Photosynthesis (Rosalyn Montgomery)

Research Question and Hypothesis

Does the color of the light effect the rate of photosynthesis on spinach disks?

Standards

Include the NGSS standards that this addresses.

Experimental Design

Spinach leaf disks float, normally. When the air spaces are infiltrated with solution the overall density of the leaf disk increases and the disk sinks. The infiltration solution includes a small amount of Sodium bicarbonate. Bicarbonate ion serves as the carbon source for photosynthesis. As photosynthesis proceeds oxygen is released into the interior of the leaf, which changes the buoyancy--causing the disks to rise. Since cellular respiration is taking place at the same time, consuming oxygen, the rate that the disks rise is an indirect measurement of the net rate of photosynthesis.

rovide a description of the experimental design

Independent variable

The independent variable is time. The students will use the timer on their phones to measure time intervals of one minute. What is the independent variable? How will you measure the independent variable?

Dependent variables

The dependent variable is the number of floating disks? Students will count the number of spinach disks that float in the solution

Series

The series to be tested will be water color. The leaves will be submerged in clear, red, green, and violet water. The students will test the number of floating disks over time in the different colored watersWhat series will you be testing. Example: Study the height of plants (dependent variable) as a function of time (independent variable) at various temperatures (series).

Constants and Controls

The amount of sodium bicarbonate solution and the concentration of dissolved sodium bicarbonate will remain constant. The clear water serves as the control.

Materials

Sodium bicarbonate (Baking soda)

Liquid Soap

Plastic syringe (10 cc or larger)—remove any needle

Leaf material

Hole punch

Plastic cups

Timer or smart phone

Light source

Procedures

Make a 0.1% bicarbonate solution by mixing 2 grams baking soda with 2000 mL water. Add a few drops of liquid dish soap to this solution and mix gently, trying to avoid making suds in the solution. Make 500 mL of each color red, green, violet by adding 5 drops of food coloring. Leave the remainder of the solution as clear.

Using the straw or hole punch, cut out 10 circles from your leaves.

Remove the plunger from the syringe, and remove the cover from the tip, if there is one. Put the leaf disks into the barrel of the syringe, and tap them down to the tip. If you have a straw, you can blow the discs gently into the plunger.

Replace the plunger into the syringe, being careful not to touch or damage the leaf disks.

Pour 50 mL of clear bicarbonate solution into the 50 ml beaker. Try to avoid making suds.

Draw about 6–8 mL of bicarbonate solution into the syringe. The leaf disks should float in the solution

Hold the syringe with the tip up, and expel the air by gently pushing on the plunger.

Plug the tip of the syringe tightly with your finger, and gently pull on the plunger, creating a slight vacuum. You should see tiny bubbles coming out of the leaf disks. Hold the vacuum for a few seconds, and then release the plunger, letting it snap back (see photos below). Some of the disks should begin to sink.

Repeat the previous step several times, until all of the disks have sunk to the bottom of the solution. (You may need to tap on the plunger to release the bubbles in order to make all the leaf disks sink.)

When all the leaf disks have settled to the bottom of the solution, carefully remove the plunger and pour the disks and solution into the 50 mL beaker. They should settle to the bottom of the cup (see photos below). If any leaf disks float, remove them from the beaker. Repeat these steps for each colored water

Go outside and place the beakers in sunny area.

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.

Sample data and graphs

Analysis & Conclusions

There was no significant difference in or relationship found between the rate of photosynthesis and the color of the water as expected. In the future, I will have students use colored cellophane instead of colored water to restrict the light wavelengths.

Photos & Movies

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References

Include links to all relevant references.

Exploratorium Link to Lab Procedures

floating leaf disk reference sheet