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Plants has the capacity to take in both CO2 and oxygen, that's during day CO2 and at night oxygen. Why don't humans have such a capability?

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Remember that O2 is also taken by the plants during the day ! –  biogirl Feb 14 at 15:35

2 Answers 2

You should have a look biological processes which are:

  • cellular respiration
  • photosynthesis

Loosly speaking, these two processes are just the opposite but deal with the same chemical compunds.

$$C_6H_{12}O_6 + 6O_2 <=> 6CO_2 + 6H_2O (+ Energy)$$ or in words... $$glucose + Oxygen <=> Carbon Dioxide + Water (+ Energy)$$

From left to right is the process of respiration and from right to left is the process of photosynthesis.

  • Respiration releases energy from glucose (sugar). Therefore, animals, plants, fungi respire in order to get their energy from their food. More information on the chemical pathways on Krebs cycle (or citric acid cycle). The main organelle involved in celullar respiration is the mitochondrion. Note: (Loosely speaking) the glucose is the end product of the digestion

  • photosynthesis allows organisms to create its own tissues from water and $CO_2$. But this cost energy. This is why plants use capture the energy form the sun with their leaves. More information on the chemical pathways on Calvin Cycle. The main organelle involved in photosynthesis is the chloroplast.

Note: In our lungs comes air which is a mixture of different gases (just like coke (Coca-Cola) is a mixture of different compounds). The two main compounds of air are Nitrogen (~78%) and Oxygen (~21%). We use only Oxygen. More info on the air composition here

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Humans breathe in air, which contains both oxygen and carbon dioxide. But we do not photosynthesise and so we do not take in CO2. The CO2 produced from respiration in our body builds up in the blood and through diffusion escapes into the lungs and out when we exhale.

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