 2 hours.
Record the color every ½ hour.
Table 2: Change is Color (phenol red) |
| 0 time | ½ hour | 1 hours | 1 ½ hours | 2 hours |
Tube 1 | | | | | |
Tube 2 | | | | | |
Questions
What is the independent variable in this experiment?
What is the dependent variable in this experiment?
What happened to the color of the solution over time?
What is the process that is occurring to make the phenol redsolution change colors?
What is the function of the following components in the processof photosynthesis (this may differ than how this compounds wereused in the experiment)
CO2:
Light:
H2O:
O2:
Chloroplasts:
