Photosynthesis In Isolated Chloroplasts
Suggestions For Teacher
Length of Lab: 2 (40-45) minute periods.
Safety Precautions: The lab involves working with chemicals. Goggles
should be worn at all times.
1. Remind students to fully insert the I/O cable in the CBL and
2. Timing is important in this lab. Students should organize themselves
to insure that they are able to take readings at the proper time.
3. Accuracy of measurements is also crucial. Go over proper pipetting
4. It is important to place a heat sink between the samples and
the light. A large flask, beaker or even a small fish tank filled
with water will work.
Answers To Questions
1) For each sample, describe the relationship between absorbance
and time? In which sample did the absorbance drop most dramatically?
The cuvettes with the boiled chloroplasts and with the chloroplasts
kept in the dark showed little change in absorbance. The chloroplasts
kept in the light showed a drop in absorbance.
2) What is the effect of darkness on the reduction of DPIP?
There is very little reduction of DPIP by chloroplasts kept
in the dark.
3) What is the effect of boiling the chloroplast on the reduction
There is very little reduction of DPIP by boiled chloroplasts.
4) What was the initial rate of DPIP reduction by the unboiled
chloroplasts that were incubated in the light?
At first there is a linear reduction (indicated by the negative
slope) of DPIP.
5) How did the rate vary during the experiment's time course?
The curve began to level off, indicating that the rate of
reduction of DPIP was tapering off.
6) There are two major products of photosynthesis-water and organic
substances such as glucose. How would the presence of DPIP affect
the production of oxygen during the light reaction? How would
the presence of DPIP affect the production of organic product
during the dark reaction? Explain.
Using an artificial electron acceptor will allow for the production
of oxygen during the course of the light reaction. However, without
the formation of NADPH2, the dark reaction will not occur and
organic product will not form.
7) What role did another artificial electron acceptor, ferricyanide
play in elucidating the process of photosynthesis?
The phenomena described in question #6 was the basis for separating
the light reaction from the dark reaction. Hill used ferricyanide,
however, as an artificial electron acceptor. He is credited with
isolating the light reaction.
8) If time permits, design and set up an experiment comparing
how the wavelength of light would affect reduction of DPIP. Analyze
the slopes and discuss your results.
In such an experiment several cuvettes are set up similarly
to the one kept in the light in the experiment the students just
did. However, the use lights of different wavelengths. The data
for each cuvette is plotted and compared.