Here's why:
* Different metabolic pathways: Organisms have evolved different metabolic pathways and abilities. Some organisms can synthesize all the amino acids they need, while others cannot.
* Plants and microorganisms: Plants and many microorganisms can synthesize all 20 amino acids from simpler molecules. They are not dependent on external sources for these building blocks.
* Animals and humans: Animals and humans lack the metabolic pathways to synthesize certain amino acids, known as essential amino acids. They must obtain these amino acids from their diet.
* Essential amino acids vary: The specific amino acids considered essential vary between species. For example, humans need 9 essential amino acids, while dogs need 10.
In summary: Essential amino acids are crucial for certain organisms, including humans and animals, but they are not required by all life forms.
