The aim of this study was to investigate 9th grade biology students' cognitive structures about the basic components of the living organisms and association between the concepts forming these basic components. Participants consisted of 50 9th grade biology students in two secondary schools in the northwest of Turkey. Data were collected by the word association test (WAT) and the questionnaire of writing sentence (QWS). The WAT is comprised 16 key concepts related to Biology: Inorganic compounds, organic compounds, acid, base, salt, water, mineral, carbohydrate, fat, protein, enzyme, vitamin, nucleic acid, DNA, RNA, and ATP. For the analysis of the WAT, the frequencies of the number of words and the number of word varieties about each key concept in the WAT were counted. Mind maps were then drawn with the help of the technique of breakpoint according to represent the WAT data. The QWS was analyzed by content analysis method. The study results showed that the existing conceptual relationships in the mind maps of the students were actively used in the sentences in the students' written QWS. The experiences of the students such as life, laboratory studies, and the sharing of teachers in lessons on healthy diet were effective in the cognitive structures of the students. However, the existence of significant deficiencies and some non-scientific relationships in the students' cognitive structures on basic components of living organisms were detected.