This worksheet is intended to supplement two aspects of the photosynthesis lab, which due to time
constraints will be performed as demonstrations. The lab objectives and experimental details are briefly
summarized and hypothetical results are given. Analyze the results, then answer the review questions.
Experiment I. The wavelengths of light for photosynthesis (above diagram is on p. 181 in text)
Objective: to determine which wavelengths of light contribute to photosynthesis
Methods: Healthy geranium plants were used which had a number of broad, green leaves. Filters were
prepared that had the property of reflecting AND transmitting light, e.g., the green filter looks green
because it reflects green light, but it also TRANSMITS green light only.
Red, green and blue filters are used. Strips of black construction paper block all light. The rest of the leaf is
left exposed to full light. The entire plant, with strips in place, is exposed to normal cycles of strong light
and darkness for at least 5 days before the experiment, and is exposed to strong light for 5-6 hours
immediately before the experiment.
A representative leaf is removed for analysis. The leaf is sketched so that the positions of the different
strips of filter or paper are recorded. Then the strips are removed and the leaf is boiled in alcohol until it is
bleached almost white. The leaf is then placed in a shallow dish and flooded with a dilute alcohol solution.
Blue-black granules indicate the presence of starch. The results are recorded on the following table. From
those results, complete the table.
Why do we look for starch as an indicator of photosynthesis?
(+) indicates starch; increasing number of (+) indicates more starch; (-) indicates no starch
Type of filter/paper Amount of starch Wavelengths of light Does this wavelength of
present transmitted by this filter light support
(refer to diagram on photosynthesis? (yes or
previous page) no)
Answer the following questions:
1. Why does the black construction paper appear black? (i.e., which wavelengths of light does it
2. Why does the leaf appear green?
3. Why does the greatest amount of starch production appear in the uncovered area? Phrase your
answer in terms of the wavelengths of light to which it is exposed.
4. Why was it important to expose the plant to light immediately before the experiment?
5. What are the wavelengths of light that can be absorbed by human retinal (the pigments in human
6. Bees can see ultraviolet light but not red light. What are the wavelengths of light that can be
absorbed by the pigments in their eyes?
Experiment/demonstration 2: Pigments in photosynthesis
A variety of pigments are found in plants. Many pigments absorb some wavelengths of light and reflect
others; yellow pigments, for example reflect yellow light but absorb blue and green. Plant leaves may
contain several pigments; for this experiment, a variegated species of Coleus is preferred, whose leaves will
contain regions of green, purple, pink and white coloration. Some of the colors we see are actually
produced by the cumulative effect of several pigments.
Green colors are produced by the presence of chlorophylls a and b, which are photosynthetic pigments
found in the chloroplasts. Carotenoids, which produce yellow, orange and bright red colors, are also found
in the chloroplasts. In addition, vacuoles may include anthocyanins, water-soluble pigments that produce
blue, violet, purple, dark red and pink colors. These do not contribute to photosynthesis. The Coleus leaf
may have regions where each color is individually seen, and will likely have deep purple regions, where all
of the pigments are combined.
In this experiment, a Coleus plant is exposed to strong light for several hours before the experiment. The
different colors are recorded in a table and the leaf sketched. Then the leaf is boiled in alcohol as before and
the starch test performed. Complete the following table.
Color Pigments present Starch present (+ or -)
Deep purple +
Answer the following questions:
1. Which pigment(s) appear to be required for photosynthesis?
2. If you were to boil the leaf in water instead of alcohol which pigments would be removed?
3. What is the role of carotenoids in photosynthesis (if they are in the chloroplasts they must do
4. If anthocyaniins “do not contribute to photosynthesis” what do they do? (hint: in what other parts
of the plant are pigments found?)
5. If you were to combine ALL pigments what color would you perceive?