This study provides insights into the use of metaphors in protein synthesis descriptions in upper secondary chemistry and biology textbooks. Data were collected from seven Swedish textbooks and analyzed with the Metaphor Identification Protocol and categorized within the framework of Conceptual Metaphor Theory. The results reveal two main parallel metaphor systems of "construction-based metaphors" and "information-based metaphors." Five sub-systems with different emphasis on the usage of construction and information related metaphors emerged in the analysis: the "location, translocation, transportation, cryptography" and "publishing" sub-metaphor systems. These metaphors can function as double-edged swords for students' learning. On the positive side, the construction-based metaphors (location, translocation and transportation) meet the educational need to describe where the processes of the protein synthesis occur and how these take place, while the information-based metaphors (cryptography and publishing) describe how the different sub-processes of the protein synthesis are linked via the interflow of information between them. On the negative side, the identified metaphors are presented implicitly without explanations, thus making it difficult for the students to identify them. Also, textbook sentences often contain metaphors drawn from several of the five sub-systems, requiring students not only to differentiate between them, but also to connect the source and target domain of the different metaphors correctly. The results highlight the important role of the teacher in supporting students' learning by explaining what metaphors are and how they are used in textbooks. To further this end, authors of biology and chemistry textbooks are recommended to introduce metaphors early and explicitly.