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Name: _____________________________________________________
Activity: Stomata in Leaves
Goal: To observe stomata and guard cells in plant cells and
describe their function.
Background Information: Photosynthesis is the process plants use to make
food. The chemical reaction for photosynthesis requires two raw materials; they
are carbon dioxide and water. The sunlight that is captured by the chlorophyll
helps the chemical reaction take place. The chlorophyll is contained in the
chloroplasts of plants.
In plants, the roots absorb water from the soil. The water then moves up
through the plant stem to the leaves. Carbon dioxide is one of the gases in the
air. It enters the plant through small openings on the undersides of the leaves
called stomata. Guard cells control the stomata and therefore regulate the
movement of gases, especially water vapor and carbon dioxide, into and out of
leaf tissues. A stoma opens or closes in response to the changes in pressure
within the guard cells that surround the opening. When the guard cells are
swollen with water, the stoma is open. When the guard cells lose water, the
opening closes. Plants regulate opening and closing of the stomata to balance
water loss with rates of photosynthesis.
Materials: Iodine, lettuce leaf, water, microscope slide, cover slip, microscope,
professionally prepared slides (as back-up)
Procedure:
 What I Know: Write a sentence or two describing what you already know
what materials need to move into and out of plant cells during
photosynthesis? How do you think they enter the cell?

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Observe a prepared slide. If this is not available obtain a small piece of
lettuce. Using a crisp section of the leaf, bend the leaf and gently pull away
the epidermis. This is a thin transparent tissue on the underside of the leaf.
Make a wet mount of the epidermis using fresh water.
Focus your specimen under low power (40X), and then switch to 100X.
Scan your specimen for stomata and guard cells.

What I Did: Write a description of the procedure you used for this activity.

What I Observed: Draw a leaf section and label a stoma. Write a detailed
caption of your observation.
Caption:
Plant Cell
____X

What I Learned: Write what you learned about stomata and guard cells from
your observations. It may be helpful to answer the lab questions below before
you complete this section.

What I Wonder: Pose a why or how question you may still have concerning
stomata or guard cells.

Questions: Write the answers to the following questions using complete
statements.
1. Describe the guard cells around the stomata.
2. How are the guard cells different from other plant cells?
3. How are guard cells similar to other plant cells that we have observed?
4. Did you observe more open or closed stomata in your sample? What
might be a possible explanation for this observation?
5. What is the function of stomata in a leaf?
6. What evidence did you see that shows guard cells engage in
photosynthesis?
7. What substance or substances do you think enter the leaf through
stomata?