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QuietDragon2728

What is the difference between photosynthesis and cellular respiration and how to remember it?

Éveline O

Thanks for your question!

First of all, the photosynthesis reaction and the cellular respiration reaction are two inverse reactions. Indeed, the inputs of one are the outputs of the other and vice versa.

Here is the formula for the photosynthesis reaction:

$$Energy + 6CO_{2} + 6H_{2}O⟶C_{6}H_{12}O_{6} + 6O_{2}$$

Here is the formula for the cellular respiration reaction:

$$C_{6}H_{12}O_{6} +6O_{2}⟶Energy +6CO_{2} +  6H_{2}O$$

There are several differences between the two reactions (besides their inputs and their outputs):

• The photosynthesis reaction is a synthesis reaction. There is creation of a molecule of glucose (C6H12O6), a molecule more complex than those from which it was created.

• The cellular respiration reaction is a combustion reaction. Indeed, although it may seem counterintuitive, it is a reaction with oxygen which produces energy.

• The photosynthetic reaction only takes place in living organisms that have chloroplasts, that is to say, plants. Chloroplasts contain in particular chlorophyll, the green pigment which is essential for photosynthesis.

There are several tips for remembering how to tell the two apart (aside from memorizing which reaction is which). For me, the best has always been to use the very names of the reactions:

• Photosynthesis consists of synthesizing a complex molecule (glucose), hence one of its outputs is glucose.

• Cellular respiration is the same reaction that occurs in our body when we breathe. One of the gases that we breathe out is carbon dioxide, just like one of the outputs of cellular respiration.

The main thing is to remember that one reaction is the reverse of the other.

Do not hesitate if you have other questions!