2. Why are molar units preferential to % when expressing photosynthesis rates?

The principles of molarity, a topic covered in Introductory Chemistry, are important to biology as well. Molarity is the unit most commonly used when describing chemical reactions and biological processes at the molecular level.

A 'mole' is the quantity of a substance present when the weight in grams equals the molecular weight of that substance. For example,

one mole of carbon = 12 g (the molecular weight of the carbon atom = 12)

one mole of uranium = 238 g ( the molecular weight of the uranium atom = 238)

one mole of O2 = 32 g ( the molecular weigh of O2 = 32)

and so on.

In all cases, this quantity of a substance contains 6.02 x 1023 particles (atoms or molecules) of that substance.

(6.02 x 1023 is referred to as Avogadro's number.) For example,

one mole (12 g) of carbon contains 6.02 x 1023 carbon atoms

one mole (32 g) of O2 contains 6.02 x 1023 O2 molecules

Thus, if one expresses the amount of a substance (such as O2 ) in molar units, then it is always possible to calculate the actual number of atoms or molecules that are present.