Il est possible d'isoler, parmi les tentatives de définition du vivant, deux traditions concurrentes : l'une a fait de la reproduction le propre du vivant, tandis que l'autre a vu dans la nutrition puis le métabolisme (le processus matériel par lequel un organisme se maintient en transformant une matière étrangère en substance vivante) une propriété essentielle et un critère d'unification de toutes les formes vivantes. Or, le second terme de cette polarité se subdivise à son tour en un mouvement permanent d'oppositions qui semble caractériser la vie comme « tourbillon » incessant, circulation ininterrompue ou flux constant de matière entre l'intérieur et l'extérieur, les corps et leur environnement. C’est à explorer les mutations conceptuelles internes à cette seconde tradition, la nutrition et le métabolisme, que le présent travail est consacré. Comment le métabolisme s'est-il constitué en problème pour la biologie ? Cette thèse propose une analyse généalogique du concept de métabolisme compris à la fois comme pont reliant la spécificité vitale des organismes à leur.; conditions chimiques d'existence, et comme schème à travers lequel l'autoproduction et le maintien de l'identité biologique ont pu être appréhendés dans une perspective naturaliste. Cette thèse propose une histoire des développements d'une théorie matérielle, chimique, de la vie, et montre, dans le même mouvement, comment l'élaboration d'un « espace épistémique » autour du concept de métabolisme a progressivement permis de redéfinir les contours sous lesquels la question de l'identité biologique été saisie depuis. / It is possible to isolate, among the attempts to define living organisms, two competing traditions: one has made reproduction the proper of living organisms, while the other has seen in nutrition and then metabolism (the material process by which an organism maintains itself by transforming a foreign matter into a living substance) an essential property and a criterion for the unification of ail living forms. However, the second term of this polarity is in tum subdivided into a pem1anent movement of oppositions that seems to characterize life as an incessant "whirlwind", an uninterrupted circulation or constant flow of matter between the inside and the outside, the bodies and their environment. This work is devoted to exploring the conceptual changes within this second tradition, nutrition and metabolism. How did metabolism become a problem for biology? This thesis proposes a genealogical analysis of the concept of metabolism understood both as a bridge linking the vital specificity of organisms to their chemical conditions of existence, and as a scheme through which self-production and the maintenance of biological identity could be approached from a naturalistic perspective. This thesis proposes a history of the developments of a material, chemical and life theory and shows, in the same movement, how the elaboration of an "epistemic space" around the concept of metabolism has gradually made it possible to redefine the contours under which the question of biological identity has since been addressed.

This thesis explores lower secondary students’ meaning-making of epigenetic visual representations within and between biological organization levels. Data obtained from five focus group discussions where students indicated and reasoned about eight epigenetic visual representations were explored. By analyzing students’ interactions with multiple visual representations, and the impact of linking and reasoning patterns on their meaning-making, the research contributes insights to the learning of epigenetics.  Epigenetics, which is gaining rapid importance in emerging biology curricula, is communicated at different biological organization levels, and serves as the meaning-making context explored in the thesis. A compelling biology didactics context, where students are required to reason with multiple representations depicted within and between organizational levels to make meaning about epigenetics. The thesis uncovers three primary findings. First, four linking patterns in students’ meaning-making across and between organizational levels using various visual representations are illuminated. Second, five visual characteristics that influence students’ linking within and between levels were discerned. Third, students’ meaning-making processes were observed to emerge through four phases, which involved form and function attributes of the visual representations, and the transfer of scientific ideas across representations. / <p>Article 1 published in thesis as manuscript, now published.</p>