Here's a breakdown:
* Structure: This plays a major role. For example, the presence or absence of a nucleus (prokaryotes vs. eukaryotes) is a fundamental difference used to separate kingdoms.
* Cellular organization: Unicellular vs. multicellular organisms are another important characteristic.
* Mode of nutrition: How an organism obtains food (autotrophic, heterotrophic) is crucial.
* Reproduction: The method of reproduction (sexual vs. asexual, etc.) helps define kingdoms.
* Evolutionary history: The relationships between organisms, as determined by their evolutionary history, are also considered.
In essence, classifying organisms into kingdoms is a multifaceted process based on a combination of factors. Structure is a significant part of this, but it's not the sole determinant.
Think of it as a puzzle where structure is one large piece, but other pieces (like nutrition, reproduction, and evolutionary history) are needed to complete the picture.
