Question

The synthesis portion of photosynthesis is sometimes referred to as fixation; that is, the gas `CO_2` is "fixed" into an organic molecule. Of the four major types of organic molecules, into which is `CO_2` first fixed?

Answer 1

Carbohydrate.

Explanation:

`CO_2` taken in by the plant initially bonds to ribulose 1,5-biphosphate or `RuBP` to form an unstable 6-carbon compound. This then divides into two three-carbon compounds of glycerate 3-phosphate. With the addition of `ATP` and `NADH_2` for energy and oxidation, glycerate 3-phosphate becomes triose phosphate, which join into glucose, a sugar that is pivotal in biology.

Sugars are a form of carbohydrate, and often end in the suffix '-ose', though this is not systematic. Glucose and ribulose are the two examples here, both of which are sugars or sugar-containing molecules.