Most challenging chapters or topics in the Biology section of the NEET exam:

1. Genetics and Evolution:
Genetics and Evolution is a challenging topic in the NEET Biology section. It involves the study of inheritance, variations, and the mechanisms of evolution. Key points to focus on include:

- Mendelian Genetics: Understanding the laws of inheritance, genetic crosses, and the inheritance patterns of various traits.
- Molecular Genetics: Understanding DNA replication, transcription, translation, and gene regulation.
- Human Genetics: Studying genetic disorders, pedigree analysis, and genetic counseling.
- Evolution: Understanding the mechanisms of evolution, such as natural selection, genetic drift, and speciation.

2. Human Physiology:
Human Physiology is another challenging topic that requires a deep understanding of the functions and mechanisms of various systems in the human body. Key points to focus on include:

- Respiratory System: Understanding the mechanics of respiration, gas exchange, and transport of gases in the blood.
- Digestive System: Understanding the structure and functions of organs involved in digestion, absorption, and metabolism of nutrients.
- Circulatory System: Understanding the structure and functions of the heart, blood vessels, and blood composition.
- Nervous System: Understanding the structure and functions of neurons, nerve impulses, and coordination.
- Endocrine System: Understanding the functions of various glands and their hormones in maintaining homeostasis.

3. Plant Physiology:
Plant Physiology is also a challenging topic in the NEET exam. It involves the study of various processes and functions in plants. Key points to focus on include:

- Photosynthesis: Understanding the light and dark reactions, factors affecting photosynthesis, and photorespiration.
- Plant Growth and Development: Understanding the role of plant hormones, photoperiodism, and vernalization.
- Mineral Nutrition: Understanding the uptake and transport of essential minerals in plants, nutrient deficiency symptoms, and mineral toxicity.
- Plant Reproduction: Understanding the different modes of reproduction in plants, such as sexual and asexual reproduction.

4. Biomolecules:
Biomolecules is an important and challenging topic in the NEET Biology section. It involves the study of various types of molecules found in living organisms. Key points to focus on include:

- Carbohydrates: Understanding the structure, types, and functions of carbohydrates in living organisms.
- Proteins: Understanding the structure, types, and functions of proteins, including enzymes.
- Lipids: Understanding the structure, types, and functions of lipids, including their role in energy storage and cell membrane structure.
- Nucleic Acids: Understanding the structure and functions of DNA and RNA, including DNA replication, transcription, and translation.

5. Ecology and Environment:
Ecology and Environment is a challenging topic that focuses on the study of interactions between organisms and their environment. Key points to focus on include:

- Ecosystem: Understanding the components, energy flow, and nutrient cycling in ecosystems.
- Biodiversity and Conservation: Understanding the importance of biodiversity, threats to biodiversity, and conservation measures.
- Environmental Issues: Understanding the impact of pollution, climate change, and deforestation on ecosystems and human health.

Overall, these chapters/topics require a thorough understanding of the concepts, their interconnections, and their applications in real-life situations. Regular practice, revision, and solving previous years' question papers can help in mastering these challenging topics.