Reconstructing student conceptions by modelling and using the zoom map: the case of cherry rain-cracking and osmosis

Willinghöfer, Anja; Hammann, Marcus; Asshoff, Roman

Abstract

Cherry rain-cracking is a popular way of teaching osmosis in biology. For a long time, scientists and educators have explained this phenomenon by referring to the ‘critical turgor pressure model’, according to which fruit flesh volume and internal pressure increase, causing the fruit’s skin to burst. Recently, this explanation has been revised and replaced by the ‘zipper model’, which explains crack formation as a series of events in which the skin ruptures similar to a zipper. Using qualitative methods, we investigated the conceptions of 43 students aged 16–18 years regarding cherry rain-cracking, and evaluated teaching materials aimed at conceptual reconstruction. In the pretest, students predominantly used the ‘critical turgor pressure’ to explain the phenomenon and compared the cherry to a balloon whose skin bursts because of the increased pressure. In the posttest, most students favoured the zipper model and referred to more levels of organisation to explain the phenomenon. Our results have two main teaching implications. First, the modelling process supports students in reconstructing their conceptions. Second, the zoom map helps students move back and forth between different levels of organisation and construct explanations that are more scientific.