Gas Exchange in the Dermal Tissue

How are the stomata and guard cells involved in gas exchange?



A11.3 Electron micrograph of stomata
Due to the waxy cuticle and the epidermis, it is very hard for a plant to get all the gases it needs through diffusion. Plants have solved this problem by having specialized cells called guard cells. Guard cells control the opening and closing of small holes in the underside of the leaf called stomata. The stomata allow gases to pass into or out of the cell, depending on their concentration gradient. If there is more oxygen inside the leaf than outside, the oxygen will move to the outside of the leaf through the stomata.

How big the stomata are and when they are open or closed is controlled by the guard cells. It is extremely important that the guard cells do this job, as water can also escape through the stomata. If the stomata were left open all of the time, the plant would quickly dry out and die. If the stomata were never open, though, the plant would not get enough carbon dioxide to undergo photosynthesis. The plant would then starve and die.

The guard cells open and close the stoma depending on how much sunlight is reaching them. When light hits the leaves, it stimulates the guard cells to bring potassium ions into the cell through active transport. This creates a high concentration of potassium inside the cell, so water flows into the cell through osmosis to try to dilute that concentration. The side of the cell away from the stoma has a thinner membrane, so the water is able to push out farther in that direction, giving the cell a crescent shape and opening the stoma. Each stoma have two guard cells surrounding it, when the stoma is open, both these guard cells have a crescent shape, forming the stoma in the centre.


A11.4 Open and closed stoma


When the stomata are open, carbon dioxide and oxygen can flow into and out of the leaf, but so can water. This loss of water out of the stomata is called transpiration. When there is not enough water left to flow into the guard cells, they go limp and the stomata close. This lack of water signals the cell to allow the potassium ions to move out of the guard cells. This changes the concentration gradient of the potassium ions and water leaves the guard cells to try to dilute the now high concentration of potassium ions outside the cell. The guard cells go back to their original shape and the stomata close.

The number of stomata found in the epidermis of a leaf is dependent on the plants environment. Plants in a hot and dry environment have fewer stomata to stop the loss of water through transpiration. Plants in a wet environment will have more stomata since water loss is not as big of an issue. Stomata are also sensitive to the amount of carbon dioxide in the air. If there is not enough carbon dioxide in the air, the stomata will open as wide as the can to try to allow all the carbon dioxide possible into the leaf so the plant can continue with photosynthesis.

A11.5 Comparison of plants in a hot and dry vs. wet environment

  Watch This

Stomatal Closure in Tradescantia Leaf Cells © YouTube davcjal



This video will show you a stomata closing in a leaf. From this video, you can see the stomata does not snap open or closed; instead it slowly closes as the water moves out of the guard cells into the surrounding cells.





Did You Know?


A11.6 Maple tree

The underside of a leaf can have anywhere from 10,000 to 100,000 stomata per cm2! On a hot day, when all these stomata are open, an average size maple tree may lose up to 200 L of water. Just imagine how much water that maple tree needs to survive.

  Read This

Please read page 309 and the top of page 311 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on how the guard cells open and close the stomata and what the function of the stomata is. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. In your own words, explain how a stoma opens and closes.

Your answer should be a variation of the following:

  • When the sunlight touches the guard cells, the guard cells begin to actively transport potassium ions into themselves.
  • This creates a high concentration of potassium inside the cell, so water moves into the cell through osmosis.
  • This creates a high water pressure inside the guard cells and causes them to move into a crescent shape.
  • This opens the stoma.
  • As the water pressure begins to go down, the cell actively transports potassium out of the cell.
  • The water then follows the potassium ions through osmosis and the guard cells go back to their original shape.
  • This closes the stoma.
  1. Why is it important that the stomata open and close in response to the amount of sunlight present?
Photosynthesis requires light to occur, so it is important that during that time, the plant has the carbon dioxide it needs in order to perform photosynthesis. Note: There are parts of photosynthesis that occur in the dark, but those parts do not require carbon dioxide like the parts that occur in the light.