Designing an experiment to test the rate of photosynthesis

John S Olson
Dist. 88 Schools, New Ulm, MN 56073
based on "The floating leaf disk assay for Investigating photosynthesis"
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This activity will allow students to measure the rate at which the photosynthesis process occurs. Students will work in small groups to design an experiment with one independent variable and test this variable on spinach leaf disks. The punched out leaf disks will initially sink in a test tube of water but will float as photosynthesis occurs. Students will write a lab report including, hypothesis, experimental design, data collection / analysis, and conclusion (findings). Students will speculate on further investigations that could be done and discuss how the rate at which photosynthesis occurs has vast implications for human survival on the planet.

Learning Goals

This activity is designed for students to:
1) learn that the rate of photosynthesis is influenced by environmental factors that can be quantified
2) understand the equation of photosynthesis and how the structure of a leaf allows for the required gas exchange to occur through the stomata
3) properly design an experiment with one variable, analyze results and report findings to the class
Students will use higher order thinking skills throughout this activity. They will need to use critical thinking to design the experiment and analyze the collected data. They will improve their observing, questioning and communicating skills. The students will need to determine if the question they have chosen to test, is indeed testable. The teacher will guide students to problem solve as they design and modify the experiment to insure only one independent variable is being tested during the experiment.

The students will learn specific concepts during the activity. The students will learn that:
1) the variables of light intensity, temperature, carbon dioxide, light quality all influence the rate of photosynthesis.
2) photosynthesis requires carbon dioxide to furnish the carbon required to make glucose. This carbon dioxide will be supplied by sodium bicarbonate dissolved in the water with the plant leaf disks
3) the stomata opening is controlled by the guard cells and that water, carbon dioxide and oxygen all pass through the stoma openings
4) food production for animals and humans on earth is dependent on plants. By increasing the use of carbon dioxide by plants, it may help slow global climate change. In turn, climate change (warmer in some regions, cooler in other regions) may affect various species of plants differently.




-Guard cells

-Wave lengths of light

-Global climate change

-Carbon sequestration

-Independent variable

-Dependent variable

-Qualitative data

-Quantitative data

Context for Use

This 2-3 hour lab activity could be used in high school or college. Ideally, a teacher would have no more than twenty four students to coach through this activity. A student will have prior knowledge as to the equation describing the photosynthetic process as well as having a basic understanding of the scientific method. The lab could be used in a unit on, the scientific method, a photosynthetic unit or in a unit on environmental problems (global climate change and its affect on plants).

Description and Teaching Materials

Teacher and students will review the equation describing photosynthesis. What types of organisms undergo photosynthesis? Discussion will include the ideas that not all plants photosynthesize at the same rate. Brainstorming will bring out possible reasons and variables that would influence photosynthesis. This could include but would not be limited to; plant species, habitat, altitude, depth in water, temperature, carbon dioxide available, water available (desert, tundra, grassland), pH of soil / water

Prepared microscope slides of leaf cross sections will be observed/ drawn. Leaf structures will be named and discussion will relate these structures to the processes of photosynthesis that occur in each. Additional slides of the leaves of a desert plant, water lily, shade plants could be shown/used for comparison and discussion of plant adaptations.

The teacher would now discuss/ review the process of the scientific method. This would be the time to give examples of how some questions scientists raise are not testable. Students should be thinking about the question they will raise about the photosynthesis experiment and if an investigation can be designed to test their question. The teacher can now review what a hypothesis is and how to write a specific, testable hypothesis.

Basic Materials needed for lab group of 3-4 students:
-Spinach leaves
-4 mm core borer (used to cut holes in cork stoppers, can purchase ones of various sizes, use one about size of paper punch) or paper punch (see web source)

-Syringe (plastic 10+ cc)
-.2 % sodium bicarbonate (baking soda) solution
-Test tube or plastic cup
-Light source
-Prepared microscope slides of leaf cross sections; plant in sun, plant in shade, desert plant, water lily

Possible extra materials needed depending on variable chosen to test:
-Various wattage's of light bulbs
-Various types of light bulbs (fluorescent, incandescent)
-Colored cellophane (yellow, green, red, blue are typical colors one can purchase)(to change wavelength's of light the plants receive)

-Meter stick
-Baking soda
-Different species of plant (other than spinach)
-Weak acids / weak bases to change the pH of the solution
-Other materials as students design experiments would some substances inhibit photosynthesis? (act as weed killers?)

The teacher will now demonstrate to students the basic technique that will be used to measure the rate of photosynthesis. Students can follow along with the directions found in their handout / lab report (see experimental design, steps 1-5 in sample lab report within the assessment section) The main focus of the investigation depends on students knowing how to make the spinach leaf disks and make them sink in water. All students will be guided to use this technique . They can now practice this technique (see experimental design and the following web site

discuss the process of choosing a variable for the experiment. Pick a testable question to answer when doing the experiment. Give students time to design experiment and do a trial run. Teacher will help coach the students as they finalize / modify the experimental design to include a testable question with only one variable.
The final experimental design will be written out using diagrams /drawings to illustrate
the setup.
Students can now plan / design an appropriate data table . (include proper units of measure, titles, all labeling)
Finally, students will complete the experiment, analyze and discuss findings, and report findings to class. Class discussion will connect the lab to world issues; food availability, global climate change.

Teaching Notes and Tips

1) Use fresh spinach leaves, soak them in water overnight at about 4 degrees C. This
increases turgor pressure and minimizes "limp" leaves. Use only the firm, dark green areas of the leaf. Avoid major veins or damaged areas.

2) Do not over vacuum the leaf disks. Too little vacuum treatment causes the disks not to sink, too much vacuum treatment may kill the cells.


Two rubrics will be used to assess student lab report (sample of report included below) and student participation.
A written exam will be used to determine if a student has met the goals of the activity.

Example of student report (some areas are condensed down here to save space)
Student name ____________
Rate of Photosynthesis Research

Background: Where in a leaf does photosynthesis mainly occur? How does carbon dioxide get into a leaf? Where / how does oxygen leave a leaf? How does water get into a leaf from the roots?

Obtain a prepared slide of a leaf cross section (x-section). Using 100x make a sketch of
what you see.

Use the text book or internet to label the following tissues;
Upper epidermis, lower epidermis, palisade mesophyll, spongy mesophyll,
vein (label both xylem and phloem), guard cells, stoma

1) In which of the labeled structures, does most of the photosynthesis occur? (hint;
there are more chloroplasts here)

2) Through what structure does carbon dioxide get into the leaves so photosynthesis can occur?

3) What is the function of the guard cells?

4) Of what purpose does the spongy mesophyll serve to the leaf and the process of photosynthesis?

5) Through which of the labeled structures does water get to the leaves from the roots?

6) Through which of the labeled structures are sugars, that are made during photosynthesis,
transported to other parts of the plant where they can be used for energy or stored?

7) discuss the variations / adaptations that desert plants, water plants, and plants that grow
well in shade have in their leaves that allow them to survive in their particular environments.

Write out a balanced equation for photosynthesis:

Experimental Design
Your table will design an experiment to test how a selected variable affects the rate of photosynthesis. Follow the information below to make "sinking plant disks". You will measure how long it takes (in seconds) for the disks to float as a way to measure the rate of photosynthesis.

Preparation of the leaf disks:
1) Use the cork borer (to cut out the number of disks needed for your experiment).
2) Put disks in a syringe and suck up 5 cc (5 ml) of .2% sodium bicarbonate (baking soda)
3) Put finger over end of syringe, pull back on plunger to about the 35 cc mark (on a 60 cc syringe) and hold this position for 30 seconds. You should see air coming out the sides of the disks. As this is done, the oxygen is being removed from the spongy layer of the leaf and the .2% sodium bicarbonate is entering the spongy layer. This is the source of carbon dioxide needed for the plant to carry out photosynthesis
4) Carefully squirt out the .2% sodium bicarbonate. Suck up about 10 cc's of water. Check to see if the plant disks sink in the water. If they don't, remove the water and try steps 2 and 3 again.
5) Choose the disks that sink. Make sure enough disks are available to properly complete a controlled experiment. They are now ready to be used in your experimental set up. The disks will float when they have produced a measured amount of oxygen through photosynthesis. The time needed for the disks to float is an indirect measure
of the rate of photosynthesis occurring in the leaf disks.

Descriptive title of experiment:

The effect of ________________________________________________ on
the rate of photosynthesis.

Purpose / Introduction

Hypothesis; (use if/ then format)

Explain the logic of the stated hypothesis

Sketch of the experimental design used. (the sketch should be specific enough so that the experiment could be reproduced exactly as it was set up; include all measurements, angles, label materials / solutions used, wattage and type of light bulbs, etc.)

Is the data collected qualitative or quantitative data? discuss

The independent variable (manipulated) variable in the experiment is


The dependent variable (responding) in the experiment is


How was the experiment controlled?

Data chart: (you design, label and fill in with data)(you must have enough data to make a graph)

Graph of data (obtain a piece of graph paper, make appropriate graph that has a title and is properly labeled, attach graph to this lab report)

Results / discussion / analysis of data:

Findings/Conclusion/ list possible sources of error

Application to World environmental issues: (list ideas generated during brainstorming session)

List new questions that could be researched


I History and Nature of Science; Scientific Inquiry
Design and conduct a scientific investigation
1) use scientific methods
2) qualitative and quantitative data
3) sources of error
IV.A.5 Processes of photosynthesis in terms of energy flow, reactants, products

References and Resources