What is the 2nd phase of cellular respiration? what happens?

The second phage of cellular respiration is transition stage.

Process take place in transition stage:

The transition stage take place in mitochondria. The pyruvate is combined with NAD+ to form NADH and acetyl co-enzyme molecules.

After transition stage, Krebs cycle starts.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.

mass and weight are closely related because mass affects the weight of an object experiencing the effects of gravity. weight is the measure of the force of gravity on an objects's mass, while mass is the measure of how much matter there is in an object.