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LAB 9
Homeostasis in Stomates
Introduction:
Stomates are special openings on the underside of leaves that allow for the exchange of
carbon dioxide and oxygen between the tissue inside the leave and the environment.
Water vapor also passes out of the leaf through the Stomates. The Stomates are not open
all the time. Instead, they open and close according to the needs of the leaf. This is an
example of homeostasis. Each stomate is surrounded by a pair of specialized epidermal
cells called guard cells. The kidney shaped guard cells regulate the opening and closing of
the Stomates. Stomates allow for the process of transpiration to occur with in a plant.
Transpiration is the evaporation of water vapor from plant surfaces.
Materials:
Crisp spinach leaf
Forceps
Microscope cover slip
Dropper
Microscope slide
Compound light microscope
Procedure:
1. Soak spinach in water.
2. Take on leaf and tear it gently, crosswise.
3. With forceps, carefully remove a thin layer of lower epidermis from the piece of
spinach, (it should be transparent)
4. Place it on a microscope slide and add a drop of water. If the spinach epidermis
becomes folded on the slide, use a probe to gently unfold and flatten it. Add a cover
slip.
5. Observe the epidermis under low power of the microscope. HINT: you may need to
adjust the amount of light entering through the diaphragm in order to see the cells
clearly.
6. Switch to high power and identify the guard cells which are half circle-shaped cells and
the Stomates which are the small spaces or openings between the guard cells.
Record the number of Stomates in your field of view:_______________________
Make a drawing of the high power field of view. Include the epidermal cells, guard
cells, stomates, and chloroplasts.
7. Lift up the cover slip and place a drop of 4% salt solution on the spinach
epidermis. How has the stomate opening changed in comparison to your first
observation?
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When plant cells are placed in a 4% salt solution, does water move into or out of the
cells?
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As water is lost from guard cells they tend to collapse. Based on your observations,
does this cause the stomates to open or close?
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As water is gained by guard cells, they tend to swell. Based on your observations,
does this cause the stomates to open or close?
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Analysis and Observations
1. What is the function of the chloroplasts in the guard cells?
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2. While guard cells are performing photosynthesis, what happens to their sugar
concentrations? Their water concentration?
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3. What part of the 24 hour day are stomates opened, closed?
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4. How does the shape of a leaf help it use sunlight efficiently?
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5. Explain the function of the cuticle.
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Part B
Construct a line graph based on the information in the table, which shows the hourly rate
of transpiration in a plant over a 16-hour period. Plot time on the x-axis and transpiration
rate on the y-axis.
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Time (h)
Transpiration rate (g/h)
Time (h)
Transpiration rate (g/h)
8 am
190
4 pm
227
9 am
200
5 pm
220
10 am
209
6 pm
213
11 am
215
7 pm
208
12 noon
221
8 pm
190
1 pm
227
9 pm
120
2 pm
233
10 pm
100
3 pm
230
11 pm
98
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After completing the graph, answer the following questions
1. At what times are the rates the highest?
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2. At what times are the rates the lowest?
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3. How do you account for this?
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4. What is the independent variable?
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5. What is the dependent variable?
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6. What is the transpiration rate at 8:30 pm?
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Part C
1. What typical plant cell organelle would a root cell have less of than a leaf? Explain.
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2. If the atmosphere was contaminated and only green light was able to pass through the
ground, what would surviving plants look like in a thousand years?
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3. What factors regulate the opening and closing of stomates?
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4. Explain how your answer to question 3 is an example of homeostasis?
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5. What effect would a drought have on a plant regarding stomates?
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6. On which leaf surface of a water lily plant would you expect to find the stomates (top
or bottom)? Explain your answer.
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