1. Cells: The fundamental unit of life. All living organisms are made of cells, which carry out basic life processes.
2. Tissues: Groups of similar cells that work together to perform a specific function. Examples include muscle tissue, nervous tissue, connective tissue, and epithelial tissue.
3. Organs: Structures made up of different tissues that work together to perform a complex function. Examples include the heart, lungs, stomach, and brain.
4. Organ Systems: Groups of organs that work together to carry out major life functions. Examples include:
* Digestive System: Breaks down food and absorbs nutrients.
* Respiratory System: Takes in oxygen and releases carbon dioxide.
* Circulatory System: Transports blood, oxygen, and nutrients throughout the body.
* Nervous System: Controls and coordinates bodily functions.
* Skeletal System: Provides support and structure to the body.
* Muscular System: Enables movement.
* Endocrine System: Produces hormones that regulate bodily functions.
* Excretory System: Removes waste products from the body.
* Reproductive System: Produces offspring.
* Integumentary System: Protects the body and regulates temperature.
5. Organism: A complete living being made up of multiple organ systems working together. This is the highest level of organization in a single living thing.
Important Note: This hierarchical organization applies to multicellular organisms. Unicellular organisms, like bacteria, consist of a single cell that carries out all the functions of life.
