Question

Glucose is made of 6 carbons. Where did those carbons originally come from?

Answer 1

It comes from Carbon Dioxide.

Explanation:

Glucose is formed through Photosynthesis by plants and some eukaryotas.

During that process, 6 molecules of `CO_2` with 6 molecules of `H_2O` are converted into 1 molecule of Glucose and 6 molecules of `O_2`. This reaction uses sunlight as the source of energy.

Or basically like this:

`6` `CO_2 +` `6` `H_2O => C_6H_12O_6` `(Glucose) + 6` `O_6`

References and Further Reading:
Glucose
Photosynthesis