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Photosynthetic organisms capture energy from the sun and matter from the air to make the food we eat, while also producing the oxygen we breathe. In this Snack, oxygen produced during photosynthesis makes leaf bits float like bubbles in water.
Turn on the light, start a timer, and watch the leaf disks at the bottom of the cup. Notice any tiny bubbles forming around the edges and bottoms of the disks. After several minutes, the disks should begin floating to the top of the solution. Record the number of floating disks every minute, until all the disks are floating.
How long does it take for the first disk to float? How long does it take for half the disks to float? All the disks?
When all the disks have floated, try putting the cup in a dark cabinet or room, or cover the cup with aluminum foil. Check the cup after about fifteen minutes. What happens to the disks?
Plants occupy a fundamental part of the food chain and the carbon cycle due to their ability to carry out photosynthesis, the biochemical process of capturing and storing energy from the sun and matter from the air. At any given point in this experiment, the number of floating leaf disks is an indirect measurement of the net rate of photosynthesis.
In photosynthesis, plants use energy from the sun, water, and carbon dioxide (CO2) from the air to store carbon and energy in the form of glucose molecules. Oxygen gas (O2) is a byproduct of this reaction. Oxygen production by photosynthetic organisms explains why earth has an oxygen-rich atmosphere.
The equation for photosynthesis can be written as follows:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
In the leaf-disk assay, all of the components necessary for photosynthesis are present. The light source provides light energy, the solution provides water, and sodium bicarbonate provides dissolved CO2.
Plant material will generally float in water. This is because leaves have air in the spaces between cells, which helps them collect CO2 gas from their environment to use in photosynthesis. When you apply a gentle vacuum to the leaf disks in solution, this air is forced out and replaced with solution, causing the leaves to sink.
When you see tiny bubbles forming on the leaf disks during this experiment, you’re actually observing the net production of O2 gas as a byproduct of photosynthesis. Accumulation of O2 on the disks causes them to float. The rate of production of O2 can be affected by the intensity of the light source, but there is a maximum rate after which more light energy will not increase photosynthesis.
To use the energy stored by photosynthesis, plants (like all other organisms with mitochondria) use the process of respiration, which is basically the reverse of photosynthesis. In respiration, glucose is broken down to produce energy that can be used by the cell, a reaction that uses O2 and produces CO2 as a byproduct. Because the leaf disks are living plant material that still require energy, they are simultaneously using O2 gas during respiration and producing O2 gas during photosynthesis. Therefore, the bubbles of O2 that you see represent the net products of photosynthesis, minus the O2 used by respiration.
When you put floating leaf disks in the dark, they will eventually sink. Without light energy, no photosynthesis will occur, so no more O2 gas will be produced. However, respiration continues in the dark, so the disks will use the accumulated O2 gas. They will also produce CO2 gas during respiration, but CO2 dissolves into the surrounding water much more easily than O2 gas does and isn’t trapped in the interstitial spaces.
Try changing other factors that might affect photosynthesis and see what happens. How long does it take for the disks to float under different conditions? For example, you can compare the effects of different types of light sources—lower- or higher-wattage incandescent, fluorescent, or LED bulbs. You can change the temperature of the solution by placing the beaker in an ice bath or a larger container of hot water. You can increase or decrease the concentration of sodium bicarbonate in the solution, or eliminate it entirely. You can try to identify the range of wavelengths of light used in photosynthesis by wrapping and covering the beaker with colored gel filters that remove certain wavelengths.
This experiment is extremely amenable to manipulations, making it possible for students to design investigations that will quantify the effects of different variables on the rate of photosynthesis. It is helpful to have students familiar with the basic protocol prior to changing the experimental conditions.
Ask your students to think carefully about how to isolate one variable at a time. It is important to hold certain parts of the experimental setup constant—for example, the distance from the light source to the beaker, the type of light bulb used, the temperature of the solution, the height of the solution, and so on. Certain treatments may eliminate photosynthesis altogether—water with no bicarbonate, very low temperature, and total darkness.
A typical way to collect data in this assay is to record the number of disks floating at regular one-minute time intervals. This is easily graphed, with time on the x-axis and number of floaters on the y-axis.
To make comparisons between treatments, the number traditionally used is the time point at which half of the disks in the sample were floating, also known as the E50.
This experiment was originally described in Steucek, Guy L., Robert J. Hill, and Class/Summer 1982. 1985. “Photosynthesis I: An Assay Utilizing Leaf Disks.” The American Biology Teacher, 47(2): 96–99.
Even plants have their favorite colors.
Compare the brightness of two light sources with an oil spot on a white card.
As René Descartes (almost) said, "I sink, therefore I am."
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Attribution: Exploratorium Teacher Institute