What is a Stoma? - Definition & Function

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  • 0:05 Plant Stoma
  • 0:50 How Stomata Open and Close
  • 2:20 Animal Stoma
  • 3:03 Surgical Stoma
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: Brekke Peterson Munks
Did you know that both plants and animals have stomata (more than one stoma)? In this lesson, we'll explore the location, function, and importance of stomata in plants and animals.

Plant Stoma

Did you know plants breathe through pores similar to the human mouth? These structures are called stomata, plural, or stoma for singular use. Stomata are pores found on stems, leaves and other plant parts that control gas exchange. Essentially, these structures allow carbon dioxide to enter, and along with water, carry out photosynthesis in the presence of light, to produce glucose. Oxygen is released through the stomata as a waste product resulting from photosynthesis, and some water vapor also leaves through a process called transpiration.

Under the microscope stomata look like little footballs on the surface of plant structures. These structures are found in all plants.

How Stomata Open and Close

You can think of a stoma like your mouth. What opens and closes your mouth? Muscles that surround your lips. Plants don't have muscles, but they have a specialized structure that opens and closes stomata called guard cells. These cells pump ions such as calcium and potassium in and out of the cell, causing the cell to contract, resulting in the stoma opening or closing. This is similar to the way that muscles contract and release.

These cells work as a result of environmental triggers that alter the turgor pressure of the guard cells. The turgor pressure increases as a result of ions flowing in the guard cells, causing water to also flow in; then, the stoma opens. On the contrary, when ions and water flow out of the guard cells, turgor pressure decreases, and the stoma closes.

Factors that affect turgor pressure include levels of light, water vapor and carbon dioxide. On hot days when water is limited and transpiration is high, stomata generally stay closed. Early in the morning, many plants have open stomata since the temperature is cooler and the air is full of water vapor. Some desert plants, like succulents, open their stomata at night and can store carbon dioxide until the following day.

In situations where carbon dioxide and water are prevalent, stomata might open a very long time as the plant is photosynthesizing and has oxygen and water vapor to get rid of through these structures.

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