You haven't mentioned the name of the exam for which you are asking about. However, it seems this is for NEET. For neet, biology is the most important section and all of the chapters are important as well.

Class XI-
Diversity of Living Organisms 14%
Structural Organisation in Plants & Animals 5%
Cell: Structure and Function 9%
Plant Physiology 6%
Human Physiology 20%

Class XII-
Reproduction 9%
Genetics and Evolution 18%
Biology and Human Welfare 9%
Biotechnology and its Applications 4%
Ecology and Environment 6%

Go through this-
http://medicine.careers360.com/articles/neet-chapter-wise-weightage-and-important-topics

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Diversity of Living Organisms has 14%, Structural Organisation in Plants and Animals has 5%, Cell Structure and Function has 9% weightage. Plant Physiology has 6%, Human Physiology has 20%, Reproduction has 9%, Genetics and Evolution has 18%, Biology and Human  Welfare has 9%, Biotechnology and its Applications have 4% and Ecology and Environment has 6%. For Physics and Chemistry, please read our article: https://medicine.careers360.com/articles/neet-chapter-wise-weightage-and-important-topics