When mixed with water, the gliadin and gluten in proteins in wheat flour form gluten which is an　extensible, cohesive, and elastic structure that is key to the texture of dough products.

这个句子中 which is an extensible, cohesive, and elastic structure 是定语从句，限定说明先行词 gluten，那么 structure 后面的关系代词 that 是说明 gluten 还是 structure？请专家帮我把句子成分划定一下？