Fungi were separated into a new kingdom for several reasons, but one key reason is that their cell walls are composed of chitin, while plant cell walls are made of cellulose. This fundamental difference in cellular structure indicates that fungi are more distantly related to plants than previously thought.
Additionally, fungi obtain their nutrients through absorption rather than photosynthesis, meaning they do not produce their own food like plants do. Instead, fungi are heterotrophic, meaning they obtain their nutrients by consuming organic matter, which is another significant distinction from plants.
Overall, the unique characteristics of fungi, including their cell wall composition and their method of nutrient acquisition, were important factors in establishing a separate kingdom for them.