Photosynthesis and Respiration

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Author: Janice VanCleave

For survival, plants as well as animals need the energy stored in chemical bonds. By photosynthesis, plants convert the energy from the sun into chemical energy stored in the form of complex sugar molecules. By respiration, plants release this stored chemical energy.

In this project, you will examine the processes of photosynthesis and respiration and determine substances consumed and produced in each. You will also look at factors affecting these reactions, such as light and the presence of oxygen.

Getting Started

Purpose: To determine whether respiration goes on in plant tissue.


  • 3 baby food jars with lids
  • Distilled water
  • Marking pen
  • 2 sprigs of elodea or other
  • Water plant (found at a pet store)
  • Brom thymol blue indicator (see Appendix 2)
  • 1-×-1-foot (30-×-30-cm) sheet of aluminum foil
  • Desk lamp


  1. Rinse the baby food jars with distilled water.
  2. Use the marking pen to number the jars 1, 2, and 3.
  3. Place a sprig of elodea into jars 1 and 2.
  4. Fill all three jars with brom thymol blue indicator.
  5. Put the lid on each jar.
  6. Cover jar 1 with the aluminum foil so that no light can enter.
  7. Place all three jars about 8 inches (20 cm) in front of the desk lamp (see Figure 16.1).
  8. Check the color of the solution in each jar every hour for eight hours. Take care to quickly replace the aluminum foil over jar 1.

Photosynthesis and Respiration


The color of the solution in jar 1 changes from blue to green and finally to yellow. The changes in jar 1 occur quickly. The color of the solution in jar 2 slowly turns green. The color of the solution in jar 3 remains unchanged.


Brom thymol blue indicator contains water and a dye extract This indicator can be used to test for the presence of carbon dioxide. When carbon dioxide combines with water, it forms a weak acid (carbonic acid). Depending on the amount of carbon dioxide dissolved, the indicator turns from blue to green or yellow (the more carbon dioxide, the more yellow the liquid turns).

Photosynthesis and respiration are the two energy-producing reactions in plants. Photosynthesis requires light and occurs only during the day. Respiration goes on day and night These reactions are exact opposites of each other: Respiration combines sugar and oxygen to form carbon dioxide and water; photosynthesis, with energy from light, combines carbon dioxide and water to form sugar and oxygen.

In the dark, the plant in jar 1 produces energy by respiration. A large amount of carbon dioxide is released, as indicated by the changing of the solution from blue to yellow. Both photosynthesis and respiration occur in jar 2. Carbon dioxide is produced by the plant during the respiration reaction, but some of it is removed from the solution during the photosynthesis reaction. The green color of the solution indicates the presence of a smaller concentration of carbon dioxide. Jar 3 is the control. Without plants, neither photosynthesis nor respiration can occur in the jar, so the color of the brom thymol blue remains unchanged.

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