This study investigated the contributions of students' reasoning ability and meaningful learning orientation to their understanding of the photosynthesis and respiration in plants concepts. Data were gathered through the use of the Test of Logical Thinking (Tobin & Capie, 1981), the Learning Approach Questionnaire (Cavallo, 1996), and the Two-Tier Diagnostic Test (Haslam & Treagust, 1987). These instruments were administered to 117 8th-grade students to determine their reasoning ability, learning orientations, and conceptual understanding. Correlational and regression analyses revealed that both reasoning ability and meaningful learning orientation were related to students' understanding of photosynthesis and respiration in plants concepts. Reasoning ability explained more of the variance in the test scores as compared to meaningful learning orientation. Moreover, it was found that although reasoning ability was associated with both male and female students' conceptual understanding, meaningful learning orientation was associated only with female students' conceptual understanding. Findings of this study highlighted the importance of reasoning ability as well as meaningful learning orientation in students' understanding of photosynthesis and respiration in plants. (Contains 4 tables.)