BIOL - Gas Exchange in Plants

During respiration, plants take up oxygen and release carbon dioxide. 

Conversely, during photosynthesis, plants take up carbon dioxide and release oxygen.

Chemical equation for aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Chemical equation for photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Net gas exchange depends on the light intensity

Photosynthesis happens only during day when there is light.

Respiration happens during both day and night because plants need energy all day.

When it is dark...

• There will only be respiration, and no photosynthesis occurs.
• There is a net intake of oxygen and net release of carbon dioxide.

• The rate of photosynthesis equals the rate of respiration.
• The amount of oxygen released by photosynthesis equals the amount of oxygen taken up by respiration. 
• The amount of carbon dioxide taken up by photosynthesis equals the amount of carbon dioxide released by respiration.
• Overall, it seems as if neither gas is taken up or released. The net intake of both gases is zero. 

• The rate of photosynthesis of higher than the rate of respiration.
• The amount of oxygen released by photosynthesis is greater than the amount of oxygen taken up by respiration.
• The amount of carbon dioxide taken up by photosynthesis is greater than the amount of carbon dioxide released by respiration.
• Overall, there is a net release of oxygen and net intake of carbon dioxide.

👇 A summary of the effect of light intensity on the net gas exchange in plants 👇

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Much of gas exchange in plants happens at the leaves through diffusion: Oxygen and carbon dioxide diffuse in and out of stomata.

Structure of Leaves is Adapted for Gas Exchange

• Stomata are pores located at the lower part of the leaf, where gas exchange occurs through diffusion.
• To enable easy diffusion of gases, there is a lot of air space in the spongy mesophyll.
• Leaves are also very thin, which shortens the diffusion distance for gases to diffuse in and out of leaves.

How stomata controls gas exchange:

• A stoma is surrounded by two guard cells.
• During the day, guard cells become turgid (swollen) because water enters the cells through osmosis. This causes the stoma to open.
• During nighttime, guard cells become shrunken (flaccid) because water leaves the cells through osmosis. This causes the stoma to close.

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