In this research word association test was used to explore the cognitive structure of Pre-service science teachers about biological energy metabolism. The participant group constitutes 67 Preservice students at a university in Black Sea Region of Turkey. Ten key concepts in General Biology course were given as a stimulating word and a mind map was established by using the frequency values of valid response words. The results showed that the total number of different response words for the key word "enzyme" is the highest with "cell" and "energy" being higher than others and for the key word "photosynthesis" is the lowest. The mind map also showed that they based their cognitive perceptions of the biological energy mechanism mostly on scientific concepts that are emphasized in the lessons and are academically acknowledged and included in the concepts used in everyday life at the least. However, they were not able to see the key concept "enzymes" as a part of branched network and they had also problems regarding the respiratory event in the plants as well as realization process of photosynthesis in the cellular level. Implications of the results in terms of understanding the subject of biological energy metabolism are also discussed.