Graduate teaching assistants (GTAs) hold a unique positionality as instructors and research mentors to undergraduate students, research mentees to faculty members, and employees to an institution. With limited pedagogical training and teaching resources, the enactment of planned teaching activities and learning resources may be influenced by how GTAs conceptualize their teacher identity, role, and experiences. In this study, we explored how chemistry GTAs enacted a scaffolded, cooperative-learning case-comparison activity in a second-semester organic chemistry laboratory course. Our study was guided by the conceptual framework of teacher noticing. Teacher noticing -- an instructor observing "important" instructional moments and connecting their observations to theory and practice -- is a part of developing instructional responses based on students' reasoning. Pairing this conceptual framework with a case study methodology, we recruited two GTAs, and conducted a pre-observation interview, two observations, and a post-observation interview. We explored GTAs' teacher noticing -- what they observed and interpreted as well as how they shaped and responded. We exposed the tension and the resolution between learning objectives (i.e., objectives set by the instructional team for students) and teaching objectives (i.e., objectives set by the GTAs for themselves and their students). GTAs' framing seemed to influence their shaping, and their shaping seemed to balance the instructional team's learning objective and GTAs' teaching objectives. Because chemistry GTAs serve as instructors in many science undergraduate courses, understanding the unique GTA framing may support both graduate and undergraduate learning experiences. Furthermore, our study has implications for researchers who design organic chemistry learning resources to consider different ways GTAs may support students' learning. This study additionally has implications for faculty instructors to develop transformative, consistent professional development opportunities focused on transparency, collaboration, and community in teacher learning.