Complex sentence: Ogsten sieving occurs when the molecules retain there standard shape but does not account for molecules stretching or deforming to squeeze through the pores of the sieving matrix while reptation is where the molecule moves through the separation media as a snake.
Split into simpler sentences: Ogsten sieving occurs when the molecules retain there standard shape but does not account for molecules stretching or deforming to squeeze through the pores of the sieving matrix. Reptation is when the molecule moves through the separation media as a snake.