Plant Respiration

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Author: Janice VanCleave

Food Builders

Respiration is the chemical and physical process by which animals and plants take in oxygen and use it to release energy from glucose molecules. Respiration provides energy necessary for carrying on all essential life processes. Plants are autotrophic—that is, they are able to manufacture glucose through a process called photosynthesis. Photosynthesis is a process in which energy from light is captured and stored in the plant.

In this project, you will use plants to examine the processes of photosynthesis and respiration and determine substances produced and consumed in each. You will study the effect of light on these processes. You will also determine if respiration occurs during the germination of seeds.

Getting Started

Purpose: To test for the production of carbon dioxide by respiration in plants.


  • five 6-inch (15-cm) test tubes
  • distilled water
  • masking tape
  • pen
  • ruler
  • four 5-inch (15-cm) sprigs of elodea or other water
  • plant the length of the test tube (found at a pet store)


  1. Rinse the test tubes with distilled water.
  2. Place a piece of tape about 2 inches (5 cm) long from the top edge down the side of each tube. Use the pen and ruler to mark a line across the tape 1 inch (2.5 cm) from the top of each tube.
  3. Use the pen to label four of the test tubes "1," "2," "3," "4." The fifth tube will be the control. Label this remaining tube "C."
  4. Place an equal-size sprig of elodea into test tubes 1 through 4.
  5. Fill all the test tubes including C up to the 1-inch (2.5-cm) line with bromthymol blue indicator. Put a cork in each of the tubes.
  6. Observe and record the color of the liquid in each tube in a Carbon Dioxide Data table like the one shown in Table 15.1.
  7. Food Builders

  8. Cover the test tubes with aluminum foil so that no light can enter.
  9. Stand the tubes together in a bowl and place the bowl on a table in an unlighted or dimly lit area.
  10. Observe the color of the liquid in each test tube every hour for eight or more hours during the day, then again after 24 hours. Record the color in the data table.

Food Builders


The color of the liquid in test tube C remains unchanged and is blue. The color of the liquid in test tubes 1 through 4 changes from blue to green to yellow as time passes.


Indicators are natural or synthetic chemical substances that change color in response to other chemicals. Acid indicators, such as bromthymol blue, change color in the presence of a substance called acid. Acids cause the bromthymol blue indicator to turn from blue to green, then to yellow, depending on the concentration (amount of one substance dissolved in a given volume of another) of acid present. At a low acid concentration the color is green and at a higher concentration the color is yellow.

A chemical reaction is a change that produces one or more new substances. In a chemical reaction, a reactant is a substance that is changed and a product is a substance that is produced. Internal respiration involves a series of chemical reactions that liberate energy from glucose (a type of sugar) molecules. Respiration is a catabolic process that occurs inside every animal and plant cell. A catabolic process, is a chemical reaction in which larger substances in an organism are broken down into smaller ones. During respiration the catabolic process is the breakdown of a glucose to water and a gas, carbon dioxide, with a release of energy. Internal respiration is also called aerobic respiration, which means that oxygen is one of the reactants. The general equation for aerobic respiration is:

    glucose + oxygen → water + carbon dioxide + energy

The products of respiration of the elodea plant (carbon dioxide and water) mix with the liquid indicator. Some of the carbon dioxide chemically combines with the water, forming the acid called carbonic acid. The more carbon dioxide that dissolves in the water over time, the greater the concentration of the acid formed. The color of the liquid in each test tube indicates changes in the concentration of acid as a result of the respiration of each elodea plant over time.

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