Glucose is a six carbon sugar when glycolysis is over it is split into two three carbon molecules called pyruvic acid (pyruvates). When this is done the link cycle begins (second stage of respiration) . In the link reaction the pyruvic acid loses a carbon that becomes CO2. The remaining two carbons are called acetic acid. This then meets enzyme coenzyme A and becomes acetylcoenyme A. This combination is the start of the kreb's cycle.