21st century learning focuses on problem-solving skills. A higher level of education requires an understanding of a more systematic concept to elaborate problems. Problem-solving skill is one of the fundamental cognitive processes. It involves higher order thinking to discover solutions or ideas. This research aims to analyze the relationship between concept mastery and problem-solving skill of pre-service biology teacher in human physiology courses. This is descriptive correlational research conducted in Sulthan Thaha Saifuddin State Islamic University Jambi. The sample was selected using the purposive sampling technique. Respondents in this study were 33 students in semester VI. The instrument used in the study was an essay test of concept mastery integrated with problem-solving. The results showed that the average scores of the students' concept mastery and problem-solving were 56.8 and 58.0, respectively. Both included in the poor category. It also showed that there was a relationship between students' concept mastery and problem-solving skill. The correlation value was 0.76, with a significance value of p0.05; therefore, it included in the strong category.

Implementing a proper learning model during the post COVID-19 pandemic is fundamental for learning quality enhancement, specifically for students' conceptual mastery. The research aims to develop a Complexity Science- Problem Based Learning (CS-PBL) model assisted by the Sistem Informasi Pengelola Pembelajaran (SIPEJAR) e-learning platform that is valid, practical, and effective to enhance students' conceptual mastery during the post COVID-19 pandemic. The research and development model were adapted from Plomp Nieven consisted of three phases: preliminary research, prototyping phase, and assessment phase. The first phase research result was that the learning process in during COVID-19 pandemic was less interactive, which led to less effective learning. The students' score on conceptual mastery was in the poor category. The second phase resulted in a book of CS-PBL model assisted by SIPEJAR and supporting instruments considered valid by three experts. The third phase result was that the CS- PBL model assisted by SIPEJAR was considered practical in the learning process implementation. The CS-PBL model can enhance students' conceptual mastery where the N-gain was sufficiently effective. It is concluded that the CS-PBL model assisted by SIPEJAR was considered valid, practical, and effective to enhance students' conceptual mastery during the post COVID-19 pandemic.

Critical thinking skills and concept mastery are essential for every individual to solve a problem. Critical thinking is an indispensable complex skill in the 21st century. This study describe that the contribution of critical thinking and concept mastery to problem-solving skills through CS-PBL is 65.3%. It could be concluded that CS-PBL is applicable to develop students' critical thinking skills and concept mastery and impacts their problem-solving skills. Accordingly, the implementation of CS-PBL is strongly recommended.