Lesson 1
Carbon Cycle - Photosynthesis (Lesson + Lab)


In addition to producing carbon dioxide through cellular respiration, organisms also need oxygen. This, through the process of photosynthesis, is the by-product of the producers (plants and algae) fixing (trapping) carbon dioxide into sugar.

The chemical equation for photosynthesis is the opposite of cellular respiration, thus showing how carbon moves from one organism to another to the atmosphere thus completing the carbon cycle.

Begin/Anticipatory Set/Launch:

Have students hold their breaths again.

QUESTION: Why does your body react the way it does when you hold your breath?

- need fresh air/too much carbon dioxide/need oxygen

QUESTION: Where does the oxygen come from?

- lead discussion until the answer, “from plants” is reached.

How do plants produce oxygen?

Plants produce oxygen as a by-product of making their own food through a process called PHOTOSYNTHESIS.

PHOTO = Light


= To Make using Light

Plants use the energy in light to make their own food by taking water and carbon dioxide and combining them into sugar. While this is happening, the water is actually broken apart to release the oxygen into the air.

This can be written in words or as a chemical equation.

The energy of light is used to combine water and carbon dioxide in order to make sugar and oxygen.


(Hopefully the students will see that this is the opposite reaction of cellular respiration).

QUESTION: How does this equation and the equation from cellular respiration relate?

How does that make plants and animals relate?

The relationship can be seen through the movement of the carbon dioxide. This is called the CARBON CYCLE.


Plants and animals provide each other with their needs.

ACTIVITY: Proving Plants Use Carbon Dioxide

However, the amount of carbon dioxide can also be increased by some other ways. Allow students to come up with ideas.

  1. Burning materials such as wood.
  2. Burning fossil fuels
  3. Decomposition of dead things (Note: this can be further explored in the supplementary lesson “Composting”).

Which of these has the greatest increase on the amount of CO2  in the atmosphere?

Burning of Fossil Fuels.

What is a fossil fuel?

- decomposed (dead) animals and plants (mostly algae) that has been compressed underground for a long period of time until it turns into oil or coal.

- When these are burned they release large amounts of energy that we use to make electricity, heat our homes, and power our cars, trucks, and planes. However, they do make a lot of Carbon Dioxide.

ACTIVITY: Proving Combustion Uses Oxygen and Produces Carbon Dioxide.

Proving Plants Use Carbon Dioxide

Purpose:  This activity uses a pH indicator and an aquatic plant to show that plants use carbon dioxide and that light is necessary for them to incorporate it.

Materials Per Group:

  1. Two springs of an aquarium plant with large amounts of surface area (such as Elodea [Anarcharis], Abulia, Cabomba, or Foxtail). Some terrestrial plants such as creeping Jenny could be used but the results will be slower due to the thicker, cuticle layer on the leaf.
  2. Four large test tubes with screw top caps, corks, rubber stoppers, or plastic wrap and rubber bands. (Small 8 oz plastic water bottles without the labels could also be used, but this will require a larger amount of pH indicator solution)
  3. Enough pH indicator solution (Bromthymol blue works best. See NOTE for information on the Red Cabbage Solution.) to fill each test tube or container. This will vary based on the size of container and the number of groups.
  4. Beakers or large cups to bubble carbon dioxide through the indicator solution to change its color.
  5. Straws
  6. Test tube racks or something to keep the test tubes resting on an angle.(Resting the test tube tops on the edge of a tray works).
  7. White paper for under the test tubes. This reflects more light back into the tubes for a faster result.
  8. Light Source
  9. Aluminum foil

Note:  In this lab the plants will absorb the carbon dioxide from the pH indicator solution causing the solution to revert back to normal. As a control, two samples will be wrapped in aluminum foil to prevent photosynthesis. With Bromthymol Blue solution, the controls do not change. However, the Red Cabbage solution can rapidly degrade and change color whether it is in the light or dark, with or without a plant. Therefore Red Cabbage solution is not recommended. You could fake the lab by removing the controls and just use one test tube with a plant. The color will change and the students will be able to make the correct applications that the plants absorb the carbon dioxide. However, this is bad science

Also, if you are using the Bromthymol blue solution and the reaction is not occurring quickly enough because you are using an indoor light source, leaving the sample for 24 hours should do the trick. If, however, you come in the next morning before the students arrive and the solution hasn’t really changed too much, adding a weak basic solution (a drop or two of ammonia) will show the correct results. By the way, this is also bad science, but...


  1. Distribute materials (keep plants in one location in a bucket or container of water). It is helpful to have the plants cut up into 3- 4 inch segments ahead of time).
  2. Students should divide the pH indicator solution evenly into the beakers or cups and use the straws to bubble carbon dioxide through the solution until it changes color (Bromthymol blue becomes yellow green. Red cabbage solution becomes purple).
  3. Fill the four test tubes (or small plastic water bottles) with the pH indicator solution. It is important to COMPLETELY fill the tubes so that the carbon dioxide in the solution does not diffuse out into the air space.
  4. Place one section of plant in two of the test tubes. Seal all the test tubes.
  5. Wrap one test tube that has a plant section completely with aluminum foil. Repeat with one of the test tubes that does not contain a plant section. These act as controls.
  6. Place all test tubes under the light source and allow them to sit for at least 24 hours.
  7. Examine the tubes and compare the color of the solutions.

Expected Results:  The tubes with no plants should not change. If they do, it may be due to the presence of an air space in the tube. The tubes in foil should not change since there is no light to drive photosynthesis. The plant itself will carry out cellular respiration and generate more CO2 . The test tube with the plant that is in the light should change since the plant is carrying out photosynthesis and consuming the CO2 that is in solution.

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