Levels of Organization

Cellular Basis of Life: All living things are fundamentally composed of cells. The cell is defined as the basic unit of living things.
Multicellular Complexity: Simple cells come together and integrate to form complex multicellular organisms.
Progression of Complexity: Living material builds up in a hierarchical fashion, moving from simple units to complex systems in the following order: CELL → TISSUE → ORGAN → ORGAN SYSTEM → ORGANISM.

Cell: * Definition: The cell is the smallest independent unit of life. * Examples: Specific types of cells include white blood cells and sperm cells.
Tissue: * Definition: These consist of groups of similar cells that collaborate to carry out a specific function.
Organ: * Definition: An organ is a group of various tissues which work together to perform a specific function. * Examples: Recognized organs in the human body include the liver and the kidney.
Organ System: * Definition: A group of organs that work together to carry out a specific function. * Case Study (Urinary System): The urinary system is formed by the combination of the following organs: * Kidneys * Ureters * Urinary bladder * Urethra
Organism: * Definition: The complete individual made up of all the organ systems within the body. * Species Context: Represents a single individual of a species.

To understand the transition from the simplest unit to the complex human body, the following specific pathway is observed:

Level 1: Specialised Cell: Example used is the muscle cell.
Level 2: Tissue: Groups of muscle cells form muscle tissue.
Level 3: Organ: Muscle tissue converges with other tissues to form the heart.
Level 4: System: The heart acts as a central component of the circulatory system.
Level 5: Organism: The circulatory system makes up one part of the integrated human body.