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There are only 400 molecules of CO2 in 1 million molecules of air.

How do plants only take in CO2 but not the other gases?

My teacher told me that's because there is less concentration of CO2 in the leaves than the air and the CO2 rushes in the leaves.But, then how does the leaf maintain the concentration of CO2 and other gases?

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Plants don't take in only CO2, they take in whatever the atmospheric concentration of gases is: gases travel freely across membranes, so it is difficult to select for certain gases.

What your teacher is explaining is based simply on the laws of diffusion in that diffusion results in a net flow from areas of high concentration to areas of low concentration. If CO2 is being used up in chemical reactions in the leaves of a plant through photosynthesis, that means that concentration of CO2 is low there, so the net diffusion of CO2 will be from outside the plant to inside.

Calling it a rush might be a bit misleading, but in general gases mix fairly rapidly (compared to, say, liquids). The plant doesn't really have to do anything to facilitate this except to have surface area available for gas exchange.

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    $\begingroup$ where do the other gases go? $\endgroup$ – Dhruva Oct 29 '18 at 16:01
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    $\begingroup$ @Dhruva They don't really go anywhere, you will find gases of all types in the atmosphere dissolved in plant tissue, they freely come and go. $\endgroup$ – Bryan Krause Oct 29 '18 at 17:57
  • $\begingroup$ @ Bryan Krause If other gases are also found in the plant tissue why don't they combine with hydrogen or oxygen? $\endgroup$ – Dhruva Nov 5 '18 at 9:33
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    $\begingroup$ @Dhruva Why some reactions happen and others don't is what chemistry is all about. Hydrogen is not present as a gas in any appreciable quantity, it's in other compounds like water, so you are really asking "Why doesn't water react with nitrogen gas?" for example. $\endgroup$ – Bryan Krause Nov 5 '18 at 17:40

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