The second ATP is consumed in the _ step of glycolysis.

Explanation

In glycolysis, the first step involves the phosphorylation of glucose to glucose 6-phosphate, consuming the first ATP. The second step converts glucose 6-phosphate into fructose 6-phosphate through an isomerization reaction and does not use any ATP. The third step is catalyzed by phosphofructokinase-1, which phosphorylates fructose 6-phosphate to fructose 1,6-bisphosphate, consuming the second ATP and serving as a key regulatory step. The fourth step involves the cleavage of fructose 1,6-bisphosphate into two three-carbon molecules, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, and does not require ATP. Therefore, the second ATP is specifically consumed in the third step.