Systems thinking in biology education is an up and coming research topic, as yet with contrasting feasibility claims. In biology education systems thinking can be understood as thinking backward and forward between concrete biological objects and processes and systems models representing systems theoretical characteristics. Some studies claim that systems thinking can be readily introduced at the level of primary-school education; other studies claim that even in upper secondary-school education the introduction of systems thinking requires a carefully outlined approach in order not to strain students' capacities too much. In order to explain these contrasting claims a procedure of analysis was used to map and compare the conceptual frameworks of the various studies, focusing on the selection of systems, references to systems theory and definitions of systems thinking. The analysis demonstrates that the frameworks include different elements of systems thinking. In particular, the analysis shows that the studies recommending the introduction of systems thinking in primary- or lower secondary-school education did not measure students' ability to think forward and backward between concrete objects and systems models. The analysis contributes to a discussion about useful preparatory learning and teaching trajectories, preceding the formal introduction of systems thinking. (Contains 3 tables.)