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Algae Balls: Investigating
Photosynthesis
Student-Centered Investigation
Once you’ve made your algae balls and know how to observe photosynthesis by monitoring changes in the pH of
the solution they’re in, it’s time for you to use your algae balls in a scientific experiment. This handout
introduces you to four different variables that might affect the rate of photosynthesis in your algae balls. Before
you leave today, decide on one variable that your group would like to investigate and fill out the Experimental
Design Worksheet (one per group). The teacher will read your experimental protocol and discuss any necessary
changes with you at the start of the next period. Then, you’ll get started on your experiment!
Variable A: What happens if I filter out certain wavelengths of light?
The light bulbs we’ve been using throughout this activity are
designed to imitate natural sunlight, which consists of a mix
of photons from across the entire visible spectrum, including
the blue and red wavelengths that chlorophyll absorbs most
strongly. Hence, our algae balls are able to photosynthesize
when we place them next to a light bulb. What do you think
would happen if you restricted the wavelengths of light
available to your algae balls?
https://reefledlights.com/wpcontent/uploads/2013/01/ChlorophyllAB.jpg
Your group should decide on one or two different colors of light that you would like to compare to the full
spectrum put out by your light bulb. Do you think that algae balls exposed to your chosen color of light will have
higher, lower, or the same photosynthetic rates as compared to ones that are exposed to the full spectrum?
Why?
Variable B: Do the algae care what type of light bulb is used?
http://mreverydaydollar.com/which-lightbulbs-to-buy-cfl-led-or-incandescent/
As discussed in the above section, the chlorophyll
in plants selectively absorbs photons in the blue
and red portions of the visible spectrum and
mostly reflects green photons, giving plants and
algae their green color when viewed under a fullspectrum source of illumination, such as the sun.
Until now, we have been using compact fluorescent lamps as the light source for our algae balls. You may be
interested in how other types of bulbs compare to CFLs. There is a staggering array of different lighting options
to choose from, and comparing between them can be tricky. Briefly, the two most important pieces of
information regarding the quality of light produced by a bulb are 1) brightness, measured in lumens, and 2) color
temperature, which describes how “warm” or “cool” the light looks. Look at the packaging of a few different
kinds of light bulbs. Which one do you expect to maximize photosynthetic output, and why?
Variable C: Do extra carbohydrates affect photosynthetic rates?
SAFETY NOTE!!!
Halogen and incandescent light bulbs can get very hot. If you choose to use these bulbs:
keep them away from flammable materials and let them cool down before you touch them
Cells have evolved many ways of restructuring their metabolism in response to the nutrients available in the
environment. For instance, E. coli only express genes encoding enzymes that catabolize—break down—lactose
when lactose is present and glucose is absent. In yeast, starvation signals to regulatory proteins that rapidly shut
down synthesis of new ribosomes. The organisms that budget energy wisely usually have a selective advantage.
We can extend the same principle to photosynthesis in Chlamydomonas reinhardtii. Glyceraldehyde-3phosphate is the fixed form of CO2 produced in the Calvin-Benson cycle. It has two fates: conversion to starch,
which is stored in the chloroplast, or into the disaccharide, sucrose, which is hydrolyzed in the cytoplasm to its
monosaccharides, glucose and fructose. How do you think photosynthesis would be affected if the algae were
directly supplied with one of the end products? Would they respond to varying concentrations? Do different
carbohydrates elicit different responses? How could you make a controlled comparison?
Variable D: Is there such a thing as too much light?
Depending on their natural habitat, different photosynthetic organisms can be adapted to particular light levels.
For example, there are different types of a genus of ocean-dwelling photosynthetic bacteria called
Prochlorococcus that are each adapted to life at a different depth in the ocean, where light levels decrease very
rapidly with depth. Strains that are adapted to life in shallow water and bright light grow more slowly than usual
when the lights are dimmed. Interestingly, the reverse is also true; Prochlorococcus strains that are adapted to
deeper waters grow more slowly when exposed to bright light.
Our algae, Chlamydomonas, can be found in a wide variety of
natural habitats including soil, ponds, puddles, and even snow
banks! In the lab, the algae are usually grown using everyday
fluorescent lamps set a foot or two away from the culture flask as
a light source for photosynthesis What do you think would happen
if you used a screen to reduce the amount of light reaching the
chloroplasts? Would reducing the light by half also reduce the rate
of photosynthesis by half? What do you think is the lowest light
level that would still support photosynthesis?
Available Materials
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algae balls
hydrocarbonate pH indicator
Vernier LabQuest
cuvettes
colored light filters
variety of light bulbs
rulers
paper cups
distilled water
test tubes
Parafilm
shade screens
glucose and sucrose solutions
scissors, tape, Sharpies