Most Complex:
* Organism: A complete living being made up of multiple organ systems working together. Examples: a human, a tree, a dog.
* Organ System: A group of organs that work together to perform a specific function. Examples: digestive system, circulatory system, nervous system.
* Organ: A structure made up of different tissues that work together to perform a specific function. Examples: stomach, heart, brain, skin.
* Tissue: A group of similar cells that work together to perform a specific function. Examples: muscle tissue, nervous tissue, connective tissue, epithelial tissue.
* Cell: The basic unit of life, capable of carrying out all the functions necessary for life. Examples: nerve cells, muscle cells, blood cells.
Least Complex:
* Molecule: Two or more atoms bonded together. Examples: water (H₂O), DNA, protein.
* Atom: The smallest unit of an element that retains the chemical properties of that element. Examples: carbon (C), oxygen (O), hydrogen (H).
Important Note: This hierarchy isn't entirely rigid, and there's some overlap between levels. For instance, some molecules (like proteins) can form complex structures that function like organs, while cells often have specialized structures that act like mini-organs. The key point is that complexity increases as you move up the hierarchy.
