The statement "glycolysis had a net effect of two ATP molecules" is due to the production of 4 ATP molecules and use of two ATPs during these reactions.
Glycolysis is the first step of cellular respiration, which is well know to produce two ATP molecules.
The complete process generates four ATP molecules but two ATPs are used during glycolysis reactions.
In conclusion, the statement "glycolysis had a net effect of two ATP molecules" is due to the production of 4 ATP molecules and use of two ATPs during these reactions.
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