Skip Navigation

Cell Structures - How Does a Leaf Work?

Previously, you learned about the basic structure of cells. Sci-ber text also described how the functions performed by cells can affect organisms.

Leaves have special kinds of cells. One of these special kinds of cells controls what enters and exits a leaf. Plant cells need carbon dioxide to make food using the energy of sunlight (photosynthesis). Plant cells also must remove the oxygen produced as a waste product of photosynthesis. As the leaf cells perform their functions, water is lost from the leaf. The plant uses small openings in its leaves (remember that leaves are organs in a plant) to allow gases to enter and leave. This activity will allow you to determine which side of a leaf is exchanging the most gases by determining which side is losing the most water.

Often the underside of a leaf has more stomata (openings that allow gases to enter and leave the leaf) than the top side of a leaf. Your objective will be test to see if this statement is true by using cobalt chloride test paper. Cobalt chloride test paper is blue. When exposed to moisture, the paper turns pink.


  • Four strips of cobalt chloride test paper
  • Transparent tape
  • Plant with leaves
  • Watch, stopwatch, or timer


  1. Go outside and find a plant with leaves still attached.
  2. Tape a strip of cobalt chloride paper to each side of 2 leaves.
    • One leaf needs to be in the sunlight and one leaf needs to be in the shade.
    • Remember leaves need to remain attached to the plant!
  3. Every 60 seconds for 10 minutes, record the color of the paper. (Is it still blue, part pink, all pink, etc.?)
  4. Record your data in a table such as found below.
  5. When allowed to sit in a dry location, the cobalt chloride strips will return to their original state and can be reused.

Data table:

Time (sec.) Upper side of leaf in shade Lower side of leaf in shade Upper side of leaf in sunlight Lower side of leaf in sunlight


  1. Which side of the leaf (upper or lower) had a cobalt chloride strip change color the fastest?
  2. What was the difference between the changes in color of the strips which were in sunlight compared to those in the shade?
  3. Try to find a plant which will either change colors more rapidly or much more slowly than the one you tested.
    • In what ways are the first and second plants similar?
    • How are these two plants different?
    • What will cause a plant to lose less water?
utah state board of education This Sci-ber Text was developed by the Utah State Board of Education and Utah educators.